(SJonicU Mntutraity Hibrai'y 3thara. Nrm tjork Cornell University Library arV11667 Wonders of the moon. 3 1924 031 323 946 Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031323946 K\ V WONDERS THE MOON. TRANSLATED FEOM THE FRENCH OF AMEDEE GUILLEMIN, MISS M. G. MEAD. EDITED, -U-ITH ADDITIONS, BY MAEIA MITCHELL, OF YASSAK COLLEGE, N. Y. Illustrated with Forty - three Engravings. NEW YORK : SCRIBNER, ARMSTRONG AND COMPANY, 1873- Entered according to act of Congress, in the year 1872, by SCIJBNER, AraiSTRONG k COMPANY, La tlie ofEice uf the Librarian of Congress at Washington. RI\'EliSIDK, CAMBRIOOE: FF.INTKD BY II. O. HOUOTITON AND COMPANY. EDITORS PREFACE. M. Guillemin's little book on the Moon is intended for popular reading ; it is neither for the school-roora nor the observatory. It is adapted to that large class of persons, who, in an age which tends very decidedly to physical research, wish to know some- thing of scientific fads ; those whose occupations do not afford them time for study, or who, from defects in their early training, believe themselves incapable of mathematics. From the first page to the last there is not a problem ; not a triangle is drawn. Although astronomy and the laws of motion can- not be studied without the highest mathematics, the facts which observation and theory combine to make known, can be gathered together and made attract- ive to the general reader, so that the narrow boun- daries of ordinary daily life may be extended by a conception of the expansions of space, and the cycles of time. iv editor's peeface. " The Moon " is a guide iDook to explorers of tlie lunar surface, to astronomical sight-see-ers. To those who are accustomed to the use of a glass, to whom the Moon is already somewhat known, it gives the same pleasure which the returned traveller re- ceives when he takes up the book of the ready writer and finds details of scenery, sketches of views , and descriptions of ravines, narrow passes and steep ascents which he had supposed only known to him- self. For such sight-seeing and such explanation, costly instruments and delicate apparatus are not necessary. The ej-e, or at best the eye aided by a small glass, is enough. Many of the phenomena of which "The Moon" treats can be seen by the naked eye ; all of them with a glass of low power. It is much to be desired that young women would give their spare time and unoccupied thought to some department of science. Scattered all over our country are thousands of young girls, leading aimless lives, to whom science might open new worlds. They have the very reciuisites necessary for observations of phenomena ; in general their perceptive faculties have Ijeen cultivated beyond all others ; they have been trained in minute details and in routine work. They could gather in valuable facts which might lead to new views of nature and new interpretations of its lessons. They could EDITOR S PREFACE. V scarcely be observers of phenomena, without taking the nest step, and becoming seekers after law. The mission of the popular scientific works is mostly that of suggestion. My hope is, that, through them, a love of nature may be aroused ; that after looking at " The Moon " the reader may be led to look beyond its surface to the beauty of order in the universe, and may become an earnest student of the wonderful revelations of God, through Nature. CONTENTS. EdITOE's PEErAOE. Inteoduction CHAPTEK, I. THE APPEAE.U^CE OF THE MOON TO THE NAEED EYE. I.— The Phases of the Moon 23 n. — Apparent Form and Dimensions of the Lunar Disc 32 in.— Light of the Moon 43 rV.— Ashy Light 54 V. — The Moon considered as the Torch of Night 62 VL — The Large Si^ots on the Moon 65 CHAPTEK n. THE a PPT: AT? A not; of the moon THKOUGH THE TELESCOPE. Vn. — The Mountains of the Moon : General Description. 72 Vin. — The Mountains of the Moon ; Dimensions and Heights 81 rS.— Topography of the Moon 89 X — Lunar Topography ; HilLs and Clefts 9G XI. — Eadiant Craters and Luminous Bands 104 XII. — Invisible Hemisphere of the Moon 113 VUl CONTENTS. PAO& CHAPTER in. VOLCANIC CONSTITUTION OF THE MOON. XIII. — Volcanic Constitution of the Lunar Surfacd 110 XIV. — Lunar Volcanoes compared to Terrestrial Volcanoes 129 XV. — Luminous Clefts and Bands 136 XVI. — Are there upon the Moon Volcanoes still in Action ? 14"2 CELVPTER IV. METEOEOLOGT OP THE MOON. XVn. — Has the Moon an Atmosphere ? 150 XA'III. — Lunar Days and Nights 161 XIX. — Lunar Seasons — Heat and Cold upon the Moon — Extreme Variations of Temperature 171 XX. — Are there upon the Moon Organized Beings ? — The Vegetation — The Inhabitants 178 CHAPTER V. THE MOTIONS OP THE MOON. XXI. — Bevolution of the Moon around the Earth ; Distance of the Moon from the Earth ; Dimensions of its Orbit and the Velocity of the Moon 189 XXII. — notation of Ihe 3Ioon. Equal Duration of the Two Motions. Lunar Poles and Equator 199 XXIII. — True Form and Dimensions of the Lunar Globe. Gravity at the Surface 208 XXIV.— Is the Moon the only SateUite of the Earth 215 CHziPTER Yl. INTE'CENCES OF THE MOON. XXV.— Oceanic Seas, Atmospheric and Subterranean. Pop- ular Prejudices and Errors 218 XXVL— A Last Word upon the Moon 227 Appendix 235 LIST OF ILLUSTRATIONS. THE MOON AT ITS PuiiL, { fvontixpiece,') Fia. PAGE 1. First j)ha3e of the Moon. Ashy light 25 2. Fourth duy of the Moon 2G 3. The Moon at its first quarter 27 4. Bet"ween the first quarter anil full moon 27 5. The fall moon 28 G. Wane of the filoon. Between full moon and the la;t quarter 28 7. Wane of the Moon. Last qnartia- 29 8. Wane of the Moon. Between the last quarter and the ne\y moon . Ashy light 29 9. La.5t phjise of the Moon. Ashy light 30 10. Orbit of the Moon. Explanation of the phases 33 11. Geometrical form of the Innar crescent 33 12. Geometrical form of the Moon after conjunction 33 13. Variations in size of the disc of the Moon 39 11. Micrometrical measure of the disc of the Moon. Diam- eter of the Moon at the horizon 41 15. Diameter of the Moon at the highest point of its course. 41 10. Ashy hght 55 17. The earth as seen from the Koou at the end of its course 58 18. The earth as seen from the Jloon at the beginning of the lunation 59 19. Orators and lunar civquei, aecor ling to Nasmyth 7C X LIST OP ILLUSTRATIONS. Fia. PAGE 20. Copemicua 79 21. Circular and elliptical forms of the lunar mountains .... 84 22. Lunar mountain chains. The Apennines 91 2.3 . Lunar crater 94 24. Lunar crater with group of interior craters 95 25. Clefts of Hyginus and of Triesnecker 100 26. Lunar crater after sunrise 105 27. Lunar crater before sunset 107 28. The luminous bands of Copernicus, Aristarchus and of Kepler 110 29. Cirque elliptical at the bottom, in the form a bowl 124 30. Lunar cirque flat at the bottom 125 31. Crater buried on the banks of the Ocean of Tempests, according to the design of iVI. Chac mac 126 32. The peak of Teneriffe and its enrirons. Topographical details, according to Piazzi Smyth 130 33. Topographical relief of the Isle of Bombon, (the Ke- union, ) according the M. Maillard 133 34. The Cichus crater, according to Schrceter, in 1792 ; ac- cording to Beer and Miedler in 1833 146 35. The two craters of Messier, according to Beer and Moad- ler in 1834 147 36. The craters of Messier, according to M. Webb, Febraary 28, 1857 147 37. The slope of the solar crescent 156 38. Difference between the duration of a lunation and that of the revolution of the Moon around the Eaith 190 39. Sinuous form of the lunar orbit ; first, amplified ; second, reduced to its true dimensions 196 40. The motion of rotation of a sphere supposed to be mo- tionless 203 41. The lunar rotation .... 204 42. The dimensions of the Moon and Earth compared 211 43. Attraction of the Moon for the waters of the sea 221 IN TR D UCT I N. Thiety years ago a humorist, relying on tlie cre- dulity of the piibho, published a pamphlet which caused a great sensation. It treated of nothing less than the discovery of the inhabitants of the Moon, whom Herschel, by the aid of an immense telescope, had seen coming and going, not suspecting, certain- ly, that their actions and movements were witnessed 140,000 miles distant by an inhabitant of their pla- netary neighbor, more ingenious and more curious than the thousand millions of his compatriots. The author of this hyperbolical narrative, com- piled, as is well known, without the knowledge of the illustrious astronomer, gave all the evidence cal- culated to satisfy his readers concerning the accu- racy of the facts of which he had constituted him- self the historian ; the dimensions of instruments, making of preparations, a minute description of the Xll INTEODUCTION. animals, the vegetation, and finally, of the inhabi- tants of the Moon. Nothing was wanting. The discoveries were declared to have been made at the Cape of Good Hope, where it was well known that Herschel was observing. Great was the excitement of the public, so dis- posed to welcome the strangest and most impossible news, so indifferent or so disdainful toward the most positive and most fruitful discoveries. The infatu- ation was so universal, so complete, that Arago thoucrht himself oblio'ed to come before the Acade- D o my to give the lie explicitly to the author of this hoax, and to those who had been innocently its propagators. It was cold water thrown upon an enthusiasm not unlike that which three centuries and a half ago welcomed the recitals of Columbus and his companions concerning the discovery of a new world. But is not the Moon a world to be conquered — a celestial body so near us, and which seems like an appendage — a miniature of the Earth ? There it is, separated from our globe by 140,000 miles, accom- panying it unceasingly in its annual revolution, as if drawn toward it by an invincible bond of sympa- thy, turning always toward the Earth the same face, alternately dark and bright, but which no cloud ever sullies, as if to invite us to solve the INTRODUCTION. XIU enigma of tliis celestial sphinx. The Earth and the Moon receive in common the same light, and dur- ing their nights, exchange in turn its rays. A hundred and forty thousand miles, did we say ? What is such a distance compared with the abysses of the visible universe — compared even with the di- mensions of the solar system, of this family of stars which press around their common centre, which dis- penses to them light and heat ? It is less than the eighteen hundredth part of the interval comprised between the Earth and Jupiter ;'■'" it is less than the millionth part of the distance between us and the nearest fixed star of our universe. How can wo be willing to be ignorant of what the Moon is, when we think that the crossing of ten times the length of the Earth's cu'cnmference would suffice to ac- complish the journey thither ? This journey, however, which imagination under- takes so willingly, is forever forbidden to man. It is this which explains the attempts, continually re- newed, of ingenious minds, ready to substitute for the inaccessible reality their fancies and their dreams. Eor lack of knowledge they are forced to conjecture. * The Jloon's mean distance from the Earth is about 238,000 miles; that of Neptune is about 2,700,000,000 miles; that of Jupiter about 500,000,000 miles. XIV INTEODUCTION. But human curiosity cannot be satisfied with pure clrimeras ; and if it sometimes feeds upon hy- potheses, it is on the condition that these hypothe- ses shall contain, in positive, real facts, a sufficient portion of reality. Science alone is in a state to furnish these facts ; it is astronomy, then, which we must cjuestion in order to learn what tlie Moon is — in order to penetrate as far as possible into the mj'steries of its structure and of its physical con- stitution. The means of exploration which the progress of optics has placed at the disposal of astronomers are hmited ; but, as we shall soon see, the skillfulness of observers — their long patience — tlieir laborious studies, have made up, in many respects, for the in- sufficiency of the instruments ; the genius of the spirit of induction and of analogy has done the rest. Telescopes, notwithstanding their power, the effect of which is generally exaggerated by the public, seldom permit the eye of man to approach the lunar globe nearer than from 150 to 200 miles. It is pos- sible, undoubtedly, to magnify more considerably, but then the want of clearness and of liglit destroys the benefit of a greater nearness. It can be seen by this how deluded are those who beheve in the possibility of seeing living beings, trees, and edifices INTRODUCTION. XV on tlie surface of the Earth's satelhte. Telescoj^es capable of receiving eye-pieces magnifying G,000 times are the most powerful wliich have been con- structed — applied, if it were possible, to the obser- vatiou of tlie Moon, they would still, under the most favorable circumstances, place its surface 37 miles distant from our eyes. The largest terrestrial ani- mals would bo totally invisible ; still more so men, if they existed upon the Moon, having the same stature as ourselves. la this hypothesis, not yet realized, I repeat, we could at most only distinguish large masses, hke forests or monumental struc- tures. Well, notwithstanding these obstacles, which the progress of optics will some day, perhaps, overcome, the Moon is already wonderfully well known, not only in its movements, in its form, and in its dimen- sions — elements purely astronomical and determined long ago with great precision — but also in the struc- ture of its surface, whose geographical details have been pointed out wdth an exactness which is still wanting in vast regions of our planet. Lunar geo- logy and meteorology have been largely sketched ; and if they leave much to be desired, if the field of conjecture is still broad in this respect, we can even now form an idea of the physical phenomena of SVl INTRODUCTION. which oiu- satelhte has been, and still is, the thea- tre. Considered in this point of view, the Moon is a strange world. Da,ys and nights succeed each other there as on the Earth ; but their duration is so different that there ruust result contrasts of light and of temper- ature, which the absence of water and of an atmo- sphere renders still more startling. On the other hand, the changes of the seasons are, so to speak, unknown. "What would an inhabitant of our Earth say, if he were suddenly transported to the surface of the Moon ? What would irot be his astonishment at beholding the singular spectacle which would be presented to his eyes ? The configura tion of the surface, entirely covered with enormous ridges, little inferior in height to the loftiest terrestrial moun- tains, perforated here and there by deep circular cavities, bristling with abrupt peaks ; the aspect of the sky, where stars shine at mid-day in a vault en- tirely black ; the intensity of lights and shadows ; the eternal silence which reigns in these desolate regions ; the severity of the temperature, some- times freezing, sometimes torrid ; the strange con- ditions which result from such a physical constitu- tion for the existence of organized beings, if, in- INTRODUCTION. XVU deeJ, life is still possible in the midst of sucli a state of tilings ; all, in short, would unite to over- turn the ideas, which his residence on our globe had made familiar. Yet such, in its general appearance, is the v,'orld we are about to describe. Need we speak of the interest of such a subject ? If the object were only to satisfy that curiosity of which we are all possessed, which impels bold travellers, through all obstacles and all perils, to seek the ^^n- known regions of our globe, it would be sufficient reason for the publication of this monograph. But science has, to us, a higher aim. In attack- ing the great problems which Kature is contmually placing before our minds, in penetrating by means of study the secret of the eternal laws, science per- mits us to draw upon the living springs from which both imagination and poetry derive their nourish- ment. She initiates us into the harmony of the great whole, whose indescribable splendor it is oiir highest glory to comprehend. In place of that foolish vanity which compelled man to consider himself as the pivot and centre of the universe, she substitutes the noble pride of knowing how to as- sign himself to his true place, of comprehending his true mission, and of using for its accomplish- ment the knowledge of the verj' laws which ho is powerless to violate, and which ho cannot resist XVUl INTRODUCTION. with impunity. Of all the natural sciences, is not astronomy the one which furnishes us, in this re- spect, with the most instruction ? But we are not all imagination, all sentiment. We need knowledge in order to develop our intelli- gence, in order to discipline it under the imperious joke of positive methods, in order to resist the errors of sophistry and of passion. No one wiU deny that the natural sciences have, in this respect, a remarli- able power. Of all the heavenly bodies which pierce with their fires the transparent layers of our atmosphere, the Moon is the one which, by its very proximity, has demanded the greatest efforts towards a complete intelhgence of its movements. Tlie slightest pertur- bations in the element of its orbit have been render- ed sensible by the frequency of its revolutions, in connection with its short distance. The motions of the Moon furnished Newton with the elements of the great problem which he solved ; it was our sa- tellite which revealed to him the secret of the gravi- tation of worlds toward each other and of the identity of this general force with the force of gravity. One after another all the lunar inequalities found their explanation in this universal principle, and what seemed at first a departure from law, has INTRor)uc^Io^f. xix been found to be the most absolute confirmation of the same law. Such efforts of genius, to which are attached the names of the greatest astronomers of modern times, ought not to be unrewarded. We have said that curiosity alone was a sufficiently legitimate motive for astronomical investigation. We shaU see that to those who seek truth above all other things, the rest is given in addition. The knowledge, more and more vigorous, of the theory of the Moon is a striking example of this. Thanks to the tables which have been calculated to show the successive positions of the lunar disc in the starry vault, sailors and travellers can to-day find their place at sea and determine their route. The immense chstance of the stars makes their apparent distance from the Moon vary according to the posi- tion of the observer on the surface of the terrestrial globe. Our satellite (this comparison was made long ago) appears, on the immense dial-plate of the sky, to be like a movable needle marking the hour, without our fearing in the wheel work of the clock any variation, any unexpected derangement. Is not this a noble use of the scientific knowledge acquired at the price of gigantic labor ? But the Moon concerns us still more nearly. Its mass, combined with the mass of the Sun, raises XX INTKODDCTION. periodically the fluid strata of the seas, and moves the wave on the surface of the globe in proportion to its own movement and the rotation of our globe, producing thus the phenomenon of the tides. So long as we were ignorant of the cause of these movements, wo could not foresee their variations, which interest so directly maritime navigation, the coasts of the ocean, and the ports situated upon it. Since the theory of tides is only a corollary to that of gravitation, we calculate in advance the intensity of the phenomenon, and so anticipate, by precise in- dications, the favorable movements for the entrance and exit of vessels. Undoubtedly science has not yet had her last word upon all these questions. There remains much for her to do. But what has been done shows that astronomy, so interesting in an intellectual point of view, so grand when we see it revealing to us the harmony of worlds, is also important for its social utility. In this little work, it is not, be it understood, the theory and its magnificent developments which we pretend to show ; but we shall confine ourselves to the results, to the curious deductions which we can draw from it. The precise study of astronomical laws is possible onl}' by means of the rigorous methods used in mathematical sciences, and those INTRODUCTION. XXI who wish to ap]3ly themselves to it should remember the saying of Archimedes : " There is no royal road to geometry." Yet the iield which remains to us is still large. In studying the Moon with respect almost exclu- sively to its physical constitution, wo shall reap an abundant harvest of curious observations. Without inventing anything, without presenting any hypo- thesis except as conjectural, without treading upon the always dangerous ground of fancy, it is our hope that we shall make, and shall cause our readers to make, one of the strangest and most singular jour- neys which man has ever yet accomplished. TH.E MOO CHAPTER I. THE MOON AS SEEN WITH THE NAKED EYE. I. THE PHASES OF THE MOON. EvEEY one knows what astronomers mean by the phases '" of the moon ; they are the various pheno- mena presented liy the disc in an interval of twenty- nine days and a half, and whicli are reproduced periodically in the same order. The period itself is called -a lunation or lunar month ; it begins and ends at the moment of ntw vioon, at the epoch when our satellite is in conjunction with the sun, and has disappeared in its rays. Among the ancients, the course of the moon fur- nished the first natural division of time, the dura- tion of the 3'ear not being known with sufficient ex- * The wovd j'jhnse is dorivcd from t':e Greek rpadii, whicli has the Bame etymology as the Yerb (pairoo, I appear, I shine. 24 WONDERS OP THE MOON. actness. We also find in tlie history of all nations the custom of celebrating New Moon or Neomenia with sacrifices and prayers. Since the Neomenia * served as the starting-point for regulating the as- sembhes, solemnities, and public exercises, and as the lunation was counted from the day when the Moon became visible, " in order to discover it easily, the people assembled in the evening on an eminence ; when the crescent had ajDpeared, they celebrated the Keomeuia, or the sacrifice to the new month, and this was followed by fetes or entertainments. The new moons which were concurrent with tlie renewal of the four seasons were the most solemn." (De Lalande.) In our day, all trace of these ceremonies has dis- appeared,! at least among civilized nations ; but, as we see, many of the prejudices which are connected with the pretended influence of the phases of the Moon are yet to be dissipated. Let us follow the Moon through the course of one of her periods, and note the various phenomena which attends each of her phases. We say tliat there is a netv moon when our satel- * Tlie Liitins gave the New Moon the name Jaiiio. sikus or sitkns. t Among the Mahometans, the end of the fast of Pvamadan is fixed at the new moon, which begins the Bairom, or rather at the instant of the first appearance of the crescent moon. PHASES OF THE MOON. lite is not visible during the day nor the night. The reason of this invisibihty is tlie situation of the Moon, its apparent position in the heavens being the same as that of the Sun. The dark side of the Moon is then turned toward the Earth, and becomes invisible in the dazzling rays of the Sun. This dis- appearance of the Moon continues two or three days ; but the precise instant of new moon, indica- tion of which is given in astronomical almanacs, takes jDlace when the Moon and Sun have exactly the same longitude. The Moon is then said to be in ronjunction. On the second and third days after the instant of conjunction,* and in the evening, a little after sun- set, the Moon appears in the form of a very narrow crescent, the convex side of which is turned toward the sun, then below the horizon. The obscure part of the lunar disc is Ficr. 1. First phaee of the Moon. ,1 t j* <.i_ „,^ ABh ' lir-ht ih&VL very distinctly seen, * Ilevclius asserts that be never saw the Moon sooner than forty hours after conjunction and twenty-seven before, so that the minimum cluratiou of its disappearance is sixty-seven hours, a little less than three days. This period varies according to the climate and the latitude ot the Moon. 26 ■WONDEBS OF THE MOON. being of a very faint color, as if transparent ; this light, much less intense than that of the illuminated jDortion, is due to the reflection of the Sun's rays from the surface of the Earth. On account of the diurnal motion the Moon soon sets in the western horizon. The next day the same phenomena are reproduced, but the crescent is al- ready less sliarp, the ihuminated portion is larger, and the Moon, farther removed from the sun, sets, also, a httle later than on the day before, The fourth day after the vrw moon the form and appearance of our satellite, which sets only three hours after the sun, are represented in Fig. 2. The ashy light is still visible, although it diminishes more and more, until it disappears entirely in the following phase, called the first quarter. The Moon is then said to be ilichotornus (divided into Fig. 2. Fourtti day of the Moon. tWO Cqiral parts. Eig. 3.) It is between the seventh and eighth days of the Moon that it appears to us in the form of a half circle, visible during a part of the day, and the diurnal motion does not cause it to pass the me- ridian until six hours after the Sun. Even in PHASES OF THK MOuN. 27 tlie preceding phase the spots with which the Moon's disc is covered were visible ; now these spots are seen with gi'eat distinctness on the himi- nous semi-circle. Between the first quar- Kg. 3. TlieMoou at iteflxet quarter. ^^^. fm^l /!(?i mvoll SeVen more days elapse, during which the form of the illu- minated portion approaches more and more that of a complete circle (Fig. 4); the Moon rises and sets later and later during this interval, but always turns the circular part of its disc toward the west. At last the whole side of the Moon is illuminated, about fifteen days after new moon (Fig. 5); the Fig. 4. Between the flret quarter and l. ^f its rising is then full Moon. ^ ^ nearly that of the setting of the Sun, which in its turn rises when the Moon sets. It is midnight when it reaches its highest point, which is, in the language of astronomy, at the in- stant it passes the meridian ; at the same time the Sun below the horizon, passes the inferior meri- 28 WONDEKS OF THE MOON. dian, so that, relatively to the Earth, the Moon is directly opposite to the Sun. After the epoch of full moon until the following new moon (this second half of the lunation is Hg. 5. Tho full Moon. Called the ivavc) the cir- cular form of the illuminated part of the disc gra- dually decreases, and fin- ally assumes, as at the beginning of its course, the form of a very sharp crescent. But the con- vexity is now always turned toward the east, and that portion of the riR. 6. Wane of the HIoou. Be- tween full nuioii and tlic hist quar- ter. semi-circle whose light is disappearing is toward tho Sun. (Fig. 7.) In the middle of the interval wliich sc]iarates tlie full moon from the period follow- ing, tho lad qiKirlcr presents a phase similar to the /Irsl cjiituii'r, but inversely situated. Ill this second h;ilf of the lunnr period, or of tjie liiiitdidi: — lliat is ll;e proper word — the apparent position (if tlie Moon in the he.ivens reapproaches PHASES OP THE MOON. 29 nearer and nearer that of the Sun. Toward the last days the Moon rises a yery short time before the Sun, uJitil at last it is ob- scured again in the Sun's rays, disappears, becomes a new moon, and begins riff. 7. Wane of the Moon. Last a ncw lunatiou. The ashy light reappears after the last as before the first quarter, in proportion to the diminutidu of the illuminated part of the disc. (Fig. 8j. This succession of the phases of the Moon, which is reproduced in- definitely, and always in the same manner, is evi- dently caused by the motion of the Moon around the Earth. This Fis. 8. Wane of the jioon. Between is easily Understood liy the last quarter and the new Moon, , . Ashy hght. exammmg i ig. 10, and it is then plain why the phases of the successive lunations are jDrecisely the same when the Sun, the Earth, and the Moon oecujDy the same relative posi- tions : if the position of the Moon is referred to the 30 WONDEBS OF THE MOON. Fig. 9. Last phase of the Jloon. Ashy light. stars, in two or more consecutive lunations, it is seen that during the same pliases (Fig. 9) it does not occupy the same point of the hea- vens ; that it does not pass througli the same constellations ; this is due to the time, and to the motion of the Earth in its orbit, and to the variations of the motion of the Moon in its or- bit. (Fig. 10.) We have already said that the duration of a lunar month was about twenty-nine days and a half ; in reality it exceeds this by forty- four minutes and three seconds. It is therefore necessary to have a month of mean duration which evidently corresponds with the period of a lunation, just as the week corresponds with the duration of each of the four pi'incipal phases.* * The days of the week, as you kuow, take their names from the seven planets known to the ancients : the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Situru. The order of the days baa been determined, it appears, by the custom which the ancients had of consecrating to the seyen planets the twenty-four hours of tlie day. Each day, then, took the name of the planet to M'hich the first hour \yas consecrated. This explains also how it haiipeued that the order of the successive diys of the week is not the natural order of the planets, of such, at least, as were PHASES OF THE MOON. 31 known to the ancient astronomers. .That order was: the Sun, Venus, Mercury, the Moon, Satm-n, Jupiter, and Mars. The first hour of the lirat day was consecrated to the Sun, the second to Venus, etc. ; it happened that the first hour of the second day was consecrated to the Moon, the fh-st of the third day to Mars, and so on. n. FOEM OP THE LUNAK DISC. At the epoch of full moon the entire disc is illumi- nated by the sun's rays and has the appearance of a perfect circle. Any one may suppose this to be the fact by simple obseryatiou, but astrouomers are more particirlar in the matter, and endeavor to veri- fy the apparent circular form of the moon b}' accu- rate measurements of all its diameters. The result of these measurements is that the iujpr^ssion made by simple observation is exact : the disc at the eiDocli of full moon is rigorously a circle. This fact was all the more interesting to establish, because the greater part of the celestial bodies of our solar system appear in the telescope as oblate discs, of a form slightly elliptical or oval. This is true of the planets Mars, Jupiter, Saturn ; and the Earth, also, as has been proved by direct measures of several arcs of meridian, is flattened at its poles of OKIilT OF THE MOON. EXPLANATION OF THE PHASES. FORM OF THE LUNAR DISC. 33 rotation, or, which amouuts to the same, is enhirgecl at tlio equator. As for the other planets, suc!i as Uranus, Neptune, Venus and Mercury, the micro - metrical measurements have not established any sensible oblateness ; it is the same in the case of the Sun. But we regard these exceptions as merely apparent, and proving but one thing, which is that the actual oblateness is too slight to be appreciable in the instruments. l''ig. 11. Geometrical form of the lunar crescent. 12. Geometrical form of the Moon after conjunction. The circular form of the disc of a star indicates ordinarily a form really spherical. As it is thus with the Moon, shall we consider our satellite as having the form, if not rigorously, at least very nearly of a sphere ? The observation of its phases permits us to reply in the affirmative. As we have seen, some days before or after new moon, the luminous cres- cent, more or less well-defined, is always bounded 34 WONDERS OP THE MOON. on the outsklo by a half circle very nicely terminat- ed. (Figs. 11 and 12.) As to the concave edge, the cnrve which forms it is not exactly a half-circle, but more nearly a part of an ellipse, the extremities of which form with the exterior curve the horns of the crescent, which terminate in the two extremities of a common diameter. Little by little the line of sepa- ration between the light and shade extends, and at the first quarter it has become a straight line. Then the elliptical curve, which was concave, be- comes convex, resumes the same forms in an inverse direction, increases little by little, and is finally transformed, at the epoch of full moon, into a half- circle, to resume again during the wane and until the last quarter the same forms and the same appa- rent dimensions. In fine, everything takes place as if the Moon had the form of a perfect sphere, whose opposite parts were successively illuminated and thrown into darkness : the study of the movements of the Moon, and of its positions in relation to the Sun and the Earth, leaves no doubt as to the reality of this appearance. Thus the Moon, like the Earth, is a spheroidal globe whoso oblatoness, if it exist, is almost inappre- ciable, at least on the entire circumference which limits its visible half. Hereafter we will give more of the details of its true form, and we sliaU. see that it FOEM OP THE LUNAK DISC 35 is supposed to be slightly elongated in the direction of tlie earth. Now that wo are somewhat enlightened as to the form of the Moon, let us speak of its size. In order to be precise, let us begin by distinguishing the ap- parent size of an object from its true size. It is of the first that we shall speak at present. There are such confused ideas in this respect in the mind of many people, and errors are so widely spread that some explanation is necessary. I have heard a hundred times — and I do not doubt that the reader of this passage, whoever ho may be, will bear like witness — I have heard, I say, many an observer of a phenomenon, in order to give an idea of the apparent dimensions of the object he has seen, as the disc of the Moon or the Sun, the size of a meteor, the tail of a comet, etc., express himself thus : Its length ivas about a decimetre, or a foot; it appeared as large as a plcde I have also read such expressions in journals, even in scientific collections. Now it is easily seen that this way of showing the apparent dimensions of an ob- ject and not the real, in which distance is whoUy left out, is perfectly unintelligible. In fact, the same object — a decimetre if it has to do with length, a plate if it has to do with a circular disc — has not in itself a decided apparent dimension. That dimen- 36 WONDERS OP THE MOON. sion is essentially variable according to tlie distance of the eye from the object which forms the term of comparison. In order that the expressions of wliich we are speaking sliould have a meaning, it is neces- sary, then, to add to the dimension indicated the precise distance from the eye at which the object is observed. A plate placed very near the eye covers an immense portion of the field of view, or, if one desires, the entire heavens. Placed at a little dis- tance the surface which it covers is considerably di- minished. Removing it still farther it may become imperceptible. In order to cover exactly the surface of the Moon, so that one circle will not be in any way projected beyond the other, it is necessary to place it at a certain fixed distance, determined either by observation or by calculation. It is only at this distance that the apparent size of the star and the object can be compared Otherwise what would be the result of the circum- stances I have just mentioned ? If the phenomena had had several simultaneous observers, one would give the object the size of a decimetre, another of a foot, a third of a metre, each one having, moreover, at the instant he saw the object, a very vague idea of the supposed distance of the metre, the decimetre and the foot, which served as terms of comparison. Astronomers, and in general all those who have ao- FOEM OP THE LUNAK DISC. 37 curate ideas of geometry, overcome this difficulty. They do not compare the apparent dimensions of an object with a determined dimension. They indicate simply the portion of the field of view which is co- vered by the diameter of the object. They say, for example, that the apparent diameter of the Moon is about half a degree, meaning by a degree the three hundred and sixtieth part of the whole circum- ference of the horizon. It is precisely the angle formed by the two visual rays from the eye which meet the extremities of the Moon's diameter. Thus, generally, it would require 360 moons, touching edge to edge, to complete the half circum- ference of a circle which, beginning at one point of the -horizon, meets the horizon at a point diametri- cally opposite, following in the heavens any course whatever. Let us speak in more accurate terms. The de- gree is divided into 360 equal parts called minutes : each sixtieth of a minute is called a second. Well, it is found by very accurate measurements that the mean diameter of the lunar disc is 31 minutes and 24 seconds, or, as we see; a little more than half a degree. This is very nearly the apparent diameter of the San. But it must not be concluded that the true size of the Moon is nearly equal to that of the Sun : it is necessary to take into account the distan- 38 WONDEES OF THE MOON. ces, and later we shall see that our satellite is about four hundred times less distant from us than the common centre whence the planets receive heat and light. Let us pass to another question. Are the appa- rent dimensions of the Moon always the same ? If they are, it is because the distance of the Moon from the Earth is always the same. Or, do these dimen- sions Tary ? — in which case the distance will vary from one epoch to another. It is the second hypo- thesis which is the true one. In the course of a lu- nation — that is to say between two consecutive new moons, the diameter of the Moon varies constantly between two limits. This variation is quite sensible, since it reaches the eighth part. of the whole dia- meter ; nevertheless, it is difficult to ascertain it with the naked eye, and accurate instruments alone are able to show it. I should add, also, that in the course of successive lunations, the variations of the apparent diameter are not reiH'oduced with exactly the same values. Therefore the Moon does not always remain at an unvarying distance from the Earth : it moves away from and approaches, following certain very complicated laws which astronomy has suc- ceeded in explaining, but which we will not attempt to point out here. In regard to the variations in distance, and con- FORM OF THE LUNAR DISC. 39 sequently iu diameter, I shall be very much asto- nished if among my readers there is not one who has at the end of his tongue the following ques- tion : Do you say nothing about the change which is observable iu the apparent diameter of the Moon from the time it rises above the horizon until it reaches its greatest height in the heavens ? In this case the testimony of the senses suffices, and it Fig. 13. Variations of ttie size. is not necessary to be an astronomer iu order to judge. Let us look at the case. When the Moon rises, at the period when it is full for instance, and the sky is very clear in the eastern horizon, its tinted disc a2:)pears enormous. But, gradually as it rises, or, to leave the language of appearances, as our horizon, by degrees, depresses itself below the Moon, in consequence of the diurnal motion of the earth, its dimensions are diminished. 40 W0NDER3 OF T.IE M ON. the brightness of its li'jht iucreases, auil it seems to resume its normal sizo. At the highest point of its course, when the Moon passes the meridian, its disc appears smallest. The contrast between the size of the Moon at the horizon and at its highest point of the heavens is moreover all the more marked when, owing to the circumstances of its movements, or else to the jwsition of the place of observation, it also approaches the zenith. Every one, however, is struck with the phenome- non. But i"hat is the cause of it ? It is here that opinions differ — I mjan the opinions of those who ask the question ; for many see it, are astonished, and there rest satisfied, sure at least that the}' are not deceived. Some consider the fact as an optical illusion, and imagine that the mists of the atmo- sphere in this circumstance assume the character of a magnifying glass ; otliers think that perhaps the Moon moves to a greater distance from us in propor- tion as it ascends. Both are evidently wrong, for they make a common supposition that the apparent diameter is larger at the horizon than at the zenith, an erroneous supposition which micrometrical mea- sures contradict. Imagine at the optical focus of a telescope two parallel threads placed in such a man- ner that the Moon when it appears in the horizon can be exactlv contained between them, and be FORM OF THE LUNAR DISC 41 made to touch tlieni without projeotiug beyond them. Leaye them in their present position and wait until the orb has reached its highest jjosition in the heavens. Now point the instrument again upon its disc. If the apparent dimensions of the last have really diminished what will happen ? The Moon will appear to be entirely contained between the threads without touching them. Well, just the con- trary happens ; the disc projects beyond the threads. Therefore we must say, contraiy to all appearances and to all the illusions of our senses : Uic moon aji- 'pears smoUer id the. ]i<:>fi7:on tlnia at the zc/n'fh. (Figs. Fig. 14. Micronometrical measure fif the Moon. Diameter of the Moon at the horizon. Fig. 15. Diameter of the Sloon at the liighest x^oint of its cnurse. 14 and 15.) It is then very clear that the opinions above mentioned have no foundation. Astronomers, who know this very well, have none the less tried to 42 WOiiDEKS OF THE MOON. account for tbe illusion, wh'cli is certainl}' indisput- able. Some tliink it is an error of estimation, owing to the proximity of the lunar disc to the terrestrial oljjects tituated in the horizon. In the ztuit'a the abse;;co of these objects causes us to be- lieve that the star is nearer to us ; wo regard as smaller that which in preserviag the same apparent dimensions seems less distant. Euler asL-igns as a cause for the same illusion the elliptical form of the celestial vault, which causes us to estimate that part of the heavens situated in the horizon as more remote than the part directly over our heads. According to that mathematician, the comparison of the terrestrial objects situated in the vicinity nf the Moon has nothing to do with the illusion. Whichever one of these explanations is true, it does not matter. That which it is iiecessary to remember is the fact that the discs of the Moon seen in the horizon and in the zenith do not differ in apparent size, as is supposed, or, if there is any difference, it is exactly opposite to wliat is supposed. I have considered the Moon when it is full and is rising, but the phenomena are true for all the epochs, either when the Moon has the form of a crescent or of an incomplete circle, and they are the same, also, at its settincr. III. THE LIGHT OF THE MOON. THE LIGHT OF THE MOON IS DUE TO THE EEFLECTION OF THE sun's LIGHT — ITS INTENSITY — QUANTITY OF LIGHT GITEN BY THE DIFFEEENT PHASES — COLOB OF THE MOON DUKIKG THE DAY AND DURING THE NIGHT — CALOrJC AND CHEMICAL INFLUENCES OF THE LUNAE KAYS. The soft and silvery light wliicli the lunar disc diffuses over tho Earth during our terrestrial nights has inspired many a poet and many an artist. But there is no necessity of making either art or litera- ture a profession in order to enjoy the charm of a beautiful evening which the Moon illuminates by its rays, or, in order to admire the changes of light which are produced when the wind chases the clouds before its disc, and the vaporous masses, now dark, now brilliant, successively eclipse and unveil it. The character of tho landscape aids, moreover, in the 44 WONDEES OF THE MOON. impvessions, grave or gay, mild or severe, which the peculiar disposition of the individual makes still more varied. In considering natural phenomena, science has other interests than those of art or poetry ; far from studying harmonies and contrasts with personal emotions, she tries to free herself from those influences, whose power she cannot for- get. That which science desires above everything, is to study these phenomena in themselves, in noting their minute details, and in discovering their laws. And while poets and painters have long ago ex- hausted in their pictures all the varieties of beauty which the country illuminated by the hght of the Moon can offer, science has not as yet determined all the questions which can be raised concerning that luminary ; we shall see that some have been scarcely touched on. It is known that the lunar light is no other than the light of the Sun reflected in space towards the Earth by the surface of our sateUite. The proofs of this fact are entirely conclusive. In fact, the relative " situations of the Sun, the Moon, and the Earth are always exactly in accordance with the fornr of the luminous part of the disc, or with the dimensions of the phases ; it always happens that the parts are either illamiuated or non-illuminated, according to the geometrical relation which their THE LIGHT OP THE MOON. 45 situations require. This fact is so simple that every one can observe it with the naked eye with tho greatest facility. How then is the strange idea of the Chaldean astronomer, Beroze, to be explained, who considered the Moon as a globe half dark and half luminous, turning each hemisphere successively towards the Earth? There is every reason to sup- pose that he did not take the trouble to examine the positions which the principal spots occujoy in the course of a lunation ; he would have seen that these spots are always obviously on the same points of the disc, and that his hypothesis had no foundation. In the telescope it is still more easy to be con- vinced that the source of the Moon's light is in the Sun ; the innumerable irregularities with which the surface of the orb is covered are all iUuminated on the side of the solar rays, and the shadows which they cast upon the surface are shortened or length- ened in the proportions which the obliquity of these rays requires. Thus, that portion of the Moon which shines towards us, is that which enjoys the light of day ; the oljscure, invisible part, or the part which we are scarcely able to distinguish, is that which is plunged into night. Such would be the appearance of the Earth to us, if we were transported through sjoace to the distance of the Moon, for example ; casting 46 WONDERS OF THE MOON. our eyes toward our own globe, we should see that it had become a celestial, luminous body. Here, then, is the first c[uestion, as to the solution of which there is no doubt. Let us pass to some others. AVhat is the intensity of the light of the lunar disc, considered either intrinsically, or measured by the degree of illumination produced by the different phases? Of a given Cjuantity of solar light which the surface of the Moon receives, how much is re- flected to us ? What proportion has been absorbed by the surface of our satellite ? Is the light of the Moon white or colored ? does it produce any appre- ciable chemical action on terrestrial substances ? We shall speak of what is known, and of what is thought to be known on all these points of the phy- sical constitution of the Moon. Compared with the light of the Sun, the light of the full Moon is only the -s-ui'uTT''tb part; that, at least, is the number which is the result of the experiments of the philosopher WoUaston." It would recj[uire, then, 800,000 full Moons to produce the light of day when the sky is perfectly serene. * Bonguer, in the last century, obtained a very different nnm- ber from that of the English philosopher ; the light of the full Moon being, according to him, the auu'iriJuth part of that of the Sun, more than double that which we have just cited from W^ol- laston. But the phonometer has nut yet given the final decision ; these two numbers, so different, are yet to be verified. THE LIGHT OF THE MOON. 47 Lideed, the Moon is not always at tlio same clis- tfince from the Earth, but the question here is of tJie intensity of its light at its moan distance. The maximum intensity exceeds the minimum intensity by nearly a quarter, when the excess of the surface of the lunar disc at the epoch of perigee over the same apparent surface at apogee * is calculated. The luminous intensity of the full Moon being estimated, it is easy to deduce that of the disc at its different phases. At the first and last quarters the intensity is, of course, diminished one half ; at the two octants, the one of which precedes, and the other follows new moon, the light of our satellite is reduced to a seventh of its maximum intensity ; at the two octants, on the contrary, the one of wdiich precedes and the other follows full moon, it lacks only one seventh of the whole illumination of the disc. These estimations are all geometrical, and infer that all the regions of the Moon, at the east, at the west, and at the centre, are equally luminous. Now this is not the case. Arago has found that the light at the edge of the Moon is nearly three times as powerful as the light emitted by the large spots. As these dark spots are not distributed uui- * Perigee, the least distance, Apogee, the greatest distance of a star from the Earth ; from the Greek words nsfji, near to, dito from and yrj, Earth. d8 WONDEES OF THE MOON. formly over the different visible regions, it is neces- sary to measure the brightness of the disc in the whole course of a lunation. Is the light of the Moon colored ? According to Humboldt, it is shghtly yellow, or at least appears so, when it is observed in full night. During the day, it is white, and appears to be of the same color as the clouds shghtly illuminated by the Sun. Humboldt explains this difference in remarking that the natural yellow color of the Moon is modified during the day by the interposition of the blue color of the atmosphere. We know that blue and yellow are complementary colors — that is to say, that their mingling jDroduces white light. During the night the sky has a much darker tone, a little greyish, so that the color of the lunar light is less changed. At the horizon, the lunar disc is often a decided red, which is explained by the strong refraction which the luminous rays undergo while traversing the thickest part of the densest strata of the ter- restrial atmosphere. Finally, when the Moon is seen from the streets of a city lit by the reddish- yellow hght of the gas-burners, it appears a bluish- white ; but this is evidently nothing but the effects of contrast. We wiU speak further of the diverse tints which the different regions of the Moon present, as well THE LIGHT OF THE MOON. 49 as of the particular liglit of certain of its spots. Many tilings are said and repeated every day concerning the influence which the Moon exerts upon our Earth, and that which is said is so vague that it is necessary to study in a more exact and more scientific manner what foundation there can be for these free assertions. Let us see, then, whether the lunar light exerts any of this influence. First, the luminous rays which the Moon reflects towards our Earth, coming indirectly from the Sun, are without doubt accompanied by rays of heat with which our Sun floods all space. It is probable that a portion of the heat received by the lunar hemisphere tm-ned towards the Sun is absorbed by the surface. This is likewise true of our earth, and of all the celestial bodies of our universe. But another portion is, by reflection, sent back into space, and it is this which it is necessary to discover and estimate. Modern works on solar radiation establish, as a demonstrated fact, that the presence of an atmosphere, and, above all, of the vapor of water with which it is more or less pervaded, serves to diminish in a large proportion the radiation in space, and, consequently, the loss of the heat which the Sun sends us.'" The gaseous * See, on this iuteresting question, the volume of this collec- tion which treats of the Sun. 50 WONDEES OF THE MOON. stratum is a screen wliieli allows tlie rays on enter- ing to pass, and, on their return stops tliem. The Moon, having no vaporous or gaseous atmo- sphere — as we shall see further on — the caloric ra- diation must be very intense, and, as the same point of the surface remains exposed more than 350 hourg to the heat of the solar raj's, it seems as if the quantity of heat reflected towards our Earth could easily be appreciated. Is it so ? The experiments of the ancients have given a negative result. The concentration, with mirrors or liurning glasses, of the light of the Moon at the focus of these instruments has produced on the most sensitive thermometers no appreciable effect. Snice then, Melloni has stated that there is heat, very slight, it is true, but very positive. M. Piazzi Smyth, in the Scientific Expedition of 1856 to the peak of Teuerifl'e, confirmed the experiments of Melloni. This is what M. Babinet stated in the fifth vokrme of his " Studies and Lectures on the Sciences of Ohservedion :" " ' M. Smj'th,' he said, ' could easily perceive the effect of the heat of the Moon, which Melloni had taken so much trouble to render perceptible in the experiments made in the vicinity of Naples. Al- though the Moon was then very low, the effect of its rays was still the third part of the effect of the THE LIGHT OP THE MOON. 51 caloric rays of a wax taper placed fifteen English feet distant.' " Now, in order properly to appreciate this result, it is necessary to recall what we have said about the difficulty which the atmospheric strata caused in the passage of the caloric rays, which do not emanate directly from the source, or, if you will, which have already been once reflected. It is pro- bable that it is the higher beds of our atmosphere which absorb the heat of the lunar rays, and that explains, also, the saying, the Moon eats the clouds. In fact, the elevation of temperature which it sup- poses, having the effect of rarifying the condensed particles of aqueous vapor of which the clouds are composed, these will be in part dissipated by the caloric action due to the presence of our satelhte in the heavens. There might bo a method of verifying the exact- ness of this explanation, on the condition, however, of having previously established the meteorological phenomena themselves, by placing at different alti- tudes very sensitive thermometers, and there repeat- ing the experiments of Melloni, not only at the epoch of full Moon, but for each of the lunar pha- ses. It must be understood in advance that the variations, if they are established, must vary in pro- portion to the illuminated surface of the disc. The 52 WONDEES OF THE MOON. observations of M. P. Smyth, cited above, and which were effected at the altitudes of 2,700 and of 3,320 metres,* (about 9,000 and 10,900 feet,) allow us to hope that the means of verification which we propose will be efficient. The light of the Moon exerts another influence which is not more doubtful : We would speak of its chemical action on cer- tain terrestrial substances. It is this action which renders possible the lunar photographs which are now obtained with a clearness and perfection so remarkable ; but this property which the Moon possesses, it shares, except in degree, with the solar ligb.t, a result which could be foreseen, since they have the same origin. t Of late, the light of the Moon has been subjected to the operation of spectrum analysis. MM. Hug- gins and Miller have compared the spectrum ob- tained by exajnining limited parts of the Moon's surface with the solar spectrum. They have dis- covered no modification which allows the conclu- sion that the solar light has changed in nature by its reflection from the surface of our satellite. Let us conclude then : if the Moon exerts ujJon * A uietre = 3.280S9 English feet. + i'hotographs of the Suu are very easily obtained, the time necessary for the exposure to the hght being a very few seconds. THE LIGHT OF THE MOOX. 53 the meteorological plieuomona of our Earth a cer- tain inflaeuoe, that influence appears to be con- fined to very narrow limits. The heat which it ra- diates is almost entirely absorbed by the strata outside of our atmosphere ; the chemical action of its light, however slight it may be, is unquestion- able. It remains to be determined whether it has anything to do with the advance of vegetation. Finally, the calorific influence and the luminous influence must be at their maximum intensity at full moon, and at their minimum at new moon, a result which is in opposition to the popular be- Uef. IV. THE ASHY LIGHT. OEIGIN OF THE ASHY LIGHT — ITS INTENSITY — ITS CO- LOR — TAKIATIONS IN ACCOEDANCE WITH THE POR- TION OF THE EARTH WHICH 13 IN VIEW OF THE MOON. The bright portion of the moon, tliat wliicli the sun directly illumines, varies in form during a com- plete lunation, from the slight luminous crescent of the new moon and of the last phase, to the complete circle which it presents at its full. But, besides tliis sufficiently brillant light, and what the intensity of this hght is, compared with that of the Sun, we have just seen, the luminous disc presents in its dark part, during certain of its phases, a faint light much more feeble, known by the name of ashj J'ujld. The ashy light is very easily perceived with the naked eye. Every one can see it a few days before or after new moon, when our satellite appears in the form of a very slight crescent. The entii'e portion THE ASHY LIGHT. 55 of the hemisphere turned toward us, which does not come in contact with the solar rays, is yet distinctly perceptible, so that the whole circle of the disc is complete. The faint Hght is feeble and phosphor- escent. Arago has given a method by which its in- tensity may be estimated by comparing it with the constant intensity of the light of the rest of the disc, but we do not know whether any application has ever been made of the method.* The ashy light of the new moon appears from the time the crescent is visible, and disappears scarcely before the first quarter ; also, during the wane of the moon, it becomes visible a little after the last quarter, and disap- pears only with our sat- ellite itself. (Fig. 16.) According to Shrceter and Lalande, it is near the third day which Fig. 16. Ashy Light. follows Or precedes the new moon that it is the most vivid. * Except that Arago himself, who found that the intenBity of the ashy light was -fTMSu P'^'^'' of ^^^^ °^ '^^ luminous part of the Moon BIX days before the new moon and toVu P'H't o^ tl"3 seventh day of the moon. 56 W0.\D3IIS OF THE MOON. It is evident to every one tliat the oxteiior outline of the illuminateJ portion of tlie disc projects sen- sibly beyond the outline of that portion which the ashy light renders visible. Tliis is an illusion pro- duced by the optical phenomenon of irradiation, which gives objects an apparent dimension much greater when they are illuminated by a more bril- liant light. The intensity of the ashy light is strong enough to allow us easily to distinguish the largest spots even with the naked eye. But if a telescope of a certain jDOwer is used, a much larger number of de- tails becomes perceptiljle. By means of telescopes the ashy light can be seen also for a mucli longer time than with the naked eye. Shroeter has ob- served it three hours after the first quarter, but ac • cording to the report of Arago, it was by using a magnifying power of 160, applied to a telescope of seven and a half feet in focal length. Where does the ashy light come from ? Is it a glimmer belonging to the Moon ? The ancients, who had no very positive ideas on physical astronomy, regarded it as jDroduced by a kind of phosj^horescence from the surface or the soil of the Moon. But it will be seen that the true explanation is too simple to admit of the least doubt on the subject. According to most astrono- THE ASHY LIGHT. 57 mers it was Mcestlin who in 1596 discovered that tlie asliy light is only the light from the Earth re- flected upon the Moon by the phases visible from our globe. But the same explanation — let us not forgetto give due glory to a great painter — had been given a hundred years before Moestliu, by Leonard de Vinci. In fact the Earth sees upon the Moon precisely the same appearances that our satellite sees upon the Earth. But the terrestrial phases are inverse to the lunar phases, as figure 10 plainly shows. By turning to the illustration it is easy to see that the new moon corresponds to tiie full earth, so that the dark hemisj^here of our satellite receives by reflec- tion all the light of the illuminated hemishere of the Earth. At fuU moon, on the contrary, it is the dark hemisphere of the Earth which is in view ot the illuminated lunar hemisphere, so that the Earth is then invisible. In fine, between these two epochs the Moon sees portions of the luminous hemisphere of the Earth, larger, in proportion as the Moon ap- proaches new moon to us. (Fig 17.) As, moreover, the apparent surface of our globe as seen from the Moon is about thirteen times great- er than the lunar disc, it is easy to understand that the Earth-liijld must give to the nights of the Moon a light much greater than our Moon-liglit. The in- 58 WONDERS OF THE MOON. tensity will also be thirteen times stronger if the ex- terior surface of the two bodies are endowed with the same power of reflection. Therefore the ashy light is merely the light of th^ Sun reflected first from the Earth to the Moon, and second from the Moon to the Earth. It seems cer- Fig. 17. The Earth as seen from the Moou at the end of its course . tain that the intensity of the lunar reflection is stronger during the period of waning than during the first days of tlienew moon. (Fig. 18.) Galileo has noticed it ; since that great man, many other observers have confirmed the accuracy of the fact. To what is the difference due ? Here is the expla- nation generally adojated : THE ASHY LIGHT. 59 When the Moon, at the end of its course, appears in the east, the kiminous hemisphere of the Earth which, turned toward our satelUte, illumines its ob- scure part and produces the ashy light, contains a large extent of land — eastern Europe, Africa, and above all Asia ; the seas occupy relatively a smaller Fig. 18. The Earth as seen from the Moon at the beginning of the hiuation . extent. On the contrary, when it is in tlie west that we see the Moon appear, then the hemisphere which sends light to it is largely composed of the Atlantic and Pacific oceans. Now we know that the oceans absorb a much larger Cjuantity of light than the land, so that the first of the two hemi- spheres seen from the Moon is evidently more lumi- 60 WONDEIIS OF THE MOON. nous than the other ; it ilhmiinates then with greater power tlie dark regions of our satelhte. If this explariation is correct, it is clear that the contrary phenomena could be observed in Australia, whore the ashy light will be less vivid during the waning of the Moon than during the crescent period of the Moon. But we do not know whether this fact has been established. It has been said that this difference could proceed from the Moon in like manner, the eastern hemi- sphere of which presents a, mu.ch larger extent of dark spots than the western hemisphere, and is consequently endowed with a greater power of re- flection. This opinion seems quite plausible, and as it is not in contradiction with the first, it is very possible that the difference of intensity which has been observed proceeds from the two causes at the same time. The term ashy light indicates, generally, a greyish color. However, various observers have considered it of a greenish-olive tint, which was jDerhaps only accidental. Arago, one of the latter, was inclined to believe that this tint is due to the effect of con- trast produced by the vicinity of the luminous cres- cent, the color of which is a yellow-orange. But he inquires whether this phenomenon of color coidd not be attributed to the bluish-green tint reflected THE ASHY LIGHT. 61 npou tlie lunar disc by the teiTestvial atmosphere. It' so, the explanation of Lambert, also re])orte(l hj Avago, deserves more consideration. We give tl 10 words of the illustrious astronomer, which Humboldt cites also in his Cosmos : " On the 14th of February, 1774, I saw that this light, very far from being ashy, was of an olive color. .... The Moon was vertically over the Atlantic Ocean, so that the Sun cast its rays perpendicularly upon the inhabitants of the southern part of Peru. The Sun then shed its strongest light upon South America, and if the clouds intercepted no portion, that gi'eat continent reflected upon the Moon a sirifi- cient cjuantity of greenish rays to give this tint to that part of the Moon which the Sun did not direct- ly illumine. This, I think, can be stated as the reason why I saw the light of the Moon an olive co- lor, while it usually is called ashy. So the Earth seen from the planets could appear of a greenish light." V. THE MOON CONSIDEEED AS THE LUMINARY OF NIGHT. INEQUALITY OF THE ILLUMINATING POWEK OF THE MOON — lEKEGULAE DISTKIBUTION OF ITS LIGHT DURING THE NIGHTS OF THE DIFFEBENT SEASONS — • THE DUKATION OF ITS NIGHTLY VISIBILITY IN A "WINTER LUNATION. In speaking of the Moon, poets never fail to call it tlie torch of night ; this is periodically very true, that is, at the epoch of full moon, but it is very far from being correct for the rest of the lunation. If it is true that the function of the Moon is to illuminate the terrestrial nights, and thus to supply the absence of the solar light, it must be admitted that it i^erforms the task in a very inadequate man- ner ; I do not speak of the intensity of its light, so remarkably inferior to that of the Sun, but of its regularity and constancy. Let us first observe that the phases of the Moon CONSIDEKED AS A LUMINABY. 63 are so arranged tbat in a lunar month it passes tlirongii all degrees of size — from new moon, when the Earth receives no light from its satellite, to full moon, when it receives nearly all. The terrestrial nights are thus seen to be very unecjually supplied. In reality, the quantity of light reflected by the disc of the Moon is exactly equal to that which it would send us if it were constantly at its first or at its last quarter, since, in considering any phase whatever, during the increase of the Moon, that phase has its exact complement during the wane ; the illuminated portions of the disc united at these two epochs, would exactly form a full moon. This is not all. During the whole time that the Moon is visible, between two successive new moons, the nights alone can profit by its light. Now all the time that it is above the horizon at the same time as the Sun, the light is entirely wasted, just as is that of a candle when lighted in full day. In view of this the nights of our planet are very dif- ferently illuminated, according as they belong to one or the other of the four seasons. It is during tlie long nights of winter that the light of the Moon could show its greatest utility. Well, it is just at this very epoch that it discharges in the worst manner its pretended functions. I will give an example. The seventh of November, 1866, at 64 WONDEES OF THE MOON. about half past ten in the morning, the Moon had reached the epoch of conjunction ; it was new moon. The lunar month terminated the seventh of December following, at half past five in the morning. In this interval, the total duration of the nights, counting from the setting to the rising of the Sun, amounted to about 466 hours and a half ; now at the latitude of Paris, the Moon is not visi- ble, or, at least, is not above the horizon at night more than 218 hours, less than half of the total duration. That is not all. By the simple fact of its own motion, combined with that of the Earth, our satel- lite is a very poor illuminator of our nights; but there is more than this, if, with the irregularity and the insulticiency of its light, we join the obstacles which arise from the inclemency of the atmosphere. The clouds and the fogs aid, also, too often in our climate in intercepting its feeble rays. We can by this instance see how puerile and vain are the claims of those who wish to interpret phenomena espe- cially to aid their own systems, and to substitute for the views of nature their feeble explanations. The Moon certainly has a reason for existing, but it is by studying that which is, not by imagina- tion, a priori, that man can hope to lift a corner of the veil which hides the truth from us. VI. THE LARGE SPOTS ON THE MOON. POPULAK OPINIONS ON THE FOKM OF THE MOON — PEKMANENCE OF ITS SPOTS — THE DAF.K SPOTS : SEAS, LAKES, AND SWAMPS — THE BKIOHT SPOTS ON THE CONTINENTS. The principal spots on tlie Moon are very dis- tinctly perceptible with the naked eye. Some por- tions, of a more sombre tint than the general light of the disc, are cut with clearness on a backgrouud, the luminous intensity of which seems to be itself unequally distributed. There is no one who may not, even without mak- ing a regular study of the spots visible to the naked eye, familiarize himself with the appearance which these differences of color give to the lunar disc. Every one can observe, also, that this appearance does not vary, or, at least, does not vary much, gg WONDEES OF THE MOON. either in the same hmation, or in the course of suc- cessive lunations. The Moon, in fact, alv?ays pre- sents the same side to tlio Earth; it is the same hemisphere which we see always. We shall see later that this permanency of the spots testifies to the motion of rotation of the Moon, the duration of which is exactly equal to tliat of its revolution around the Earth. A popular opinion, very widely spread and very ancient, perceives in the figure of the full moon a human face or body, for, accurdiug to the imagin- ation of the observer, it is either the one or the other of these two appearances wliich it most readily represents. " The dark and luminous parts," said Arago, " delineate vaguely a sort of human figure, the two eyes, the nose, and the mouth." Others see in the same spots a head, arms, and legs ; in our country, it is Judas interred in the Moon, in punishment for his crime of treason and felony. We will not dwell upon these passing remarks, the sole merit of which is to prove that this fact, to which we would call the attention of the reader, has long been established. In the course of a lunation, the disc not being entirely visible except on the day of full moon, it is to this epoch tliat wo must give the preference for studying the general distribution of the spots. THE LAEGE SPOTS ON THE MOON. 67 At the first quarter we see the western part of the visible hemisphere ; only the last quarter shows the eastern part.* When the crescent is very slight, it is clifficirlt to distinguish any spots with the naked eye. Let us then take the instant of full moon for our description. Let us first remark that the large greyish and dark spots occupy especially the northern half of the disc, while the southern regions are white and very luminous ; this luminous color is seen again on the northwest edge, as well as toward the centre ; and, on the other hand, the spots invade the south- ern regions on the eastern side, while at the same time they continue around to the west, though less dark. Except a slight portion of the northwestern * In order to observe the Moon in its passage over tlje meridian, ■we must evidently turn toward tlie soutL./m part of tlie horizon. Tlien the two extreme points of tlie diameter of the disc perpendicu- lar to the horizon give the northern and southern points. At the left we find the east, and at the right the west. If it is observed with an astronomical glass, the image is inverted, the south is at the top, the north at the bottom of the disc ; the west is at the left, and the east at the right. In order to distinguish the differ- ent regions of the Moon, we consider either the southern and northern regions, or the western and the eastern parts, each of these parts comprehending the half circle, at the vertex of which we find the point which gives it its name. The northern and southern poles are situated, the first in the northern region, the second in the southern, liut without coinciding exactly mth tho north and south points of the disc. 68 WONDEES OF THE MOON. edge, the entire curve of the Moon is white and luminons, and partakes of the tone of the southern regions. Let us now enter upon certain de- tails. You will see toward the west, and very near the edge, a large greyish spot, of an oval and regular form, isolated in the midst of the more luminous color of the edge ; this is the Mare Crisium. Do not attach to the name of this sea any special meaning ; it is the common name by which the first observers designated all the large greyish spots of the Moon ; farther on, we shall give the reasons which impelled them to consider these s^Daces as large extents of water, while they considered the brilliant parts with which they were surrounded as the lunar continents. The situation of the Mare Crisium, on the western boundary of the Moon, enables us to recognize it from the time of the first phases of the lunation, until the full Moon ; for the same reason it is the first to disappear at the be- ginning of the wane. Between the Mare Crisium and the centre of the disc, a large dark space cut at its lower part by a kind of pointed promontory, has received the name of 3Iare TranquilUtatis. It throws towards the west two appendages, of which the most westerly and the largest forms the 3Iurc Fwctinditatis, so THE LARGE SPOTS ON THE MOON. 69 that tlie other, smaller and nearer the centre, is the McLTc Ncdaris. If now from the Mare Tranquillitatis we look to- wards the north, we find the Mare Serenitatis, smaller than the first, but also a little more regu- lar in form than the Mare Crisium. This spot is crossed in its entire length by a brilliant ray, almost rectihnear, which gives it a certain resem- blance to the Greek letter phi ($j. The 3Iare Va- jjori-iiu is like a prolongation towards the centre of the Mare Serenitatis. rinally, the Mare Imbrium, round in form, is the largest of aU those which we have mentioned, and terminates at the north, the series of greyish spots, called seas, which improper name we have agreed to preserve. We must now cross towards the east in order to find the Occaniis Procdiaruin, the boundaries of which are more indistinct, and are lost towards the south in the Marc Hninoruni and the Mare Nuhium, at a very short distance from a luminous point, at which whitish streaks of great length separate in all directions. We perceive, also, above tlie Mare Serenitatis, and in the vicinity of the north pole, a narrow spot elongated from east to west, and known by the name of 3Iare Frujoris ; at the limit of the northwest edge, a spot of an oval form, very much 70 WONDEKS OF THE MOON. elongated, is the 3Iare Austrah ; and, finally, on the extreme edge of the southwest portion, the Mare Humholdtimmm, of which we, without doubt, per- ceive only a part. All these pretended seas form on their shores, or in prolongations, smaUer dark spots which have re- ceived the name of Sinus, Lacus, or of Palus. We will mention a few. Between the seas Serenitatis and Frigoris, the Lacus Somniorum and the Lacus Mortis extend. The Palus Putridinis and the Pcdus Nebularum occupy the western portion of the Mare Imbrium, the northern bank of which forms a round guK known by the name of the Sinus Lridium, or Bay of Painhoics. The Sirius Boris is the prolonga- tion towards the centre of the extreme northwest corner of the Oceanus Procellarum. Finally, to end this nomenclature, which by-and- by win be very useful to us in the geographical de- scription of our satellite, let us mention the Palus Somnii, to the west of Mare Tranquillitatis. The Sinus Medii, which is the southern prolongation of the Mare Vaporum ; and, also, the Sinus JEslmmi, which is on the southern border of the Mare Im- brium. As to the large, luminous, and brilliant spaces which encompass the grey spots, they have, for THE LARGE SPOTS OX THE MOON.- 71 some unknown reason, received no general name ; they are quite unlike them, as we shall see by the details, invisible to the naked eye, which the tele- scope shows. CHAPTER II. THE MOON SEEN THROUGH A TELESCOPE. VII. THE MOUNTAINS OF THE MOON. GENERAL DESGEIPTION. Tpie lunar spots which we are about to describe, when examined by the naked eye only, teach us nothing of the real structure of our satellite. It is through the telescope that we must now study these, as well as the brilliant regions which surround them, of which wo have as yet said nothing, except that they differ in brightness from the first. Place the eye, then, at an instrument of medium power, that is to say, one magnifying from 30 to 60 diameters. Choose the period when the Moon is in one of its quarters ; that is, wdien the disc pre- sents to us its eastern or its western illuminated half. THE MOON SEEN THROUGH A TELESCOPE. 73 An astonisliing spectacle immediately offers itself to our view. Ail the white or brilliant parts of the disc appear to us studded with an immense multi- tude of cavities of a circular or oval form, and of very different dimensions. It is in the central regions, or rather on the limits of the illuminated parts of the Moon that these ir- regularities of the surface seem the better to indicate the structm'e of which we speak, and which it is im- possible not to recognize. They are like so many hollows, the verges or edges of which, in the form of ramparts, raise themselves both above the gene- ral level, and also above tlie-bottom of the cavity. Each one of them is vividly lighted on the same side of the luminary, that is,, on the exterior of the half circle which presents its convexity to the solar rays, and on the interior of the other part of the circumference, which presents its concavity to them. On the obscure side of the disc, on the contrary, we perceive very marked shadows, which serve to show wonderfi;lly the general form of the irregular- ities of the surface ; yet the bottom of the cavity is sometimes very luminous, sometimes of a more sombre hue, and in some of the cavities we perceive very clearly eminences wliich cast shadows upon the interior ground. Their dimensions, we have said, are very varied. 74 WONDERS OP THE MOON. Some seem like little hollows, from wliicli tlie soil is sifted ; others are like vast circles or circular enclos- ures, which contain, sometimes on the inside, and sometimes on the edge, cavities of much smaller dimensions. This first glance cast by the aid of a telescope on the disc of the Moon shows clearly to us that the lunar ground is covered with depressions and eleva- tions. These elevations are nothing else than the mountains of the Moon. Let us continue our investigation. We have seen that the form of the irregularities of the surface is sometimes circular, sometimes oval. Is there a real difference between the two aspects ? No, as we can readily convince ourselves. Notice this circumstance. The accurately circu- lar form belongs to all the cavities, to all the spots situated in the central regions of the disc. When we examine those which recede from the centre, ap- proaching little by little the edge, we perceive that this form becomes insensibly oval or elliptical, and the oval is all the more elongated as the cavity which we examine is nearer to the edge, whatever may be the direction we have chosen in making this examination. Moreover, the greatest diameter of each ellipse is always parallel to that portion of the arc of the circle of the lunar edge, which we THE MOON SEEN THKOUGH A TELESCorE. 75 obtain by joining the centre of tlie disc with tlio centre of the cavity examined. The least reflection upon these singular circum- stances forces us to recognize that the real form of each cavity is circular. The elliptical appearance is only due to an effect of perspective, arising from the fact that every circle is drawn on different parts of a half sphere. The portions of the surface which are opposite to our visual ray, perpendicular to its direction, do not appear to us distorted ; the others, on the contrary, are seen obliquely, and their distortion is greater in proportion as they arc seen at a greater obliquity. Therefore, when we speak here of circle and ellipse, it is understood that we speak exclusively of the particular irregularities which the outlines of this cavity present. Suppose, now, that we have observed the Moon at the precise period of its first c|uarter. The next day and the days foUowing, if the sky permits, let us continue our examination. We I shah see the light invade by degrees the east- ern regions of the disc, and little hj little, new ele- vations appear, the tops of which alone were first lighted by the Sun. Nothing is more curious than to see at tlie bottom of the shadow the interior side of a new cavity first appear in the form of a cres- cent, then the Ught increase, ])cnotrate to the bot- 76 WONDEKS OF THE MOON. torn of the cavity, and finally light up the whole outline. At other times it is an isolated, luminous point, the summit of which shines, although the base of the eminence is still entirely plunged in darkness. In proportion as the Moon follows its course, and its illuminated phase enlarges, we see, as we then expect, the shadows of mountains diminish in ex- tent, the bottom of the plains become hghted with greater brilliancy, and the structure of our satellite display itself to our eyes in all its details. Let us say at once, to simplify the language, that we have given to the lunar cavities of small and medium dimensions the names of craters or volca- noes ; to those of larger dimensions, the names of " cirques"'^' and that the isolated mountains of pyr- amidal or conical form are peaks. We shall soon see what legitimacy there is in these different deno- minations. Let us now see how the mountains are distributed on the surface of the Moon. At first we are struck with the inequality of this distribution. Notwithstanding that in general the luminous regions are scattered over with cavities, * We have kept this word, "cirques," although it might per- haps haye been translated "circus, "or "amphitheatre." Webb uses the term " walled plains." THE MOON KEEN THROUGH A TELESCOPE. 77 and bristling with elevations, the great greyish spots which we have called seas are almost totally desti- tute. Where they are the most numerous is princi- pally in the southern part of the Moon, in the large space which is surrounded on the north by the Mare Nectaris, the Mare Tranquillitatis, and the Mare Vaporum, and on the east of the Marc Nubi- iim, and the Mare Humorum. There the craters and the circpies are so numerous that they barely leave narrow valleys between their ramparts or boundaries. At the north pole, beyond the Mare Frigoris on the southeast, on the borders of the Ocean of Tem- pest, and lastly, in the northwest region, near the Mare Crisimn, we find the same character of sur- face, covered with the same crater-like elevations, although the borders of the west and east are evi- dently the prolongation of regions occirpied by seas. At the period of full Moon the lunar mountains appear entirely illuminated ; some, those of the cen- tral regions, because they receive the solar rays ver- tically ; the others, those of the regions near the border, because their shadows are thrown, relatively to us, behiucl the eminences which form them. Nevertheless, all are easily distinguished, thanks to the more vivid light with which their edges shine. 78 WONDERS OF THE MOON. Among them are some notably more luminous than the generality of the others ; we shall describe them, because they will serve us as signs or land- marks for a more detailed description of the oro- graphy of the Moon. In the southern part of the disc, south of the Mare Nubium, at an apparent distance from the lower edge of the Moon, nearly equal to a sixth part of the diameter of the orb, a crater, whose di- mensions exceed the medium, is distinguished at once by its brightness, by the presence of a peak in the centre of its circumference, and by the multi- tude of white and brilliant beams which radiate all around it to a great distance. This is Tycho, which appears to be the centre of a vast system of crater- like mountains. Copernicus, Aristarchus, and Kepler are three other remarkable craters, all situated in the midst of the region of seas, towards the northwest, and all surrounded by luminous, radiant beams. Their position causes them to be easily distinguished ; the first appearing to be the centre of a small system which separates the Mare Nubium from the Mar e Imbrium ; and the other two, very brilliant, stand- ing out in relief from the greyish bottom of the Ocean of Tempests. Nearly in the middle of the disc, to the south of THE MOON SEEN THKJUOH A TELESCOPE. 79 the Sinus Medii, (Fig. 19,) three large cirques, the circumferences of wliich are ahuost contiguous, and whose dimensions are ahuost equal, have received Fig. 20. Copernicus. the names of Ptolemseus, Albategnius, and Arzachel. Their ramj^arts render them distinctly visible, espe- 80 WONDEBS OF THE MOON. cially at the period of the first quarter of the Moon. (Fig. 20.) Other craters or cirques, instead of being distin- guished by their brightness, are remarkable on ac- count of the sombre shade of their bases. Such are especially the crater Plato, on the northern shore of the Mare Imbrium, which appears like a black oval spot ; Endymion, a large crater, near the northwest, between the Mare Humboldtianum and the Lacus Mortis, and which appears very dark even in full moon ; finally, the large crater Grimaldi, on the shore of the Ocean of Tempests, whose dark oval rises from the luminous bottom of the eastern border of the Moon. Before studying more intimately the lunar topo- graphy, and stating what we know of the principal formation of our satellite, let us give some details of the dimensions and areas of the large spots, called seas, and of the heights of the lunar moun- tains. VIII. THE MOUNTAINS OF THE MOON. DIVISIONS OP THE ANKULAE MOUNTAINS, OF CKATERS AND OF CIKQUES — iU^TITUDES OF KAMPAETS AND OF PEAKS. In coDsidering tlie Moon rigorously a sphere, v/e find that its total area is about 14,681,000 square miles. This calculation gives 7,300,000 square miles for the surface of the visible hemisphere (about 7,3±2,000j. Three tenths of this last ex- tent are occupied by dark spots, seas, lakes and marshes, or rather by the lunar plains, and the other seven tenths belong to the mountainoas regions, that is to the brilliant parts of the disc, which are covered by the numerous craters, cirques and walled plains visible through the telescope. But, as we have already said, and as we shall see farther on, if the seas and plains compared with the mountainous regions seem unitedly and relatively destitute of these asperities, otherwise so numerous, here and 82 WONDEES OF THE MOON. there crater-like mountains are seen in the midst of their precincts hke so many elevated summits which a general inundation has not been able to reach. While, the eye at the glass, we contemplate the multitude of cavities with which the surface of the Moon is marked, it is impossible not to bo struck by the resemblance which those circular openings bear to the terrestrial volcanoes. Hence the name of volcano has alwaj's been given to most of the moun- tains of the Moon. But the analogy of form may lead us into error when we endeavor to seek the causes of the lunar phenomena if we do not first give an exact idea of the dimensions of these moun- tains, the surfaces which their bulwarks enclose, and the height of their elevation above the surrounding surface. Let us begin with the large excavations which have received the names of " cirques." The most considerable of all appears to be Schick- ard, an immense circ[ue situated near the south- west border of the Moon, a little below the Mare Humorum. Its diameter is estimated at 100 miles, which would give to the embankment surrounding it an extent of 507 miles and to the enclosure itself a surface of more than 19, .500 square miles : it is the 7G0th part of the whole area of the Moon, and the 11th part of the area of THE MOUNTAINS ON THE MOON. 83 France. Clavius, a large irregular cirque which we perceiv, -^1 - ^ "'■^ .r -- - ; A J= "-^ - -~ ,-^ ^ r-— s '*#■/- - 5,. ■»\ ■"■'* ._ " ■^ ^^f^:^^ ■^ j'-<^' ^ ^ '*'"' 1 ^ 1 - - , fi . ^ ^ \- o ^.: llf. J 5- ^~y^ "^r _ i Ti "" /?-"^' 'Hj' ~ \ - ~lJ _ ^ ^-^o' ^ " " - - » - : '' J . ^^ «. .; _ j-V :;!:* -. .i^ ■^ ^ ' . -1 r "^ J « -^ . Fig, 21. Cii'cular and elliptical forms of the lunar mountains . This fragment represents a section of the moun- THE MOUNTAINS OF THE MOON. 85 taiuous region which covers a hii'ge portion of the southern part of the disc betweeu Tjcho and the Mare Nectaris. Five hirge cirques, Geber, Tacitus, Almanou, and Abulfeda, and Descartes are accom- panied by numerous smaller craters. The valleys which separate these annular mountains are them- selves very irregular ; a number of small hills grouped in parallel ranges extend in every direc- tion. As for the intrenchments of the cirqu.es, we can see that they are not contiguous through- out ; some peaks tower over summits of different altitudes. Finally we may remark that the bounda- ries are sometimes formed of two lines or layers, pa- rahel or circular, forming tiers. But what are these heights ? How much are they elevated above the surrounding surface, above the bottom of the circular spots, and above the ex- terior valley ? These are questions wliicn are not only interest- ing, but are of groat importance in the topographi- cal and geological study of our satellite. Wo have a g(jod deal of data on these points. Without speaking of the measure of the altitmles of a certain number of mountains, inaccurately estimat- ed by the astronomers of the last century, let us say that Beer and Mtedler have determined the al- 86 WONDERS OF THE MOON. titude of eleven hundred points on the surface of the Moon. We will not enter upon an exposition of the methods adopted for this kind of measure- ment, although they are very simple and based upon elementary geometry. We will content our- selves by giving the results. It is in the neighborhood of the southern pole that the most elevated summits of the lunar moun- tains stand. Two peaks belonging to Mounts Dor- fel and Leibnitz attain a height of 25,000 feet, much higher, as we see, than those of our Mt. Blanc (15,800 feet.) From the summit of one of these mountains the eye embraces an horizon of more than 50 miles, a great distance upon a globe whose curvature is so marked. Four other mountains exceed 19,000 feet in height. One of the peaks which rises on the west from the enclosure, called Clavius, measures 23,000 feet above the base of a crater situated in an im- mense cirque. The annular mountain of Newton, near the southern pole, is bordered by embankments which tower over 24,000 feet above the base of the crater: this is the altitude of the highest summit of the Andes. " The excavation of the crater of New- ton is such," says Humboldt, " that the base is never lighted either by the Sun or by the Earth," a THE MOUNTAINS OF THE MOON. 87 circumstance wliicli arises, also, from its extreme position on tlie disc of tlie Moon. Mounts Casatus and Curtius rise to the heights of 22,800 and 22,200 feet. In the northern regions, we find, also, extensive elevations. Calippus, one of the peaks of the Cau- casian chain, and Huggins in the Apennines, attain respectively 20,400 and 18,200 feet in height. The ridge of this last chain is bordered on one of its sides by precipices of a frightful depth, and the peaks of which it is formed cast their shadows a distance of more than eighty miles. The mountains, in the form of isolated domes and pyramids in the centre of cirques and craters, are generally less elevated than the summits of the boundaries. But, if we measure their heights from the level of the lower surface we still find summits which exceed the highest mountains of Europe : the peak of the crater of Tycho is 16,400 feet high, and that of Eratosthenes, at the extremity of the Apen- uine chain, rises 1G,000 feet above the base of the cirque. According to Humboldt, Beer and M;edler have measured o9 summits whoso heights are greater than that of Mont Blanc, and, as we shall see, four exceed 19,000 feet, that is to say, they compote with the highest peaks of the Cordilleras and Andes. 00 WONDERS OF THE MOON. Strictly speaking, the measure of the lunar moun- tains cannot be compared with that of terrestrial mountains. These are all computed from a common level, the ocean. Such a common surface does not exist in the Moon ;* the heights are there counted from the surface of the surrounding plains. When craters are in cjuestion, the height is generally greater above the base of the spots than above the exterior level; both of these altitudes have often been measured. However this may be, it is certain that, all propor- tions regarded, the elevations of the surface of the Moon are greater than those of our planet. Mounts Durfel and Leibnitz arc, it is true, lower by nearly 4,000 feet than the famous Gaurisankar of the Himalayas. But while this giant of terres- trial mountains does not exceed the 720th part of the radius of our planet, Mou.nts Leibnitz and Dor- fel have a height equal to the 229 th part of the lunar radius. That is relatively more than triple. * Tiio lai'gc plains called seas a}ipear in general to be of a lower level tliau the valleys whicli esteud between the cirques and the ci'atcvs of the mountainous regions. The surface is otherwise more even, no that they can serve as a common level for the measurements of height, but we do not know that the ditferenee in the level of the two regions has been accurately determined. IX. TorOGEAPHY OF THE MOON. CEATERS, CIEQUES, AND CHAINS OF MOUNTAINS. Theke ave, then, two cliaracteristics which distin- guish the elevations of our sateUite from our terres- trial mountains. First, their very general circular form ; second, the prodigious height of a great num- ber of them. Let us enter into details which are more particular and more characteristic, and which, as we shall see, show more plainly these differences. Most of the mountains of the Moon, as we have said, assume an annular form, their tops either col- lecting together and forming immense circles, sur- rounding a level and smooth plain, or, more likely, one bristling with isolated peaks, or else resembhug a volcanic cone, whose interior cavity or crater is rounded in the form of a bowL A very small num- ber are of a decidedly oval form ; the niimerous 90 WONDEES OF THE MOON. ellipses wliicli we see on the borders being nothing but circles foreshortened to our sight. There do exist, however, a certain number of elliptical forms, such as those of Figs. 21 and 25 (pp. 84 and lOOj. One, very regular, appears between the cirques Abulfeda and Almanon. Two others, the cirque Godin, south of Agrippa, and an inclosure sur- rounded by hills, slightly elevated between Tries- uecker and Agrippa, present also plainly an ellipti- cal form. It is similar in the other regions of the Moon ; but after all these are few in number. The large plains are limited by circular arcs, and bordered by very high and steep mountains, whose great expanse has caused them to be regarded as chains. The Sea of Crises bears in form a strong resemblance to a cirque. The Sea of Serenity and that of Eaius have elevations in a largo part of their craters, thus showing the character of all the irregu- larities of the lunar surface. We shall now speak of mountain chains. There exist, in truth, on the Moon, certain series of eleva- tions, which we can liken to our terrestrial moun- tain chains. We shall name the most important. The greater part are found in the northern regions of tlic disc. At the southwest of the Mare Imbrium, over a length of 4G0 miles, rises a succession of peaks topoctKaphy of the moon. 91 and blnffs, wliicli separate this great plain from tlie Hea of Yapors, tlio immense lieiglit of wliicli wo liavo already eited ; they are the Apcuniues. Their i!:eni-'ral direction is from northwest to srnrtlieast. At this last limit another chain hegins, which runs from Avest to east, and which, under the name of Carpathians, is only an extension of the Apennines. It is at the south, and about 100 miles from the Carpathians, that the radiant cirque of Copernicus is found. The Caucasian ]\Ioxintains and the Alps limit, on the west and northwest, the Sea of Kains. (T'ig. 22.) Fig, 22. Lunar Mountain diains. The Apennines. The first of these cliains is formed of a series of iso- lated peaks or needles, some of which rise to a height 92 WONDERS OF THE MOON. of 19,000 feet. The Alps, also, are analogous in structure. For the most part, the remaining chains of lunar mountains, such as we are aliout to describe, seem to be portions of immense circuits, reminding us, iu their general form, of cirques of the smallest dimen- sions. Such are Mounts Taurus and Hemus, on the shores of the Sea of Serenity, whoso highest sum- njits attain an elevation of 9,000 and 6,300 feet. West of Taurus is seen a crater 25 miles in diam- eter, the terraces of which reach a height of 11,000 feet. It is known by the name of lloimer. Such, also, arc the Altai Mountains and P^'r^nees, which surround the Sea of Nectar ; the Altais offer an ex- tent of about 100 leagues from north to south and southwest ; next to the Apennines, it is the largest chain. The Urals and Riphees seem to be fragi ts of a chain, at one time more extended, which doubt- less separated the Mare Nubiuni from the Oceanus Procellarum, and that from the Mare Humorum. On the extreme eastern border of the visible hemisphere, two ranges of mountains have received the names Jf Cordilleras and Alemberts, and are prolongeLi *^oward the south by the Rook Mountains. Their summits stand out in relief on the border of the disc, at elevations of 19,000 feet. TOrOGRAPIIY OF THE MoON. 93 We have already noticed the prodigious lieiglit of the two chains wliich sniToniid the south polo of the IMoou, Leibnitz and Dori'cl. In recapituhxtion, the conligiiration of tlie hmar nionntains differs materially from the mountains of our globe. While the terrestrial chains exicnd ofteuer in a straight line, or parallel to a great circle of the sphere, forming a scries of systems which intersect each other at different angles, each one of ■ndiich correspoirds to a particular period of up- heaval, the mountains of the Moon are all or nearly all developed in arcs of circles, from the small cra- ters and circ[ues to the immeirse circumvallatioiis which surround the plains. [NiJTE TO Chatter IX. — The accompanying en- gravings of lunar craters, or walled plaius, are from .vings by Miss E. R. Cofdu, of Brooklyn, March nth and May IGth, 1S7'2. The instrument used for this was the equatorial telescope of th(; observa- tory of Yassar CoUege. In both cases the time is shortly after the first quarter of the Moon. The sunlight appears to be faUing upon the Moon's surface from the left, illuminating t' i raised por- tions of the disc so that they form siroi _, contrasts with the unilluminated portions. ^J'he peculiarities of the first figure are the two " WONDEES OP THE MOON. interior small peaks, one of which throws its pointed shadow on the floor of the enclosure ; and the nar- row spurs or ranges of elevations which seem to run out from the walls. bcA-^^^/^^S ri Lm ar C atei M on jbout ten daj 6 uld. April 1 ,th, 1372 The second figure (which I think remarkably faithful ill detail) is of a crater just at the termia- ator ; the blackness of the unilluminated Moon is seen ou the right. On the left, the sunlight is falling upon an irregu- lar wall, ^hich throws its black shadow upon the enclosed jJain. Within the enclosure are four simi- lar formations ; raised walls, surrounding elliptical depressions, into whose depths the sunlight has not yet shone— craters within craters. Near these TOPOGKAPHY OF THE MOON. small craters are several small, bright jjoiuts — peaks wliicli, like the walls of the craters, throw dark shadows on the right. — Ed.] Fij,'. 24. Lunar Crater, wit'j group of interior craters. Moon about nine day-g old. Jlay leth, 1872. X. TOPOGRAPHY OF THE MOON. GROOVES AND HILLS. At the period of full moon we perceive in some regions of the disc long, whitish furrows ordinarily rectilinear, or, at least, presenting only slight curves, ajad, for the most part, so narrow that it requires great attention, eye-pieces of high powers, and very favorable atmospheric conditions to distinguish them from all other irregularities of the lunar surface. During the phases, these furrows appear like black lines. These are " rainures " or grooves. Their dimen- sions vary in length from 10 to 180 miles, and in breadth from 1,500 to 10,000 feet. In the en- tire extent of their course this breadth vai'ies very little, and when it increases, it is never at either of the extremities, but at an intermediate point. The TOPOGEAPHY OP THE MOON. 97 narrowness of these grooves suffices to distinguish them from the bright, radiating bands which we have akeady mentioned, and which we shall soon describe more minutely. But we must notice be- tween them a difference altogether characteristic. While the luminous bands have no projections or bluffs, and are altogether superficial irregularities of the surface, the grooves, on the contrary, are formed by excavations, the parallel edges of which are very steep, but without exterior embankments. This structure is very apparent when we observe them in the phases which follow or precede full moon. Then each one of them appears like a black Une, indicating the shadow cast by the edges on the bottom of the crevice. Most of the grooves are isolated, sometimes run- ning through the plains, sometimes passing by the sides of the craters, sometimes even traversing their enclosures. Some are bounded by mountains, but there are also some which terminate without any obstacle opposed to their prolongation. They are to bo met with in all the regions of the surface of the Moon, in the mountainous regions, as well as in the plains, and if they are more numerous towards the centre of the disc, it arises, without doubt, from the fact that we perceive such delicate objects with much greater facility when they are 98 WONDEES OP THE MOON. seen in the front, without being distorted by the ob- hquity of the visual rays. In several points the grooves appear in groups of parallel lines ; such are, for example, the grooves which extend to the northwest of Gutenberg. More rarely they cross each other and unite hke veins ; such are the grooves in the neighborhood of Tries- necker, on the border of the Sinus Medii. Finally, among the isolated grooves there are some which are entirely situated in the interior of cirques, like those which traverse the great circular valley of Petavius, without bordering otherwise on its ramparts. The cirques of Almanon and Abuifeda (see Fig. 21) are united by a groove tangent to the two boundaries, which stretches from one to the other, traversing a succession of small and medium-sized craters, which are on the borders of the two circ^ues. The rectilinear form is the most general. We find, however, some grooves of sinuous form, such as that which extends on the northwest of Aristar- chus. This remarkable groove commences near a mountain in the neighborhood of Herodotus, at first narrow and of slight depth, then describes two sharp angles and becomes steeper and larger. Near Aristarchus it rises abruptly to more than 3,300 feet above the neighboring plain ; then changing direction, it winds itself, and, finalty, at a ax)roGCAPHi: of the moon. 99 breadth of two aud a half miles, it coutracts consid- erably, aud terminates in the crater of Herodotus, which it enters as by the mouth of a river. The depth of the grooves is considerable, often reaching fi-om 1,000 to 1,500 feet. Such are the most interesting j^eculiarities offered Ijy these hollow furrows, or clefts, as it were, of the lunar surface, whose form contrasts so completely with most of the mountains which cover our satel- lite. It was not till toward the end of the last century that the grooves were observed for the first time, and it is to Schroeter, one of the most successful modern observers, that their discovery is due. On the 5th of December, 1788, the astronomer of Li- henthal recognized the groove of Hyginus (Fig. 25), one of the most curious of all, because it traverses ten craters from one and a quarter to two miles in breadth, and breaks the embankments of the largest of these, called also Hyginus. Beer and Mtedler have more recently noticed this. In our illustration nearly the whole of this remarkable cleft can be seen. Other observers, Pastorf, Gruithuysen and Lohr- man, have discovered several more, but the industri- ous authors of the selenographical map have ob- served the greatest number of these singular forma- 100 WONDERS OF THE MOON. tions. Thanks to them, we know to-day nearly one hundred grooves spread over all the regions of the visible hemisphere. " =^ c ■' -^ t "■ ^ '^'"i ^--^i 1 ^ 1/ x^ . 3" - ^. ' *\ - ■■ *" JO 1 '1 1 1 1 i * r' ^^Z^,--'"^- J .^ Fig. 25. Clefts of Hyginus and TrieanecUer, TOFOGKAPHY OF THE MOON. 101 But what is the origin of these long and narrow valleys ? Schroeter, who believed the Moon to be inhabited, who snp]3osed a city to be situated north of the crater of Marius, who, in his works, dwelt continually on the arts, the industry, and the culture of the inhabitants of the Moon, Schroeter, I say, could not doubt the artificial origin of the grooves. According to him, they are canals dug by the inhabitants of the Moon for their commercial needs. In accordance with this idea. Doctor Giuithuysen, another partisan convinced of the existence of in- habitants of the Moon, finds no difficulty in admit- ting the explanation of Schroeter. But the wise professor has more than once taken the fancies of his imagination for realities. It has also been said that the grooves are nothing but the beds of the rivers and streams of the Moon. These two hypotheses are both improbable. How can we imagine, for example, that the inha- bitants of the Moon have been able to produce such gigantic works of art ? The canals of our civilized country, many of which seem to us so considerable, demanding so much time and labor in digging, would be only small jjits compared with the canals of the Moon. Grooves, several miles in breadth, and several hundred yards in depth, are extended over 102 WONDERS OF THE MOON. lengths of 150 miles aud more. We see the impossibihty of digging such trenches. Moreover what could become of the materials from these immense excavations ? Evidently Schrreter and Grruithuysen had not re- flected upon these difficulties, or perhaps they did not consider the real dimensions presented by most of the grooves. The other explanation does not appear more pro- bable. We shall see that it is almost certain that there does not exist on the Moon water, or anything resemblinir water. o The grooves then can only be the beds of dried-up rivers, whose existence goes back to primitive times. But their rectilinear form, with one or two excep- tions only, appears at least singular on a surface as iiTegular as that of the Moon. Moreover it is diffi- cult to conceive that a running stream could wear beds of such depth, so vastly superior in this respect to the beds of terrestrial rivers ; above all, if we reflect that on the surface of the Moon the weight is six times less than on the Earth. Evidently, we can only reason by analogy of the phenomena presented by celestial bodies compared with the phenomena which we observe on our globe. But the laws of matter are the same on the Moon as on the Earth, and it is impossible for us to con- TOPOGRAPHY OF THE MOON. 103 ceive of rivers whose greatest width is in the mid- dle of their course, wliich ascend the sides of moun- tains, leap over their tops, and terminate abruptly at beginning and end. On many grooves the length does not exceed ten or twelve times the breadth, although in terrestrial rivers this ratio is hundreds of times greater. It aj^pears then altogether probable that the grooves owe their origin neither to artificial work nor to the movement of water. It remains to be seen, if the natural forces which have produced all the other irregularities of the lunar surface can ac- count for these, so entirely different in their forma- tion. XI. TOPOGEAPHY OF THE MOON. RADIATING CEATEES — LUMINOUS BANDS — DIFFEEENT HYPOTHESES ON THE NATUEE OF THESE BANDS. The plij'sioguomy of the different regions of the lunar surface changes from one clay to anotlier, ac- cording as the rays of the Sun, falling more or less oblicpely on the surface, produce a more or less marked contrast of light and shade. All the irregu- larities, cirques, craters or hills cast opposite to the Sun, that is, on the eastern side of the disc,* sha- dows which diminish in length from the period of the rising of the Sun, to that of the lunar midday. The shadows are then thrown toward the west, iu- * lu our description of the Moon the cast always has refevi'iice to the terrestrial ohservcr ; lor an iuhahitaut of the Moon, the words west and east should be reversed. TOPOGRAFIlY OF THE MOON. 105 creasing in length as tbey liaTC before decreased, until the Sun sets to those regions under consider- ation. From the new to the full moon, it is the rising of the Sun which we observe on our satellite ; during the waning, on the cont}'ary, we have the view of the settings of the Sun for aU the meridians of the visi- ble hemisphere. But it is easy to conceive that the shadows cast by the mountains (Fig. 2G) of the western border Eig. 26. Lunar Crater after sunrise. towards the setting of the Sun, and by those of the eastern border towards its rising, are almost invisi- ble, concealed as they are from us by the lighted slopes. At oj^posite periods, the shadows of the regions on the borders are visible, but we onlv see 106 WONDERS OF THE MOON. tliem very obliquely, on account of the perspec- tive. It is, above all, the central regions, situated on both sides of the first meridian, that the solar illu- mination permits us to distinguish clearly, especially when it is near the line of separation of light and shade. All the craters and cirques are then sharply defined ; one part of the black shadow invades the interior hollow of the cavity ; the other part, the exterior slope of the embankments ; the peaks them- selves cast their elongated shadows to a great dis- tance. On the opposite side a bright light vivifies the same objects, and renders their form and all their contours clearly visible. We have seen else- where that it is in measuring the shadows cast by the mountains that we are enabled to measure with precision their height above the surface which sur- rounds them. At the epoch of frill moon, it is no longer by the contrast of the lights and shades that the irregular- ities of the surface are visible. At this period it is only by the intensity of their brightness that these irregularities appear to stand out in relief. This intensity depends upon two causes ; one, purely op- tical, arises from the angle at which the objects re- flect to us the luminous rays (Fig. 27), and this angle is also in conformity with the inclinations of TOroGllAI'HY OF THE MOON. 107 Fig, 27. Lunar crater before sunset. the different sides of tlie mountains ; tlae otlier cause is due to the nature itself of tlio substances which compose this or that region of the Moon, and to tlie difference of their reflecting powers. It is to this last cause that we must certainly attribute the sombre hues which characterize the largo grey- ish spots of the seas, or rather of the plains, the appearance of which presents such a forcible con- trast to the surface of tlji' mountainous regions. It is C(|ually proljabh; that several circpies, such, as Plat(> and Grimaldi, owe. to the same cause the dark color of their cavities ; wliile other mountains aie so brilliant that they have given the idea of active volcanoes. Such is Aristarchus, which we perceive distinctly during the eclipses, or in the midst of the ashy hght, a little before the first quarter. 108 wondehs of the moon. Let us now speak of the singular appearances known by the names of luminous bands. It is principally during the full moon that these irregu- larities of the lunar disc are visible. The luminous bands are distinguished from the spots in this respect : the oblique light of the solar rays causes them to disappear, or at least renders them difficult to see, although they shine with all their brightness when this light falls .perpendicularly on the surface. Most of them form radiating systems w.hich have for centres some of the principal lunar craters or cirques. Of all these singular systems, the greatest is that which belongs to Tycho. Imagine more than a hundred luminous bands, of various sizes, diverging in all directions from north to south, and from east to west, like so many meridians drawn around Ty- cho as a pole, running with the same intensity over mountains and jjlains, crossing the embankment ot cirques, and losing themselves at various distances, the greatest of which reaches 1,865 miles, more than a quarter of the circumference of the Moon. These luminous bands are not, as were known early, chains of mountains; neither are they long valleys. In both cases their boundaries would cast shadows, now on this side, now on that, as the Sun's TOPOGRAPHY OF THE MOON. 109 rays fell. But they are at all times equally brilliant tlirougliout tlieir length, -which reaches even twelve to eighteen miles. Some astronomers have said that the radiating system of T^'cho is not visible excepting at about the period of full moon. It is an error. I have before me the charming photographs of Mr. Warren de la Eue, obtained by him at different periods of a lunation. It is easy to distinguish the luminous bands, at least the most brilliant among them, from the first to the last quarter of the Moon. But it is true that it s at the jjeriod of fuU moon that they are shown with the greatest clearness and brilliance. Tyeho is the only radiating mountain of the south- ern hemisphere, but it is also the most remarkable of these systems. In the northern hemisphere the radiating moun- tains are numerous. Copernicus, Aristarchus, and Kepler, all situated within or upon the Oceanus Procellarum, are among the most brilliant. (Fig. 28.) The luminous bands which radiate from each of these craters are not only shorter than those of Ty- eho, but they are also less regularly distributed ; those of Aristarchus, for example, radiate only from the southwest to the southeast ; they are wanting upon all the northern periphery of the crater. On liO WOXDEKS OF THE MOON. the other band, the three radiating systems appear to be in connection with each other, and many of their bands unite, which arises, perhaps, from their })roximity. Like the bauds of Tycho, those of Kepler, Co- pernicus, and Aristarchus are visible even after full moou. Euler, Mayer, Timocharis, and Eratosthenes are also mountains, situated, like the preceding ones, in the eastern part of the northern hemisphere ; but their bands attain less length. The western part of the same hemisphere con- tains, also, Proclus, east of the Mare Crisium, Cas- sini, and Aristillus, and AuLolycus, three craters, situated at a little distance from each other in the Palus Putridinis and the Palus Nebularum. The radiating bands of these last mountains unite like those of Kepler and Copernicus. We must not confound these luminous bands with the slopes which separate the embankments of Ar- istillus and Autolycus, and which have been com- pared to volcanic eruptions and currents of lava. Besides these radiating sj'stems, each having an annular mountain for its centre, we perceive, also, upon the disc of the Moon, luminous bauds which appear isolated, and are not attached to any visible system. 8*P*SS»S l».wl 1^ \ij ,., T* ^fi itr -i^-^X**4 jii ^^ H A II 1 fi II i^i wjii'iiCi'^'i 1 '^iMi^'p «» tim TOPOGRAPHY OF THE MOON. Ill Sucli are seen in the neigliborliood of Copernicus, whose direction is not that of rays fi-om a central crater. Another traverses the Marc Serenitatis, though from north to south. Beginning at tlie steep crater of MeneU^us, it runs in a straight line over the even surfaces of the sun 3unding plains, traverses the crater Bessel, and loses itself near the Lacus Somniorum. Although Menelaus is the centre of some radiat- ing bands, this of which we speak does not appear to belong to its system. Mr. Webb considered it, with much reason, as the prolongation of one of the bands of Tycho, which, as we have seen, extends to a distance of 1,865 miles from its centre. What is the nature of these singular appearances, what is the origin of their systems ? This is a very interesting problem, but a difficult one to solve. We have said that it was impossible to confound them with the white furrows of the lunar hills, because those forming projections on the surface cast, when the incidence of the solar rays permits, shadows on each side. It has been supposed that the bands were produced by the currents of lava, whose bril- liant traces are imprinted on the surface which they have traversed. But how shall we explain their im- mense length ? How account for their course above the highest craters ? 112 WONDEKS OF THE MOON. Some have supposed the himinoas bands to be composed of white crystalUiie matter, of strongly reflecting power, which has been thrown upon the surface of the Moon through fissures caused by vol- canic action. This hypothesis is liable to strong objections, and does not appear more probable than the former. According to M. Babinet, it is entirely to the structure of the surface that we must attribute these mysterious appearances, which arise from the reflec- tion of the solar light thrown upon peculiar forma- tions, a phenomenon analogous to that which is presented by certain crystalline geodes. Finally, an eminent observer, Chacornac, has pre- sented a theory which is connected with a system of lanar geology ; we shall soon explain it in develop- ing the ideas of that scholar on the jDeriods of the formation of the surface of the Moon. XII. THE INVISIBLE HEMISPHERE OF THE MOON. IS THEKE A Dli-'FERENCE IN PHYSICAL CONSTITUTION BETWEEN THE VISIBLE AND INVISIBLE HEMISPHEltES '? The Moon always turns tlie same face towards the Earth, Has this alwaj'S been the case, aud will it continue to be so ? Arago, who propounds the same question in his Popular Astronomy, cites in support of the affirmative some verses of an ancient poem collected by Plutarch, the vague import of which, it seems to us, is far from proving anything. That Age- sianax and his contemporaries saw in the disc, co- vered with dark and brilhant spots, a human figure, which the people of oiu' day still think they see, is very slight evidence in favor of the invariableuess of the fact in question. Modern observations, and above all the modern Ill WONBKIt.:i OF rilK MOON. tlieury, are more convincing. Laplace lias shown that " the cause wliicli has established, a perfect equality between the mean motions of rotation and revolution of- the Moon, deprives the inhabitants of tlie Earth forever of all hope of discovering the parts of the surface opposite to the hemij^shere which is presented to us. The terrestrial attraction in continually drawing towards us the large axis of the Moon, makes its motion of rotation participate in the secular inequalities of its motion of revolu- tion, and constantly directs the same hemisphere towards the Earth." Thus our curiosity is bounded, and the imagi- nation of framers of hypotheses apparently put to rest. That the ancients supposed the invisible side of the Moon to be of a concave form, or again, half transparent, is not to be wondered at ; their know- ledge of physical astronomy amounted to almost nothing. But what appears more strange is that the moderns have imagined that the hemisphere opposite to the Earth is possessed of water, air and inhabitants of which the hemisphere turned to- wards us is entirely destitute, and that the latter only has the advantage, or disadvantage as maybe, of being covered with abrupt and rugged elevations. It would be difficult to refute assertions so im- probable, which moreover, are purely imagiufitive, THE INYIalBLE HE.UISPIIEUE OE TilE MOON. 115 if positive observations diJ uot prove tUom false. Wlien, iudeed, we say tljat the Moon alwajs tuin? tlie same face to the Earth, this asiortiou is not rigorously accurato, and liore is the reason : The revolution of the Moon around the Earth is performed with a variable rapidity, while its motion of rotation is uniform. The result of this inequality of the two motions is that the Earth is soinetimjs east and sometimes west of that point in space op- j)osite to the same point of the surface of the Moon, considered as the centre of the visible hemisphere. We thus discover, either on the east or west regions, near the edge, which, were it not for this circum- stance, would remain concealed from us. Besides, the inclination of the plane of the lunar orbit, added to that of its equator on the plane of the terrestrial orbit, causes the moon sometimes to present its north and sometimes its south pole, and thus discovers to us certain portions of the polar regions. The result of these two librations, the name given to these motions, is that of 1,000 parts of the surface of the Moon, 569, that is more than half, would be visible to an observer at the centre of the Earth, while only 431 would remain concealed from him. But it is not from the centre of the globe only that we observe the Moon, it is, on the contraiy, 116 WONDERS OF THE MOON. from points of its surface often remote from eacli other, and, as the dimensions of the Earth are ap- preciable compared to its distance from the Moon, the result is that two observers placed at different points of the terrestrial spheroid would not see the centre of the lunar disc at the same point of its sur- face, or, which amounts to the same thing, would perceive different parts of its edges. This increases still more the dimensions of that part of the Moon which is accessible to us, so that by an accurate cal- culation, of 1,000 parts only 424 remain absolutely and positively concealed from us, while 576 are visible. Of the 14,670,000 scpare miles, which, as wo have seen, compose the total sm'face of our satellite, we are enabled to observe nearly 8,500,000. The pai't of the Moon totally concealed from the Earth embraces from east to west about 2,800 miles ; from north to south 2,820 miles ; from 40 degrees of north latitude to the same number of de- grees of south latitude 2,700 miles. While the same dimensions reckoned on the visible surface are resi3ectively 3.200, 3,100 and 3,300 miles. (Beer and Mfedler.) A large zone, then, of that part of the Moon which is opposite to the Earth is visible to the eyo of man. THE INVISIBLE HEMISPHEKE OF THE MOON. 117 " But observations have not enabled us to per- ceive — it is two of the most industrious observers of the Moon who speak — any essential difference be- tween the regions which form the seventh part of the lunar surface which is concealed from us, and that with which we are acquaiated ; we find the the same mountainous countries and the same Mares (the plains called seas). Beyond the north pole we perceive some large circular valleys sepa- rated by chains of mountains of a medium height and by level plains of a less extent, resembling those which we perceive in the arctic regions on this side of the north pole. On the northwest the Mare Humboldtianum, the high surrounding mountains of which we can readily perceive, extends into the hemi- sphere which is invisible to us ; the same is true of some parts of the Oceanus Procellarum on the east, of the Mare Australe on the southwest, and of the great plain called Kastner, which ap- proaches the smallest seas in extent on the west. . . Almost directly to the east rise the high summits of the Alembert Mountains, similar to the less elevated ones of the Cordilleras on the hemisphere which we see. tSo, at the south pole we perceive on the two sides an equal accumulation of colossal heights and of enormous depths ; the gTeat inequahtios of the 118 WONDERS OF THE MOON. lunar edge whicli appear iii tliis part belong mostly to the invisiljle liemispliere." (Beer and Maedler). Thus nothing warrants us in considering the he- mis])here of the Moon which is turned from us un- hke that which we see ; this may well lessen our regret that we can never see the whole surface ot our satellite. Besides, the visible regions are suificieutly remark- able, on account of the variety of their configuration, to present a vast field for the study of lunar geology and topography. But to those whose interest is not Kmited to mere facts, who wish to inquire into causes, to discover the origin of these peculiarities of the lunar surface, to whom knowledge means know- ledge of laws, to the true i^cholar, the visible hemi- sphere of the Moon ofl'i is al)undunt opportunity for research. CHAPTER m. GEOLOGY OF THE MOON. XIII. VOLCANIC CONSTITUTION OF THE LUNAE SUEFAOE. IGNEOTJS OEIGIN OF THE LUNAR MOUNTAINS — PERIODS OP FORMATION. The lunar mountains are of volcanic origin. This is a leading fact, which is shown directly by the rounded, annular form of the large valleys, of the plains, and of all the smaller cavities to which, as we have seen, the name of Craters has been given. Astronomers have long agreed in considering the formations of the lunar surface as dtie to a reaction of internal forces against the exterior crust of the globe. Robert Hooke " attributed these phenome- na to the effect of subterranean fires, to the eruption of elastic vapors, or, which is the same, to an ebul- 120 WONDERS OF THE MOON. lition setting free bubbles which biirst at the sur- face. Experiments made by boihng calcareous earths appear to him to confirm his views ; and since then the circumvallations and their central moun- tains have been compared to the forms of iEtna, the Peak of Teneriffe, of Hecla, and the volcanoes of Mexico." (Humboldt.) Su- John Herschel is not less positive in this re- spect. " The lunar mountains," he says, " present in the highest degree the true volcanic character, such as is presented by the crater of Vesuvius and the volcanic districts of the Champs Phlegreens, or of the Puy-de-Dome." (Outlines of Astronomy.) Bu-t if the igneous origin seems to be the only very probable one by which all the mountainous irregularities and crater-forms can be accounted for, this does not imply that they are whoUy the result of volcanic eruptions in the hmited sense of the word. The Moon was primitively, like the Earth, a fluid globe on whose surface the cooling, due to a calorific radiation, has caused the formation of a solid crust. It is the crust which has been the seat of subsequent phenomena whose traces remain to- day under the form of prominences of all sizes ; and the causes of this series of productions are, without doubt, the expansive forces of gases and vapors ■which the high temperature of the centre incessantly IGNEOUS ORIGIN OP ITS MOUNTAINS. 121 developed. Originally, the solid crust of the Moon (Fig. 29) being thinner was, for this reason, less re- sisting ; and, as it had not yet been agitated by previous convulsions, it would present at all points nearly the same homogeneity and the same thick- ness. The expansive force of the gases acting then perpendieillarly to the superficial layers, and follow- ing the lines of least resistance, would break the envelope and produce upheavals of a circular form. It is to this period, doubtless, that the formation of the immense circumvallations whose interior is to- day occupied by the jolains called seas, should be referred. We have sjaoken of the circular form of the Mare Crisium, and of Sereuitatis, Imbrium and Humorum. Their walls, half ruined by subsequent convulsions, still form the longest series of asper- ities on the lunar surface, the Carpathian, the Apen- nines, the Caucasian and the Alps among the chains, and Mounts Hsemus and Taurus. Then came new upheavals, but these, occurring at an epoch when the crust of the lunar globe had acquired a greater thickness, or perhaps in conse- quence of a decrease in the elastic force, formed the largest cirques, yet much smaller than the primitive formations. The cu-ques of Shickardt, of Grimaldi and of Clavius seem to belong to this class. 122 WONDERS OF THE MOON. Subsequently a multitude of cirques of smaller dioiensions appeared, whose walls covered the en- tire surface of the Moon, and appeared even in the very heart of the primitive circumvallations. We understand readily the reason of the successive diminution in dimensions of the annular mountains, craters and cirques. Each crater is due, as we have seen, to the uprising of a bubble or blister, whose depression has produced in the interior a cavity of elliptical form, and upon the borders one or several walls in the form of ramparts. But the dimensions of these inflations should be in proportion both to the intensity of the internal force which produced them, and to the resistance of the solid, or rather plastic crust of the lunar globe. It is probable that these two causes have united to produce the most marked results, therefore the largest circumvalla- tions, and the greatest cirques or craters were form- ed first. (Fig. 30.) But it is time to make a distinction between the two kinds of soil which characterize the surface of our satellite. The first comprises that which has been called from the beginning the continental soil ; it is that of the mountainous regions which cover almost all the southern portion of the visible hemi- sphere. "Its porous structure," says an observer very familiar with selenographical studies, M. Cha- IGNEOUS OrJGIN OF ITS MOUNTAINS. 123 cornac," "its great reflecting power, and, above all, its elevation above the plains, make it easily dis- tiiiguisliable from the levelled ground, whose dark color and smooth surface, according to Sir John Herschel, give it all the appearance of plains of al- luvium. Are, then, the lunar mountains plains of alluvium ? No, not exactly in the terrestrial meaning of the word. Even the astronomer whom wo have just quoted, rejects this expression as improper. But he relies upon numerous aud very interesting phenome- na in order to prove that the early period when the largest circumvallations appeared was succeeded by a kind of general diluvium or muddy overflowing. " This overflowing would have buried under a brown mass more than two- thirds of the visibhi sur- face of the Moon, and, in spreading itself from one extremity to the other, filled the bottom of all the large craters, obviously, to the same level." Indeed, among the innumerable craters whoso cavities indent the surface of the lunar body, some present in the interior an excavation of regularly conical or rather elliptical form, perfectly sloped, whose borders or walls are unimpaired. (Fig. 31.) Others, on the contrary, have their walls broken, * Note upon the a2)pearauces of tlie luuar Burface. 124 WONDERS OF THE MOOX. aud the bottom of the cavity is flat and ou a level with the ground of the surrounding 'valleys. It is especially upon the borders of the seas that these half-ruined craters are found, and it apjDcars evident that the cavity has been filled by the over- flow which M. Chacornac describes. " The config- uration of these shores presents vast semi-circular Fi^'. 29. Cirqiie, elliptical at the bottom, in the form of a bowl. bays, whose entrance is partly obstructed by the de- bris of the ruined wall in the exact direction of the width, as has happenetl in the bottom of the crater of the island of St. Paul (Indian Ocean), covered in our day by the waters of the ocean." The Sinus Iridium, on the borders of the Mare Imbrium, is one of the most remarkable examples of this encroach- IGNEOUS OEIGIN OF ITS MOUNTAINS. 125 ment. But many examples of others can bo cited, among which we shall mention at random Hippalus and Doppelmaycr in the Mare Humorum, Davy and Bonplaud in the Mare Nubium, i-nd Fracastorius upon the southern border of the Mare Noctaris. Several of the craters which are elevated in the ^H ^^^^^B V H ■ ^^^^^B H 1 ^^[ji^^^^^^^^^g E^^^ ^^'ITJtffl^gs^i^^^^^^^^g; g»Se m ^^M 1 1 ^^^here turned towards us. A cu- rious circumstance, and one which contrasts with the constant motion of the Moon in our sky ; the disc of our planet shines always in the same part of the starry vault, suspended above the horizon like a lamp, and oscillating around this almost invariable position only in an insensible manner. Let us transport ourselves in thought to a place on the visible hemisphere, for instance just opposite our Earth, that is on the central meridian. At mid- night — at the hour when the new moon begins for the inhabitants of our planet — the full earth shines in all its glory. More than thirteen times as large as the lunar disc seen from the Earth, the terres- trial disc presents to us varied spots which mark its continents and seas, here and there hidden by other shining and moving spots, the atmospheric clouds. Two whitish caps, like those of Mars, surround the ITS METEOEOLOGY. 167 poles ; the seas are of a deep bluish shade, while the continents are spotted in parts by a pale green, and the whole contour of the disc, more luminous than the central parts, is sHghtly reddish, the natural effect of atmospheric refraction. The Earth remains almost immovable at the same point of the sky, more or less near the zenith, according to the latitude ; but the aspect of its disc varies with a rapidity relatively very great. We see spots passing from the eastern to the western edge (with regard to the south point of the lunar horizon). If the Asiatic continent were first seen, it would disappear first to make room for Europe and Africa, and finally for the New World and the Pacific Ocean. Every 24 hours this procession re- commences, and the Earth seems thus like a clock with a movable dial-plate, whose hours correspond to the difi'ereut spots. As the night advances, the terrestrial disc flat- tens, and the cu'cular shape becomes oval upon one of its halves, until at sunrise it is presented under the form of a half circle. The reverse takes place in the first half of the lunar night, so that in 354 hours the Earth has passed from the first to the kist quarter. The other phases are comjJetud in the entire day, and our planet appears in the midst of the stars like a great crescent whose curve is 168 WONDERS OF THE MOON. bounded by an obscure tint like the greyish light ; such are the lights of the terrestrial Moon. These phenomena, we have said, are unknown in the days and nights of the opposite hemisphere : nothing is known there of the Earth. A zone only of a certain extent sees our globe appear on the horizon, re- main there some time and then disappear, never rising more than a few degrees. Still this appear- ance of the Earth does not take place every night, so that in this zone there are bright nights and nights entirely dark. Everywhere else, the nights have an intense obscurity which no twilight relieves, but where the magnificence of the starry vault per- mits the observer to witness, for 350 hours, the most delicate celestial phenomena. The duration of the day and of the night varies within very small hmits on the surface of the Moon, a circumstance which arises from the small inclina- tion of its axis to the echptic. The parallels de- scribed by the Sun are but a short distance. from the equator. But it is necessary to except the regions quite near the poles, where the duration of the night and that of the day may be much shorter or longer. At the very poles, the mountains are per- petually illumined by sunlight. " The Sun descends below the true horizon of a lunar pole only by a quantity equal at most to the inclination of the ITS METEOROLOGY. 169 Moon's etpator, that is by 1° 30', but the smallness of the Moon's globe is such that, even at an eleva- tion of 1,900 feet, the eye penetrates 1° 30' below the true horizon. Now there are at the North Pole mountains 9,500 feet, and at the South Pole, 13,000 feet high ; consequently the summits of these moun- tains can never be hidden from the Sunlight." (Beer and Msedler.) We shall obtain in another way a more exact idea of the little difference which exists in the dura- tion of the shortest days at different latitudes of the Moon, by the following table, whose elements are borrowed from the Popular Astronomy of Arago, and which may of course be applied to the longest and shortest nights. At the equator the days and nights do not vary, and are constantly 'of 355 hours, 22 minutes and 1 second. Latitude, N. or S. Duration of longest day. Duration of shortest day 0° 354 h. 22 m. 1 s. 354 h. 22 m. 1 s. 15° 355 h. 9 m. 19 8. 353 h. 34 m. 43 s. 30° 35Gh. 3 m. 54 s. 352 h. 40 m. 8 s. i5° 357 h. 18 m. 30 s. 351 h. 25 m. 38 s. 60° 359 h. 27 m. 47 s. 349 h. 16. m. 15 s. 75° 362 h. 21 m. 40 s. 343 h. 22. m. 22 s. The difference between the longest and shortest days is, as we see, very little at first, and becomes 170 WONDEHS OP THE MOON. very sensible only after the 60th degree. At 88° of latitude, 2° from the poles, this difference is more considerable ; it has already increased to 190 hours, and finally at the very poles the Sun is visible for 179 days, a little less than half a terrestrial year. The invisible hemisphere has days a little shorter than the hemisjohere turned towards the Earth. The greatest difference is at the points situated on the two halves of the central meridian, and reaches, for the average duration of the day, 1 hour, 7 mi- nutes, 54 seconds. XIX. THE MOON'S CLIMATE. THE SEASONS — HEAT AND COLD — TOEEID TEMPEE- ATUKE OF THE LUNAE DAYS ; INTENSITY OF COLD DURING THE NIGHTS — TAEIATIONS IN THE COURSE OF A REVOLUTION ABOUND THE SUN. The Moon has days and nights, hke the Earth ; has it, hlie our planet, a year and seasons ? When we consider a celestial body which moves directly around the Sun, describing an orbit of an elliptical or nearly circular form, the year of this celestial body is the interval of time which elapses between two consecutive returns of the body to the same point of its orbit. For the Moon, the move- ment being more complex, the definition of a year can be understood in two ways, and each of these is subject to different interpretations. Do we consider the revolution of the Moon around the Earth ? In this case, the lunar terres- 172 'WONDEES OF THE MOON. trial year may be understood as the return of our satellite to the same point of its orbit, and its du- ration is 27 days, 7 hours, 43 minutes, a little less than a lunation ; or again, as the return of the Moon to the same position bi regard to the Sun and the Earth, and in this case the lunar year and the lunation are one and the same thing. That would amount to saying that the Moon's day is identical with its year, which is composed in all of a single day and a single night. Finally, if we consider the lunar year relatively to the movement of the Moon around the Sun, it will be very nearly of the same duration as the terrestrial year. But it is not the purely geometrical side of the question which we have in view here ; what it con- cerns us to know is the influence which the varia- tions in the position of the Moon may have on the temperature and the climate of the different lunar regions. These variations are very slight. This results from the axis of rotation, as it moves in space, re- maining always parallel to itself, and nearly perpen- dicular to the plane of the ecliptic. The Sun, whether in the course of a lunation or in successive lunations, varies Uttle in its altitude above the same horizon. Oscillating from one degree and a half below the equator to one degree and a haK above, ITS METEOROLOGY. 173 its total variation reaches only three degrees. Hence a uniformity in the climate of each region, so much the more sensible because there are no cor- responding atmospheric phenomena. But let us try to form an idea of the Moon's cli- mate, with reference to the variations of its temper- ature. For 354 hours, nearly 15 terrestrial days, the Sun darts its rays without intermission to the ground, at first obhquely in the hours which follow its ris- ing ; then more and more vertically as mid-day ap- proaches. The temperature of the surface, under the prolonged influence of such an intense radia- tion, must attain an extraordinary elevation, " per- haps much greater," says Sir John Herschel, " than that of boihng water." After the lunar noon, the heating of the soil continues, and attains, undoubt- edly, its maximum between this time and sunset, as on the Earth. In truth, the absence of an atmosphere must per- mit the radiation of heat to be developed with an extreme intensity, depending also on the nature of the substances which compose the soil. It follows that the Moon's climate offers a certain analogy to our Alpine climates. We know that if we ascend high mountains, the Sun's heat directly received is insupportable ; the soil itself is easily heated ; but 174 WOKDERS OP THE MOON. the layers of atmosphere have a lower temperature, and we experience distinctly a sensation of cold, specially noticed when we are in the shade. In the elevated regions, " the soil radiates rapidly, and if it is heated more than the air under the influence of the solar rays during the day, it cools off sooner when the Sun's rays no longer strike it directly — ■ that is, in the shade, and during the night." (Mar- tins.) It is the rarity of the atmosphere which fa- vors the heating of the soil on the summit, and which, rendering also the radiation more intense, favors stiU more its cooling off in the shade or dur- ing the night. On the Moon the air is wanting, or, if there are traces of it, its rarity is much greater than that of the air in Alpine regions. The contrast is still more striking there ; we ought to find at the same time a torrid temperature in places exposed to the solar light, and an intense cold in places which are de- prived of it ; for instance, in the shade of cavities, of craters, and of circular enclosures. During the 354 hours of the night, aU this accu- mulated heat being no longer retained by a gaseous envelope, the temperature decreases with extreme rapidity. It sinks, undoubtedly, much below that of our polar winters. From the equator to the two poles, the difference ITS METEOROLOGY. 175 of climates is due only to the greater obliquity with which the rays of light and heat strike the soil. So that the decrease of temperature with the increase of latitude ought to have much analogy with the thermometric variations which characterize the dif- ferent i^eriods of a lunar day. On our Earth, the extremes of winter, as well as of summer, are often mitigated by atmospheric phe- nomena, by aerial or oceanic currents, by the pres- ence of clouds, by rain or storms. In the Moon there is nothing of the kind. A leaden Sun darts its piercing arrows without pity upon all points ex- posed to its action. If the lunar surface were cov- ered here and there by sheets of water, by seas, by lakes or rivers, how could these liquid masses es- cape a rapid evaporation, especially as no atmo- spheric presence would hinder this change of state ? So much for the day. During the night, every trace of water or of aqueous vapor would disappear by an inverse phenomenon, and the liquid sheets would be found transformed into frozen lakes, and the va- pors, also, rapidly condensed, would fall suddenly under the form of snow. Sir John Herschel has expressed an opinion of this kind ; but, if he does not consider such phenomena as impossible, at least he con.siders them as confined to very narrow limits. The alternation of two opposite tempera- 176 WONDERS OF THE MOON. tures, both excessive, he says " should cause a con- stant transfer of whatever moisture may exist on its surface, from the point beneath the Sun to tliat opposite, by distillation in vacuo, after the manner of the little instrument called a cryophorus. The consequence must be absolute aridity below the vertical Sun, and constant accretion of hoar frost in the opposite region. It is possible, then, that eva- poration on the one hand, and condensation on the other, may, to a certain extent, preserve an equi- librium of temperature, and mitigate the extreme severity of both climates ; but this process, which would imply the continual generation and destruc- tion of an atmosphere of aqueous vapor, must, in conformity with what has been said above of a lunar atmosphere, be confined to very narrow Hm- its." If the hypotheses of the illustrious astronomer were a statement of facts, it would account, in a certain degree, for the feeble radiation of heat from the Moon to the Earth ; since it is the vapor of water, according to phjsicists, which forms an obstacle to the radiation of heat emanated from a source not incandescent. But what destroys in our eyes much of the probability of this hypothe- sis, is that the evaporation of which Herschel ITS METEOKOLOGY. 177 speaks ought to give birth to clouds at least in the regions situated near the hmits of light and shade — that is, during the lunar nights and morn- ings. We know that nothing of the kind has ever been observed. To finish what we have to say of the lunar tem- peratures and their variations, considered thus far in the interval of one revolution only around the Earth, let us see what these same variations are in the course of a terrestrial year. As the Moon un- ceasingly accompanies our planet, its distance from the Sun varies as the distances of the Earth itself — that is, nearly in the ratio of the numbers, 1,019 and 0,980. The intensity of the solar heat will then vary in the inverse ratio of the squares of those numbers ; so that if we represent this inten- sity by the number 1,000 at the mean distance, it will be 1,038 at the maximum distance or aphelion, and 960 only at the minimum distance, or perihelion. That is a difference, more or less, of nearly ^l, a very appreciable and sensible quantity. XX. IS THE MOON INHABITED ? ■VEGETATION— HABITABLENESS — EXAMINATION OF THE CONDITIONS NECESSARY TO THE EXISTENCE OP OE- GANIZED BEINGS ON THE MOOn's SUEEACE. Aee there upon the Moon vegetables, animals and men ? In a word, is the Moon inhabited ? These are questions which human curiosity has been agi- tating for a long time, and such as we never fail to put to ourselves whenever our imagination and our thoughts transport us to any of the celestial bodies which illumine the starry vault. The answer is generally very diiScult, so long as we do not wish to leave the solid ground of obser- vation and of facts. Undoubtedly, when we con- sider the problem in its generality, and from an en- tirely philosophical point of view, it appears very probable that the Earth is not the only planet of our universe where the proper conditions for the development of animal and vegetable life are to be ITS METEOROLOGY 179 found. Still more, we must believe that the suns, scattered through the infiuite space, and which are the sources of heat for so many planetary worlds invisible at such great distances, do not spread light and heat over their satellites in vain ; we re- fuse to imagine the silence of death, where the principal sources of life appear to us in full activity, and seem to spread themselves abroad in such mar- veUous profusion. Indeed it is impossible for us to conceive of this universe otherwise than as a har- monious whole, where are grouped in infinite num- ber the sources of. life and of power. But we also know that if the laws of nature have an undeniable character of universality and of unity, their manifestations are indefinitely varied. The historjr of the Earth itself shows us that it has not always been the abode of life, that before the periods when the first attempts at organization appeared upon its surface, other periods had elapsed in which the conditions necessary for the appear- ance of the simplest organisms did not exist ; that then, in a word, the Earth was not inhabited. Per- haps, without quitting our solar system, we may find planets which have not yet passed out of their embryonic state, and where life has not yet been generated. It is also possible that revolutions, of which we have no idea, may have destroyed, on this 180 WONDERS OF THE MOON. or that globe, all animated beings, or even that a planet may have been from its origin so constituted that animal or vegetable life may be forever impos- sible upon its surface. All these hypotheses seem to us equally probable ; but they are only conjectures, and we might reason for a long time upon snch suggestions without ever drawing from them anything but vague and arbi- trary conclusions. We must resign ourselves to this for the unknown jjlanets of other systems, per- haps even for several of the celestial bodies of our own system ; but we may hope to solve the question in a jjositive manner as far as concerns our sateUite, too near to escape decided investigations. Let us see wdiat is already known or supposed concerning the existence of animated beings upon the surface of the lunar globe. Conjectures are not wanting : there have always been people who have given the Moon inhabitants ; they have been appropriately called selenites (from GeXi'lvii, which in Greek signifies the Moon.) But they have had no other reasons than the analogies Avhich the Moon and the Earth present in an as- tronomical point of view, and they have hastened to extend these to all other physical phenomena. Not more than a century ago, one of the most learned ITS METEOROLOGY. 181 astronomers of the period reproduced from the Eng- lish Encyclopedia the following lines : " The Moon is in every respect a body similar to the Earth, and which appears adapted to the same end ; in short, we have seen that it is dense, opaque, that it has mountains and valleys ; according to many writers it has seas with islands, peninsulas, rocks and promontories ; a varying atmosphere where vapors and exhalations rise and fall again ; finally, it has a day and a night, a Sun to lighten the one, and a Moon (the Earth) to lighten the other ; a Summer and a Winter, etc. Hence we can infer, from analogy, an infinite number of other properties in the Moon. The changes to which the atmosphere is subject ought to produce uinds and other meteoro- logical 'plicriomena ; and, according to the different seasons of the year, rains, fogs, hail, snow, etc. The inequalities of the Moon's surface ought to produce on their part lakes, rivers, springs, etc. Now as we know that nature produces nothing in vain, that the rains and the dews fall upon our Earth in order to make the plants vegetate, and that the plants take root, grow and produce seeds in order to nourish animals ; as we know besides that nature is uniform and constant in her processes, that the same things serve for the same ends ; why should we not conclude that there are plants and 182 WONDEKS OF THE MOON. animals on the Moon ? If not, for what end this display of provisions ^vhich seem so well adapted to their existence ?" (Encyclopedia, Art. Moon.) We have nothing to say of the value of the rea- soning in itself, which draws all its power from the principle of final causes, generally abandoned to-day by learned men ; but it is very evident that the data are far from being incontestable. We have stated the reasons why astronomers no longer beheve either in the existence of hquid masses, or in that of a gaseous envelope, composed either of aqueous vapor or of atmospheric air, and that there results from this the impossibility of all the meteorological phe- nomena, the enumeration of which we have just read. What are, at least upon the Earth, the primary conditions indispensable to life ? Water, air, a cer- tain temperature. Now, if it is not rigorously proved that the Moon is totally dejDrived of an atmosphere, it is at least certain that its density is extremely slight, compared with the density of the earth's atmosphere. Yet, rare as it may be, it would sufBce to furnish vege- tables with the gaseous elements which their nutri- tion and their develojoment require. But we have no knowledge of an animal organism capable of living in a medium analogous to the air which re- ITS METEOEOLOOY. 183 mains under the bell-glass of our air-pumps, when the pressure is only a few hundredths of an inch If the lower layers of the supposed atmosphere were sufficiently dense to permit animals to live there, the inhabitants of the Moon would be obliged to inhabit the bottoms of craters, in groups separated from each other by impassable ridges. Neither does wa- ter exist upon the Moon's sirrface ; we have seen two conclusive reasons for this : first, the absence or the feebleness of atmospheric pressure, owing to which every liquid would be spontaneously reduced to vapor ; then the prolonged intensity of a torrid temperature, which would parch the soil at each lunation. Have we any idea of organized beings whose tissues could neither preserve nor renew their supply of moisture ? On our earth undoubtedly we see vegetation developing itself with astounding rapidity in the warmest climates of the torrid zone ; but such a development is due to a mingling of moisture and of heat : vegetation ceases almost en- tirely wherever dryness and heat are found com- bined. Finally, hfe disappears, even in regions where air and water are certainly far superior in quantity to what our satellite possesses, when the altitude is sufficient to jaroduce a glacial tempera- ture analogous to that of our polar climates or of the Alpine regions. 184 WONDERS OF THE MOON. Now such a temperature certainly prevails on the Moon during its long night of fifteen terrestrial days. How can living beings resist these alternations of excessive heat and cold ? Besides, we know that it is the vegetable kingdom which, directly or in- directly, furnishes to animals the assimilable ele- ments necessary to their existence ; so that the first question to solve would be to know whether the physical constitution of the Moon is adapted to ve- getation. We have just seen how unfavorable this constitution seems in connection with the three most indispensable elements : air, water, and the degree of temperature. Let us add that the soil itseK does not appear favorable to the development of the vegetable king- dom. Its eminently volcanic nature, and the absence of lands analogous to the tertiary and sedimentary formations of the terrestrial globe, except perhaps in the vast open plains caUed seas, cause the Moon's actual state to agree with the primitive geological epochs ; and we know that vegetation did not appear upon our Earth until in later periods, when the atmospheric agents, by disintegrating the rocks, had rendered the soil fit for the production and for the life of the lowest organisms. Are there, then, many probabilities against the possibility of the existence of Hving beings in the ITS METEOEOLOQY. 185 Moon ? Is there an absolute impossiblity in it ? No, undoubtedly ; only we are left to imagine con- ditions of vitality different from those of which we have any knowledge, that is, to enter into pure hy- pothesis. Attempts have been made to solve in other ways the interesting question of the Moon's habitability, I mean by direct observations. Have they succeeded any better ? Does the telescope permit objects as small as animated beings to be seen with sufficient distiactness on the surface of the Moon? Let us see. Much has been said of large telescopes or of powerful glasses, possessing fabulous magnifying powers — for example, of 6,000 diameters. For the observation of the Moon, a magnifying power so considerable is simply impossible, and that for sev- eral reasons. We must not forget that with an in- crease of power, we obtain a more than proportional diminution in the intensity of the light of the ob- served object, when that object, like the Moon, is not self-lummous. Hence a limit to the magnifying power of an instrument. Moreover, the terrestrial atmosphere is never stiU enough to permit the em- ployment of such very powerful instruments, and the undulating images, badly defined, destroy all the nicety of vision. 186 WONDERS OF THE MOON. Thus, in the actual state of applied optics, we cannot use for the observation of the Moon this magnifying power of 6,000 diameters, which would make us see the star as if it were at a distance of only forty miles. We must content ourselves with magnifying powers of 1,100 or 1,200 times, which put, in the case of the minimum distance, the cen- tral parts of the lunar disc at 200, or, at best, at 180 miles from our eyes.* With an instrument sufficiently perfect to permit the employment of the strongest of these magnify- ing powers, we can perceive an object of 1,300 feet in diameter. The highest of the pyramids of Egypt would stiU be invisible. Supposing that we reach a magnifying power of 6,000 we could still perceive only objects presenting at least 250 feet in dia- meter. We must not expect then to be able very soon, in observing the Moon, to recognize hving beings, animals or vegetables. Vast forests would certainly be visible, but, like dark spots, poorly defined ; we could judge of their existence by their color only. Have we seen, on the lunar disc, colorings which * Jlons. Guillemin supposes a power of 1,100 or 1,200 to be used. lu practice this is very rarely done. Very few nights, at least in our Northern States, are suftieiently still and clear to admit of the use of a power greater than 400. — Ed. ITS METEOROLOGY. 187 TTOulcl lead us to believe in the existence of vegeta- tion, covering vast spaces ? Several observers agree in saying that indepen- dently of the variations in brightness which the different lunar regions present, they have also found, in several places, differences of tint. We will name the principal. Mare Crisium is of a grey color, mixed with dark green, according to Beer and Mjcdler. According to Webb, it is during the full moon that this green- ish tint shows itself. Mare Serenitatis is of a bright green color ; Mare Humorum offers distinctly the same tint, surrounded by a narrow greyish bor- der. Are these colors, as Arago is incHned to think, simple effects of contrast arising from the opposi- tion of the brilliant and slightly yellow light of the luminous parts of the disc with the feeble light of the dark spots ? If it were so, why should not this green color be common to all the plains, or at least to the parts which border upon the mountainous regions? Mare Foecunditatis, Mare Nectaris and Mare Nubium would be admirably situated for pre- senting the same effect of color by contrast, and yet observation says nothing with respect to them. What seems, moreover, to establish the fact of a real color is, that other places seem reddish. The 188 W0SDEK3 OF THE MOOS. crater Liclitenberg, in the neighborhood of Montes Hercynii, and of the northwest border, offers this tint, and Ptilus Somnii is of a yellow shade ap- proaching brown. Vitruvius, a crater very dark in the inside, is sur- rounded by a region of a pale blue color. Then, too, " the circular plains, whose centres are not oc- cupied by mountains, are for the most part of a dark grey approaching blue, which resembles the brilliancy of steel." Humboldt, in referring to these last facts, adds that " the causes of these dif- ferent shades on a soil formed of rocks or coYered with movable substances are entirely unknown." We are ignorant, in short, whether the rocks them- selves are thus closed, which would be very natural, or whether these tints are due to this or that lu- minous incidence ; finally, whether this has any con- nection, as was at first supposed, with spaces cov- ered by vegetation, forests or prairies. Undoubtedly, this last hypothesis would appear probable, if the reasons, which have made us be- heve in the total absence of air and water from the surface of the Moon, did not render the existence of organized beings upon its surface very hypo- thetical. CHAPTER V. THE MOTIONS OF THE MOON. XXI. THE EEVOLUTION OF THE MOON ABOUND THE EAETH. DIFFERENCE BETWEEN THE LENGTH OF A LUNATION AND THAT OF THE EEVOLUTION OF THE MOON — DISTANCES FROM THE EARTH — DIMENSIONS OF THE DEBIT, AND "\TiLOCITY OF THE MOON — EEAL FORM OF THE CURVE DESCRIBED IN SPACE BY ODE SATEL- LITE. What have we learned of the phases of the Moon and their succession in periods of about 29^ days ? It is that the Moon moves in space around the Earth from west to east, and, at the end of a hination, is again in the same position with re- spect to tlie Earth and Sun, considered as fixed. If 190 WONDERS OF THE MOON. the Earth were really motionless, the length of the lunar revolution would be that of the lunation — that is, 29 days, 44 minutes, and 3 seconds. But while the Moon describes its orbit, the Earth itself de- scribes its own around the Sun, or, at least, describes a portion, which is about 29 degrees of arc. And, as the direction of the two motions is the same, the evident result is that the Moon has effected an en- tire revolution before the lunation is accomplished. This is shown by the accompanying figm-e. (Fig. 41.) The Moon, starting from the point, L, at the time of conjunction, reaches L', where its revolution is Fig. 38. Difference between the duratinn of a lunation and that of the reTO- Intion of the Moon around the Earth. terminated,* before reaching L", where it will again • To understand clearly the difference that Ave mention, vre must have a definite idea of the independence of the two simul- MOTIONS OP THE MOON. 191 be in conjunction. Now, from L' to L", there re- mains an arc of about 29 degrees — that is, an arc equal to that described by the Earth during the Moon's revolution. This revolution is then less than a lunation, and an easy calculation shows that it is 27 days, 7 hours, 43 minutes, and 5 seconds. Now, what is the precise form of the curve de- scribed by the Moon ? This is a question which astronomers have answered by measuring the ap- parent dimensions of the lunar disc, during its whole revolution. If their dimensions, reduced to the centi-e of the Earth, remained constant, the distance of the Moon would not vary, whence we should conclude that it moved in a circular orbit. But this is not the case ; their dimensions vary, and, in calculating the corresponding variations of distance, we perceive that the curve has the form of an eUipse, and that the Earth is at one of the foci. We give an idea of the real form of the lunar taneous movements. If the Earth were motionless the Moon would finish its revolution at the moment it returned to the point L. But, during this time, the Earth has moved, ftnd the Moon with it ; consequently, the point L becomes L' in a direction T' L' parallel to the line T L. But this point L' is no longer in the di- rection of the Sun, and, in consequence, new moon happens afterwards, when the Moon has advanced to L", in the direction in which, seen from the Earth, the Moon would go around the Sun. 192 WONDERS OP THE MOON. ellipse — that is, of its eccentricity or the quantity by which it differs from a circle, by means of the follow- ing numbers, which are the extreme and mean dis- tances of the Moon from the Earth, the mean dis- tance being taken as unity. Greatest distance, or at apogee 1.05i9 Mean distance 1.0000 Least distance, or at perigee 0.9i51 These numbers indicate relative distances only ; but the true distances have been calculated, and may be expressed in values of the radius of the Earth, in ordinary measurements. The methods employed in solving this very interesting problem of the distances of the heavenly bodies cannot be explained in this place, but we have given elsewhere a sufficiently minute explanation to make one readily understand the principle.^'' The greatest distance of the Moon from the Earth is about 64 times the equatorial radius of our planet, (more exactly, G3.583,) while at the epoch of j^erigee, or at its least distance, it is no further removed from us than 57 radii of the Earth (56.9G4). The mean distance of our satelhte is GO^ radii (or GO. 273) — that is, nearly equal to the 400th part of the distance from * See the third part of the work on " The Heavens." MOTIONS OF THE MOON. 193 the Eartli to the Sua, which is, as we know, about 24,000 terrestrial radii.-'- Between the Earth and the Moon, or rather, be- tween the centre of the Earth and the point of the Moon nearest to us, there must be phiced a chain of thirty globes equal to the terrestrial globe, in a straight line, touching one another, to fiU the interval which separates the Earth from its satellite, when the Moon is at its mean distance. One hundred and ten lunar globes would be ne- cessary to fill the same space. Let us now change these spaces into distances in miles. The centre of the Eartli and Moon are 251,947 miles distant at apogee ; at perigee, 225,719 miles ; and finally, at the mean distance, it is 238,793 miles. It is readily seen that we must subtract from all these numbers the two radii of the Earth and the Moon, if we would obtain the distances of the two nearest points of the surfaces of the globe ; so the preceding numbers become the following ; Distance at apogee 246,910 miles. Distance at perigee 219,68'2 miles. Mean distance 233,756 miles. 233,750 miles ! This is not nine and a half * The distance of the Earth from the Sim was formerly con- sidered to be 95,274,000 miles. It is now calkd 91, 130,000. 194 WONDERS OF THE MOON. times the entire circumference of tlie Earth !* There are sailors, clovibtless, who, in the course of their voyages, have traversed distances as great, which our express raihoad trains would pass o\ er in less than 300 days. Suppose the space which separates the Moon from the Earth to be fiUed with air, so as to allow the passage of sound from one globe to the other ; then, at the time of full moon, if a volcanic erup- tion took place on its surface, the noise of the ex- plosion would not reach us until 13 days and 8 hours after the event, so that we should not be aware of it until nearly the next succeeding new moon. This calculation supposes the temperature of space to be at the freezing point of water, 32° Eahr. It would require a little less time, about eight or nine days, for a cannon ball to pass through the same distance, supposing that it kept a constant velocity of 1,610 feet a second. Light, which has the most rapid of all motions, comes from the Moon to the Earth in Ij seconds. In these famihar comparisons, which are useful in fixing, in the memory and imagination, distances so difficult for the mind to conceive, we consider only the constant velocity of moving bodies. We can also calculate the time which it would take a MOTIONS OF THE MOON. 195 body to fall frona the r 'iitre of the Moon to the centre of the Earth, or, what amounts to the same thing, the time which it would take for the Moon to reach the Earth, if the tangential force, which, combined with the force of grayity, causes it to de- scribe its orbit, were to be suddenly destroyed. At the end of 6 days, 5 hours, 40 minutes, and 13 seconds, the catastrophe, the terrible consequences of which we need not describe, would be consum- mated. Suppose the Earth to be motionless in space, the Moon describes around her an ellipse, the circum- ference of which is 1,500,000 miles ; the velocity with which it moves through this orbit is variable, but in general it is 3,350 feet a second. In reality, the lunar orbit is much more intricate, because our planet, in moving around the Sun, draws the Moon through space with it. It is thus that a person placed upon the deck of a ship in motion, in walking around the mast, appears to himself to move in a circle, while the line which he describes on the surface of the sea is a sinuous curve, the form of which is analogous to that of the real orbit of the Moon. In reality, the path which this person pursues is still more comi^li- cated, and to draw its true form we must combine his own motion, the motion of the ship on the sea, 1% WONDERS OF THE MOON. and the double motion of the ro- tation of the Earth on its axis, and of the revolu- tion of the Earth around the Sun. We shall see, fur- ther on, that the Sun also moves in space, drawing with it the Earth, the other planets and their satel- Utes ; hence result the sinuous forms of the orbits of all these bodies, the degree of com- plexity varying with the number of motions by which they are impelled. Fig. 42 gives an idea of the sinuous curve which the Moon MOTIONS OF THE MOON 197 describes, upon the supposition tliafc we remember the true proportions of the distances of the Moon from the Earth, and the Earth from the Sun. Yet the ciu've given above, appearing sometimes as con- vex and sometimes as concave, toward the Sun, is really always concave. But the Moon is not always in the plane of the ecliptic, in the plane of the Earth's orbit. The plane of its orbit is inclined to the plane of the ecliptic, at an angle of about 5° 9'. In consequence, our satellite is sometimes above, sometimes below, the plane of the figure, through which it passes twice in a revolution. Each of these two particular positions is called a node ; it is the ascending node when the Moon passes from the south to the north, and the descending node when it passes from the north to the south of the ecUptic. No two consecutive nodes occupy positions diame- trically opposite on the lunar ellipse, and these positions vary from one revolution to another. It is only after an interval of eighteen years and eight months that the nodes are again in the same rela- tive situations. This is one reason why the eclipses of the Sun and Moon do not repeat themselves at every lunation. The motion of the Moon presents many other in- equahties which we have not mentioned, and which 198 WONDERS OF THE MOON render the theory very difficult. Their explanation would not have been possible if the theory of gravi- tation, permitting us to separate the causes of these inequalities, had not found observations to compare with calculations. But this is not the place in which we should even touch such difficult ques- tions. xxn. THE MOTIONS OF THE MOON.— ROTA- TION. THE EQUALITY OF THE TWO LUNAB PEEIODS OF RO- TATION AND EEVOLUTION — LUNAR POLES AND EQUA- TOK. The Earth revolves on its axis in the interval of a sidereal day ; the diurnal motion of the stars and of the other heavenly bodies from east to west, testifies to the reality of this rotation. The spots on the Sun, those which are visible on the disc of the planet Mars, on that of Jupiter, the slopes of the crescents of Mercury and of Venus, long ago made known the fact that these bodies are subjected to the same motion, which affects them in the same manner, but whose periods are of very different du- rations. The Moon does not escape from this law, which seems common to all celestial bodies. At the same time that it performs its monthly revolution around 200 WONDERS OF THE MOON. tlie Earth, it turns, also, upon an invariable axis , and, by a singular circumstance, tlie duration of its rotation is exactly equal to its motion of revolution. The Moon, like the Earth, has also its poles, an equator, and meridian, and parallel circles. Hence the phenomena which we have already studied, and which make it necessary that each point of the lunar globe should possess a night and day, according as the light of the Sun leaves it im- mersed in darkness, or illuminates it by its rays. As upon the Earth, there is reason for distinguish- ing upon the Moon two different days, unequal in duration ; the sidereal day, comprising the interval of time between two successive rotations, and whose duration is 27 days, 7 hours, 43 minutes, and 11 seconds ; and the solar day, comprising the interval between two successive returns of the Sun to the same meridian, and whose duration equals that of a complete lunation — that is to saj, 29 days, 12 hours, 44 minutes, 3 seconds. The difference lJet^yeen the sidereal and solar days, as we see, is 53 hours, 51 minutes ; while the terrestrial, sidereal and solar daj's do not differ by 4 minutes (3 minutes, 56 seconds). The cause is, how- ever, the same, and it depends entirely upon this circumstance, that each of the two orbs, the Earth and the Moon, at the same time that they are turn- MOTIONS OP THE MOON. 201 iug on tlieir axes, are carried through space, and describe an arc around the Sun. But the real duration of the rotation of the Moon being more than 27 times greater than that of the terrestrial rotation, there results from the difference in the sidereal and solar days of the two bodies an iuequaUty doubly proportional. How is this motion of lunar rotation manifested to our eyes ? Do the spots iipon its disc move from one edge to the other, as is the case with the spots on the Sun, and with those on the other planets ? Not at ail ; we know, ou the contrary, that the Moon always presents the same face to the Earth, with the exception of the slight periodical oscilla- tions which present, sometimes at the north and south, sometimes at the west and east, certain re- gions of the invisible hemisphere. It would appear, then, at first sight, that the Moon does not turn on its axis, and, unlike all other celestial bodies, has no rotary movement. This is indeed what has been claimed ; it is still claimed by certain savants "" who have never accurately calcu- * An Euglish scientific review, T/;e Aslronomtcal Berjlsier, has published on this subject, iu its numbers of 1864, a series of arti- cles, in which the arguments for and against the lunar rotation have been largely developed in verse and prose. The discussion, The Moon Controversy, threatening to become interminable, the 202 WONDERS OF THE MOON. lated the geometrical conditions of the question. Doubtless, if the Moon and the Earth formed an immovable system in space, and if the first had not the motion of revolution around the second, the ro- tation of the Moon would manifest itself by a uni- form change in the position of its spots, which we should see had a parallel motion upon its disc. Without doubt, the permanency of the same face would be an evident indication of its immobility, of the absence of all movement of rotation. But, on the one hand, the Earth and Moon together move around the Sun, and, on the other hand, the Moon revolves continually around the Earth. Under these circumstances, the rotation of the Moon follows from the very fact to which it seems opposed — the permanency of the visible spots — and this permanency proves only one thing, that the time of rotation and that of revolution are the same. What is, indeed, a motion of rotation ? How can one know that a body — a Si:)here, for example — has performed an entire revolution around one of its diameters ? Evidently, when the sphere has presented succes- editor eaw the necessity of cutting short the British tenacity of the disputants. MOTIONS OF THE MOON. 203 sively one of its faces to all the points of space which smround it. Fig. iO. The motion of rotatiou of a sphere supposed to lie motionless. If we divide the complete rotation into four periods, Figure 40 shows how the sphere would ap- pear, at the beginning of each of these periods, to a motionless observer. Now, whether or not the sphere rotates about its axis in precisely the same time that it performs the motion of revolution around the motionless observer, it is not less evi- dent that the complete rotation woultl be performed if the face, of which the point A forms the appa- rent centre, is successively presented to all the re- gions of space. Now, this is exactly the case with the IMoon after it has performed a complete revolution in its orbit. The comparison of Figures 40 and 41 shows this in- disputably. We see in the second that the point A, which marks the centre of the lunar disc turned towards us at the time of full moon, takes the same positions in the successive phases as the point A, 204 WONDERS OF THE MOON. of the first figure, until the following full moon ; and this is always the central point of the disc with respect to the Earth. Fig. 41. The lunar rotation. Fig. 41 shows, also, that the complete rotation is effected before the lunation is entirely accomplished, which explains the duration of 27^ days assigned to the lunar rotation and revolution, instead of the 29.J days necessary to restore our satellite to the same place. In comparing the duration of the lunar rotation with that of the Earth, we find that the first is about 27.4 times greater than the second. This delay explains perfectly how it happens that any appreciable flatness at the poles of the Moon can- not be established, since it is the centrifugal force developed by the motion of rotation, which is the MOTIONS OF THE MOON 205 cause of the equatorial enlargement of celestial bodies. The angular velocity is the same for all the points of a globe which turns on its axis ; but the path described by each depends on its greater or less distance from the axis ; nothing at the two poles, it continues to increase as the points have less lati- tude, tiU, at the equator, it attains its greatest power. A point of the lunar equator does not travel more than 54,000 feet, or about 10§ miles an hour — that is to say, about 15 feet a second. This is a velocity 100 times less than that of a point of the terrestrial equator, which is 1,522 feet. As the velocity of the Moon in its orbit is about 3,350 feet a second, more than 200 times greater than its velocity of rotation, the result is that the Moon glides through space, according to the expression of M. Saigey, " Comme une roue enrayee par un point se deplagant lentement sur la circonference " — hke a wheel traversed by a point moving slowly upon the circumference. But it must not be forgotten, if we vnah to have a correct idea of the motion of the Moon in the heavens, that it participates at the same time in the motion which causes the revolution of the Earth around the Sun. Its true velocity is then sometimes equal to that of our planet, (about 18| miles a 206 WONDEKS OF THE MOON. second), sometimes greater, sometimes less, accord- ing to the relative directions of the two motions. The axis of rotation of the Moon is nearly per- pendicular to the plane of its orbit ; but, as this piano does not coincide with that of the terrestrial orbit, (it forms with it an angle of 5° 8',) the result is that, in consequence of the relative positions of the two bodies, we perceive sometimes the north pole and sometimes the south pole of the lunar globe. The north pole is a httle bejond a crater called Gioja ; and the south pole occupies a position very near the Dorfel Mountains, which extend a little to the east of this point. Finally, tlie mean meridian, which gives the direction of the polar axis, traverses the disc, following a line which, to the Earth, never deviates much from a straight line, leaving Tj^cho in the east, grazing the western ram- part of Ptolemseus, traversing the Sinus Medii, cut- ting the Apennines, the Alps, and the Mare Frigoris, a little distance from the dark circpe of Plato. As to the equator of the Moon, its position is determined by a diameter perpendicular to the line of the poles ; starting at the eastern edge, fi'om the crater Eiccioli, the equator traverses the Oceanus Procellarum and the Mare Nubium, leaving the ra- diant cirques of Copemicus and Kepler a little to the north, passes, like the polar axis, the Sinus MOTIONS OF THE MOON. 207 Meclii, separates the Mare TranquiUitatis from that of Nectaris, and terminates in the west in the Mare Fcecunditatis. XXIII. FOEM AND DIMENSIONS OF THE LUNAE GLOBE. INAPPRECIABLE FLATTENING — FOEM ELONGATED TO- WAEDS THE EAETH — COMPAEATIVE DIMENSIONS OF THE EARTH AND MOON — MASS AND DENSITY — WEIGHT AT THE SUEFAGE. The Moon has the form of a sphere, of which we do not perceive much more than half. This proceeds, as we have seen above, from tlie constantly circular appearance of its disc, and from the ellip- tic form of the line which separates the regions plunged in darkness from those which receive the solar light. All the diameters of the disc are of equal size, excepting the slight inequalities \ihich arise from the indentations produced on the edges by the profiles of mountains. The lunar globe is not flattened, as is our globe, at its poles, or, if it is, the flatteniug is insensible and inappreciable to us. That the Moon, like the Earth, was originally fluid, MOTIONS OF THE MOON. 209 is a very probable hypothesis, but tlie slowness of its motion of rotation explains sufficiently how it happens that it has not tho form of an ellipsoid bulging at the equator. It appears certain, however, that the lunar globe is not rigorously spherical : the attraction of the Earth has caused its elongation in the direction of the centre of our globe, so that its greatest diameter is always turned towards us. It is to this circum- stance, which is, in other respects, a result of the laws of gravitation, that Laplace attributes the per- fect equality of the two motions of the rotation and the revolution of our satellite — motions which, with- out doubt, were originally somewhat unequal. Per- haps a day will come when we can measure the dif- ferent lunar meridians with sufficient accuracy to render the elongation of which we speak sensible. It is not enough to know the form of a heavenly body. We desu'e to know, also its real dimensions. Nothing is easier when we know both its distance and the angle under which we see. We have only to solve the most simple trigonometrical problem. The radius of the Moon is about three elevenths — a little more than a quarter — of the mean radius of the Earth, or more accurately, the 0.273,125th part of that radius. The lunar diameter is 2,159 miles. 210 WOKDEES OF THE MOON. The circumference of a meridian is 6,788j miles, the length of a degree is 18j miles. Such is the distance which those who would make a tour of the lunar world would have to traverse in a straight line. I say in a straight line, because the obstacles pre- sented by the mountainous irregularities would ma- terially increase the length and duration of the journey. As for the total area, it is a little less than the thirteenth part of the surface of the Earth, (0.0746,) and comprises about 14,685,000 square miles, nearly four times the area of the European continent. This allows 7,342,500 square miles for the area of the hemisphere seen from the Earth. Indeed, owing to the motions, of which we will treat farther on, Vfe can see a little more than the half of our satel- lite, and, in calculating the area of all the visible parts, we reach the number of 8,000,000 square miles, — about 41 times the extent of France. If we i^ass from the linear and superficial dimen- sions to the volume, we find that the Moon is about 49 times smaller than the Earth. It is even then about 5,750,000,000 cubic miles. Compared to the volume of the Sun, the size of our satellite is a very small part of the immense globe from which it reflects the light to us. We should have to collect 70,000,000 of Moons to fill MOTIONS OF THE MOON. 211 the prodigious sphere, vet the discs of the two orbs appear to occupy almost equal portions of the starry vault. But, as we have seen, this arises from the great inequality iu the two distances, one of which is 400 times greater than the other. Fig. i2. The dimensions of the Moon and Earth. The numbers which we have just quoted teach us only the geometrical importance of the lunar globe ; we shall say nothing of the material of which it is composed. The telescope shows us what form this material has taken under the influ- ence of internal forces, and how it has accumu- lated, here in vast plains, there in a multitude of irregularities, hills, circular mountains, pyramidal peaks and points bearing more or less analogy to 212 WONDERS OF THE MOON. our teirestrial mountains. But what is the nature of the rocks of which these irregularities are formed, and of the level surface of the valleys and plains ? These are questions, the solution of which would be as interesting as it is in reality difficult. However, the laws of celestial mechanics furnish on this point some data. The accurate knowledge of the motion which draws the Moon around the Earth, and the certainty, since the great discovery of Newton, that it is gravitation which holds the body in its orbit, have permitted us to calculate the mass of our satellite. We have given elsewhere * an idea of the methods which belong to a calcula- tion of this kind ; we have explained how astrono- mers have been able to weigh the celestial bodies, and to measure their masses and their densities. Applied to the Moon, these methods have shown us that the mass of our satellite is the 75th part of the mass of the Earth, and, what consequently follows, that the density of the matter of which it is com- posed is equal to 653 thousandths of the mean density of our globe. Change these values into numbers expressing known quantities. The weight of the Moon amounts to nearly 86,310,040,000,000,000,000 tons of 2,000 * See tbe chapter which treats of universal gravitation, in our work, The Heavens. MOTIONS OF THE MOON 213 pounds. Its mean density, compared with tLat of ■n-ater, is 3.55 — that is, the lunar globe weighs more than three and a half times a globe of water of the same dimensions. But it must be added that the Moon is without doubt formed, like the Earth, of heterogeneous layers, the density of which increasses in going from the surface to the centre. Therefore the strata which form the soil are lighter than indi- cated by the density of 3.55. How much ? We do not know. Yet this mean density compared with that of some of the minerals of the terrestrial crust permits us to form an idea of the composition of the lunar mat- ter. The carbonate of manganese, epidote, the glass known by the name of flint, and the diamond have nearly the same specific weight as the lunar matter. The compound substance of which aerohtes are formed is perhaps more fit to furnish us with a term of comparison : and it is interesting to find that the numbers 3.57, 3.54, express the density of some of the meteorites picked up after their fall to the sur- face of the Earth. Such an identity would be pre- cisely of a nature to sanction the opinion that aero- htes are rocks thrown by the volcanoes of the Moon, if the cosmical origin of these bodies was not now well known. 214 n'ONDEKS OF THE MOON. There is still another element which should be taken into consideration when we wish to compare the physical constitution of the Moon with that of our terrestrial globe. I speak of the force of gra- vity at the surface. This intensity varies in differ- ent celestial bodies, being greater in proportion as the total mass is larger, but, at the same time, less as the radius of the orb is greater, or, which amounts to the same thing, as the surface of the body is farther removed from the centre. In apply- ing these principles to the Moon, we arrive at this result, that the weight at its surface is between |- and i of that which bodies at the terrestrial surface weigh. If then we imagine a man transported to our satellite, if we suppose, moreover, that his mus- cular force remains the same in his new abode, he can there raise, without additional eflbrt, weights five or six times heavier, and his own body would appear five and a half times lighter. We have seen above what important results we can gather from this fundamental truth, when we consider the forces which could raise the masses of rocks which form the lunar mountains, to heights relatively so immense. XXIV. IS THE MOON THE ONLY SATELLITE OF THE EARTH? The Moon, we have said at the beginning of this work, is the celestial body nearest to us. Is this as- sertion quite true ? It is what astronomers have long beheved, and, indeed, aU the planets known to oui solar system, all the bodies which have the Sun for the centre or focus of their motions, and with still greater reason, all those luminous points with which the celestial vault sjjarkles, are at incompar- ably greater distance. Venus at its least distance from the Earth is yet more than 21,000,000 miles away — that is to say, a hundred times farther re- moved than the Moon. Mars does not come nearer than 3.3,000,000 of miles, and this occurs very rarely ■ — a distance 145 times gTeater than that of the Moon. These are the planets nearest to the Earth. But besides the celestial bodies individually known 216 WONDEKS OF THE MOON. wliose orbits have been calculated, there are in our solar sj'stem immense numbers of small bodies ■which move around the Sun, at a distance from it which is not far from that of the Earth. These are such as appear on pleasant nights in the form of luminous trains, of sparkling globes : falling slews and holides. Their orbits seem to pass near that of the Earth, sometimes crossing it. When the meeting occurs they either only graze the outer regions of the at- mosphere, take fire by their contact and continue their course ; or, attracted by the mass of our globe, they fall upon its surface : such are the stones known by the name of meteorites or aerohtes. In this way there are numerous small bodies which periodically approach us much nearer than the Moon does. But what means have we of recog- nizing them in such an irregular crowd ? Some astronomers, among whom we must cite M. Faye, believe that a certain number of falling stars, those which are seen hero and there on every clear night of the year, are so man}' satellites of the Earth, which are drawn away, so to speak, by the Earth from the more crowded hosts which revolve in troops around the Sun. Is this hypothesis based upon jjositive observations apart from mere probability ? MOTIONS OF THE MOON. 217 The following fact shows it. A French astrono- mer, M. Petit, of the Toulouse observatory, has cal- culated the orbit of a bolide of which he has been able to obtain a sufficient number of data. This singular satellite of the Earth, this compan- ion of the Moon, revolves around us in a period which W'Ould not exceed 3 hours and 20 minutes, and its distance from the centre of our globe would be at least 9,000,000 miles.* In such a case the Moon would not be the only companion of the Earth in its journey through the ethereal regions ; and we would have very near to us, at all events much nearer to us than the lunar globe, miniature moons whose illuminated faces would be visible to us whenever they were not eclipsed by the cone of the terrestrial shadow, which would occur quite rarely, since their orbits would not be very much inclined to the orbit of the Earth. * I liave omitted a passage in the origia)iL If sao'a a body is recognized by astronomers I do not know it. — Ed. CHAPTER VI. INFLUENCES OF THE MOON, XXY. OCEANIC, ATMOSPHEEIC AND SUB- TEREANEAN TIDES. One could fill a large volume with all the nonsense which has been spoken in connection with the Moon and its pretended influences upon our planet and its inhabitants. A still larger book could perhaps be written if one wished to mention all the super- stitions of this kind which are current to-day, even among civilized people as well as among the bar- barous tribes of which the most unenhghtened nations are composed. Most of these, more or less absurd, have had the .honor of a serious discussion, and we will not revert INFLUENCES OF THE MOON. 219 to them. Arago, that generous scientist, so dis- posed to collect popular traditions, not for the pur- pose of admitting them into science, but in order to separate those that might contain any truth, and interpret their real meaning, has devoted several chapters to examining the different influences attri- buted to the Moon. He has shown beyond a doubt that some have no foundation and no probability, that others are exceedingly weak, and bear no pro- jiortion to the magnitude of the results attributed to them, although they have been supposed to explain them. Let us enter into details concerning the only in- fluences which are scientifically established. The Moon, by its mass, attracts the Earth. The phenomena which are the result of this incessant action are the tides. All the particles of matter which together form the lunar globe, attract at the same time all the par- ticles composing the terrestrial spheroid, and thus counterbalance 'in a certain measure their actual weight. But the amount of this attraction is not the same for all particles : it depends both on theu' distance from the centre of the attracting mass, and on the angle which the direction of the force makes with the direction of the terrestrial gravity. It is easily understood that the points situated 220 WONDEES OF THE MOON. vertically imder the Moon are those whoso gravity is most diminished, and that this decrease lessens in proportion to the distance of the particles from that position on the whole surface of the terrestrial hemisphere which is turned towards our satellite. There is no change of the solid portion of the globe, but it is not so with the fluid portion, that is to say, with the waters of the ocean nor with the strata of the atmosphere. By reason of their fluidity and their freedom of motion, the liquid and gaseous par- ticles rise under the influence of the lunar attraction, and the liquid sheet of the ocean is extended and heaped up on the side of the Earth towards the Moon. (Fig. 46.) Instead of preserving its form, which is almost spherical, it takes the form of an egg, with all the proportions perfect.-' It is the same with the gaseous sheet which en- circles the Earth. It is this elevation of the fluid portions which has received the name of tide. If the Earth had not its own particular motion, the tide would be permanent, and the waters would preserve their ec[uilibrium, which purely meteorolo- gical influences alone would disturb. But the Earth in turning presents to the Moon its whole periphery, and the maritime wave thus moves * Prolate sphoroid. INFLUENCES OF THE MOON. 221 ou tlie ocean, following the parallel which corres- ponds to the position of our satellite. Ou the opposite hemisphere the same phenomena take place simultaneously ; the liquid sheet is lengthened Fui. 43. Attraction of the Moon for the waters of the eea. in opposition to the Moon, the most distant, and in consequence the least attracted particles are left behind, so that a similar effect is produced by con- trary circumstances. A'lZ ^YO}^DEKS OF THE MOON. This is not the place to describe all the periods, diurnal, luoutbly, yearly periods, which the pheno- mena of the tides present, the multiple causes of which depend upon the simultaneous motions of the Moon and Earth and upon the action of the mass of the Sun."' Bat, if you desire to know what the intensity of tliis force is which produces upon a mass, as large as that of the ocean waters, changes as violent as those of the great tides, j'ou will be surj^rised undoubtedly to learn that it does not dimini.sh the weight of the bodies on the surface of the Earth more than a six- teen millionth part. Thus a body which weighs 36 pounds exerts a less pressure when the Moon is passing the zenith than when the body is in the horizon, but how much? At most 0.0151 of a grain. This number enables us to form an idea of what the most insignificant force might become wdien it is re- peated and is incorporated in a mass as large as that of the waters of the sea, and accumulates inces- santly at each moment of its duration. But to show all that an action of this kind has the power of producing on the terrestrial globe, it is not by daj's nor by years, but by centuries and thousands of centuries that one must count. •Refer ou this subject to the ehapter devoted to tides, in our iistronomieal worlc ou Tlie Heavens. INFLUENCES OF THE MOON. 223 Thus we can understand how the structure of con- tments and the outhne of coasts have been slowly but irresistibly modified by this many-headed battering ram, which rushes twice a day with pitiless force upon the downs and the cliffs. "The Moon," says Humboldt, "by reason of the attraction which it exercises in common with the Sun, sets the ocean in motion, displaces the liquid element on the Earth, and by the periodical risings of the sea and the destructiTe efiect of the tides, changes Httle by little the outline of the coasts, helps or hinders man's work, and furnishes the greater part of the materials of which the sandstone and conglomerates are comj^osed, covered in their turn by loose and rounded fragments of rocks. The Moon acts incessantly as a source of change upon the geological conditions of our planet. The Moon, we have said, not only acts by its mass upon the motions of the sea, but also dis- places periodically the gaseous strata of which the terrestrial atmosphere is composed. From this we have the atmospheric tides. " In order to reach the ocean," says Laplace, " the action of the Sun and Moon passes through tlie atmosphere, which should in consequence feel their influence and be sulijectL'd to motions similar to those of the sea. "WlieiiC'.! '.'L'sult some periodical 224 WONDERS OF THE MOON. variations in the height of the barometer, and some winds, the direction and intensity of which are pe- riodical. These winds are inconsiderable, and, indeed, almost imperceptible in an atmosphere al- ready greatly agitated. The extent of the fluctu- ations of the barometer is not 0.04 of an inch even at the equator, where it is the greatest. At Paris, ob- servations of eight years have confirmed the theore- tical deductions of the great geometrician, and have proved that the action of the Moon on the atmos- phere does not cause the barometer to fluctuate more than the 2,000th part of an inch. What is to be said, after these results, concerning the ridiculous theories of our almanac-makers, who, devoid of all knowledge of the mechanism of fluids, lay to the account of the lunar attraction the most violent atmospheric perturbations, and variations of temperature from 20 to 25 degrees. According to some savants the Moon produces, not only oceanic and atmospheric tides, but, also, subterranean tides. The nucleus of the Earth be- ing, in all probability, fluid, would periodically be raisedj being excited by the lunar attraction, and this mass of great density having struck the exterior solid crust Avould be the cause of the greater jjart of the earthquakes. Statistical researches have been made for the purpose of proving the accuracy of INFLUENCES OF THE MOON. 225 this theory, and their author, M. Perrey, of the uni- versity of Dijon, thought to find in the frequency of seismic phenomena a periodicity which should correspond with the hmar periods of motion. Opinions differ greatly on this subject. Accord- ing to M. Babiuet, " the attracting force of the Moon would not produce much more effect than the weight of a stratum one foot in thickness," which is to say that this effect is entirely insignifi- cant. Such was the way in which Poisson looked at it, although he did not deny the action of the JM.oon or the existence of subterranean tides. All the preceding has referred merely to the in- fluence of the Moon considei-ed as a mass. We have considered before what effect its light and heat could have upon the Earth. They have a power, but it acts within such narrow limits that it hardly explains the popular prejudices on the phases, with which it is necessarily connected. At the time of the new moon the lunar globe sends us neither rays of light nor caloric rays. The full moon, on the contrary, corresponds to the maximum effects of this nature. Between these two periods the influ- ence increases or diminishes by insensible grada- tions, in such a manner that we cannot understand how sudden changes can be ascribed to it. Before looking elsewhere for the cause of these changes, it 226 WONDERS OF THE MOON. would be well for observation to establisli them, which has not yet been clone. Does there exist between the Moon and the Earth some hidden mysterious bond of a magnetic nature for instance ? Nothing proves or contradicts such a hypothesis, and it is possible that study in this direction might lead to interesting results, bat we do not know that such has been undertaken. This is all that can be said as positively known concerning the influence of the Moon and its con- nection with our planet. Beyond this we should pass into the realms of fancy, of mysticism or of ignorance, which would lead us far away. XXVI. A LAST WOED ON THE MOON. THE ANTIQUITY OF THE MOON— BOOKS THROWN OUT BY THE LUNAR VOLCANOES — WHY THE MOON HAS NO ATMOSPHERE — THE MOON DRAWING NEAR TO THE EARTH — WILL IT EVER FALL TO IT? — SUPPOSE THE MOON SHOULD ABANDON US. In reviewing this work on the satellite of the Earth, we percen e that the reader will not here find the answer to many of the questions which we have heard asked, or which wo have read in old and new works. We shall not excuse ourselves hj saying that it was for want of space ; it was due, rather, to the want of an occasion, and, in certain cases, to the resolution taken to say nothing. Concerning some of these problems, however, we have decided to say, without arrangement, a few words. 228 WONDERS OP THE MOON. And first, is the Moon as old as tlie Eartli ? Is there any authority in the tradition of the ancient Arcadians, wlio believed their ancestors older than the Moon ? Writers who have not known more than others on this subject, do not hesitate to say that there is. From this we have the hypothesis that the Moon made its appearance in the form of a comet, which began by causing agitations, and geo- logical revolutions upon our globe — the dekige, for instance, or rather, the deluges — and which finally settled quietly into a regular orbit. All this is purely fancy, put together for the purpose of ex- plaining a fable, and having only the value of a fable. That which is known of the cometic constitution, of the slight mass of these gaseous agglomerations, of the immense condensation necessary to transform a comet into a globe of the volume and mass of the Moon, of the boundless lapse of time which such a condensation would render necessary, does not per- mit us to believe for an instant that all these events, besides being mere suijpositions, date from an epoch so little removed from our time that they are within the memory of man. How much more philosophical and probable is the theory of Laplace, which assigns to all the bodies of the solar system, to planets and to satel- INFLUENCES OF THE MOON. 229 lites, to the Earth and to the Moon, a gaseous origin ! It is then not by thousands but bj- millions of years that one must count, in order to measure the periods necessary to such transformations. Another question is, why the Moon, if of gas- eous origin, liad not the power of preserving its atmosphere ? Could it have been deprived of this atmosphere by the Earth ? We could answer that the gases of which the lunar atmosphere were com- posed are chemically hxed, and were absorbed in order to produce its solid crust, the oxidizing power of the substances being considerably increased by the high temperature to which each one of the lunar hemispheres is periodically exposed. Perhaps even the slight weight of the atmosphere, or rather, of the gases which formed it, enabled it to spread itself to such a distance that it was in part dissipated in space, or, in any case, remained in a state of extreme rarity. This is a hypothesis which Laplace has admit- ted as very probable. This slight weight reminds us of the stones which are said to be hurled from the volcanoes of the jIoou, and which we now know to be a quantity ol very small bodies, forming planetary rings, which revolve together around the Sun. The hypothesis 230 WONDERS OP THE MOON wliich has been made is exceedingly improbable. Calculation proves that if we take a point on the line which joins the centres of the Moon and Earth, 217,940 miles distant from the latter, which distance is not more than 20,000 miles from the surface of the Moon, this point marks the respective limits of attraction of the two bodies. According to Laplace, a lunar projectile, having an initial velocity of 8,200 feet a second, would be enabled to reach or pass the point of which we speak. Then either the direction of its primitive impetus would jDush it into the terrestrial atmosphere, or else, without falling ujDon our Earth, it would become a satellite. Per- haps this is the origin of some of the meteors of which we have spoken above, an origin which dates from a period at which the lunar volcanoes were at their maximum intensity. Let us say, however, that the great velocity of aerolites, such as have been ob- served in a large number of cases, does not seem to be reconcilable with this origin unless the projective force of the lunar volcanoes has been greater than that which we have supposed. The idea of a communication of this nature be- tween the Eartli and the Moon is very old ; it is from this, without doubt, that we have the fable of the lion of Nemeus and its fall into the midst of Peloponnesus. INFLUENCES OF THE MOON. 231 The Moon is approacliing the Earth. This is another of those facts which excite the imaginations of the lovers of the marvellous. If this diminution of distance continues indefimitelj, they say, the Earth is condemned to be destroj'ed some day by the fall of its satellite, crushed in pieces and dis- persed in space. What period must we assign to this terrible event, which would, without doubt, be the end of the universe, or of the solar world, or, at least, the end of our little world ? Unfortunately for those who, in the midst of present security, en- joy setting forth the catastrophes of the future, and for those who are eager to take advantage of such prophecies for the benefit of their prejudices, it is not possible to anticipate such an event in human affairs. The motion of the Moon is indeed accelerated ; ancient observations compared with later ones have proved it. Consequently, it approaches the Earth ; but, not only is this Bearing insensible, but it will also have a limit : in about 25,000 years from this time, this acceleration will cease, and an inverse motion will begin. The cause of this oscillation is understood, and it is known that it is held within narrow limits. Let us reassure ourselves, then, both for our- selves and for our descendants : the Moon will never 232 WONDERS OF THE MOON. fall upon our Earth, to the great disappointment of our hypotheses makers. We return to our task, and inquire : "What would become of us, if, for some reason or other, the Moon should abandon us ? We now enter the domain of fancy, but what matter ! Let us always try to reply, and perhaps this will teach us something. And first, it is clear that our nights would lose ■variety, that eclipses would be unknown to us, and that we would need to find another method of count- ing some intervals of time. But aU that is nothing. The most noticeable changes would be a great abatement in the phenomena of tides, since the in- fluence of the Sun does not amount to more than a third in the periodic oscillations of the ocean. Finally, it would cause inevitable modifications in the outline of the maritime coasts, which the motion of the tides insensibly transforms. This is not all ; the gravity of the Moon is felt in another way on the Earth. Our globe is enlarged at the equator, as if covered by a shell or ring, growing thinner and thinner on every side to the poles. Well, it is the action of the Moon upon this enlargement which produces the fluctuation of the axis of the Earth known as Nutation. This motion would be destroyed if the Moon disappeared, and the variations of the equinoxes and of the obliquity rNTLUENCES OF THE MOON. 233 of the ecliptic -would soon be reduced to tliose alone whicli are due to the influence of the Sun. Finallj', the motion of the Earth around the Sim would also be slightly modified, since it is the cen- tre of gravity common to the Earth and the Moon which impels it to move in an ellipse, having the Sun for a focus. The centre of gravity, which really is in the in- terior of our globe, at about 750 miles below the surface, would find itself transported to the centre of our spheroid, but the distance from the Earth, and the dimensions of its orbit would not appear to be altered until after long periods. Enough has been said of a mere conjecture, and we take leave of our readers in begging them to meditate on this great thought, which comes forth so clearly from the study of the laws which govern nature ; it is that she acts only by measure and gra- dation ; that violent and sudden changes are repug- nant to. her, and, as has been said, the cycles, in their slow but u'resistible evolutions, are only the seconds of Eternity. THE END, APPENDIX. NOTES. Page bd.— On the calorific effect of the Moon's rays on the surface of the Earth. The question of the iniiueiice of the lunar rays upon ther- raometric instruments has been taken up of late by various men of science. The son of Lord Eosse, using a reflector ■with an aperture of three feet and converging the rays upon a thermo-electric pile, has shown that the Moon radiates with a power equal to a surface heated to 360° Fahrenheit. The intensity of the radiation, however, is proportionate to the illuminated surface of the disc. M. W. Huggins, with an eight inch refractor, has made observations, the different results of which prevent any definite conclusion. But M. Marie-Davy objects, and with reason, that the lenses of the instrument " almost completely arrested the dai'k heat-rays of the Moon, while the mirror of Lord Bosae reflected them as well as the luminous rays." M. Mari^-Davy began, in 1869, a series of observations, by the aid of which he proposes not only to ascertain and measure the oaloriflo power of the lunar rays, but also to to solve the foUcWing questions : 1. What is the proportion 236 APPENDIX. of the Moon's diffasive powers in lunar heat ? 2. What is the proportion of its absorbent and radiating power, and what are the hmits of its variation of temperature in the course of a lunation ? 3. How much the diffusive and ra- diating powers vary on different parts of the lunar surface ? 4. What deductions may be drawn concerning the Moon's surface compared to that of the Earth. The first series of observations has given M. Marie-Davy twelve millionths of a degree centigrade (about 21,500,000ths Fahrenheit) for the direct action of the Moon's luminous rays. "It is," he says, "a sixtieth part of the result ob- tained by M. Piazzi Smyth on the Peak of Teneriffe, and experimenting with all the lunar rays." M. Marie-Davy re- ceived on his instrument only about three fourths of these rays. M. Bailie likewise obtained, in 1869, a positive result, from which he concludes that "the full moon, at Paris, dur- ing the summer months throws off as much heat as a smooth black surface maintained at 212° Fahrenheit, at a distance of about 115 feet." Page 54. — On the ashy light. Schrceter has observed the ashy light three days after the first quarter, using a telescope that magnified 160 times. Page 142. — On volcanic activity upon the Moon's surface. To the list of lunar mountains where signs of present vol- canic activity are believed to have been recognized, must be added a small crater situated in the middle of the Mare Se- renitatis. This crater, which is found indicated on old maps of the Moon, ia very distinctly figured on the map of Beer APPENDIX. 237 and MajcUer, and is known under the name of Linne, was clearly visible even during the full moon. Mr. J. Schmidt, director of the observatory at Athens, who had observed it since the year 1844, was greatly astonished when, in Octo- ber, ISGG, he noticed its disappearance. Different astrono- mers, advised by Mr. Schmidt of this singular change, di- rected their attention to this portion of the Moon's disc. By the aid of powerful instruments Messrs. Secchi, Wolf and Huggins recognized that instead of a circular mountain with well-defined borders, as it was represented on the map of Beer and Mjedler, there only remained a whitish spot or aiu'eola surrounding a black cavity, indicating the pre- sence of a crater, but a crater much smaller than that known under the name Linne. The edges, instead of pro- jecting above the surrounding plain, appeared now to pre- sent nothing more than an insensible declivity. By refer- ring to former observations and anterior drawings, it seems probable that the recent condition of the crater of Linna had already presented itself in former ages ; thus everything would lead as to believe in the reality of successive erup- tion.?, which at various times have caused a partial filhng up of the interior cavity of the crater, and, overflowing its exterior, have leveUed the surface about its walls. It would result, then, from these interesting observations, that, us- ing the expression of M. Elie de Beaumont, " geologic life still exists ixt the interior of the Moon as well as in- the inte- rior of the Earth." Page 195. — Buraiion of a Itypothetical falling of the Moon to the Earth. In estimating the time it would take our satellite to faU to 238 APPENDIX. the Earth — which we have given as six and one fourth days — we had, to abridge the calculation, considered the accelerating force as remaining constant. In reality, it is continually increasing as the distance diminishes, according to the Newtonian law of gravitation. From this it results that the fall of the Moon would be much more rapid. M. riammarion, to whom we are indebted for this observation, finds the time required to be only eighteen hours, employ- ing the complete formula given by Poisson. Page 225. — Influence of (he Moon on the time of the Earth's rotalio7i. The attractive force of the Moon's mass acting on the waters of the ocean raises them, we have stated, in such a manner as to give their total mass a constant ellipsoidal form, the principal axis of which would continually follow the radius vector that unites the centres of the Moon and the Earth, if the movement of the water encountered no resistance. As the liquid ellipsoid turns to the Moon in proportion to the movement of the Earth's daily rotation, this continual displacement of water cannot be accomplished without the results of friction and resistance arising from the liquid molecules themselves, and from inequalities of all kinds presented by the surface upon which the seas repose. Hence a retardation, which daOy observations of the tides prove, and in consequence of which high tides never occur till a little while after the Moon passes the meridian. Setting aside local iiTegularities, the effect is as if the Moon were situated in the heavens a little back of the place it really occuiaies, Iiaving reference to its daily movement. We have, then, a fact the explanation of APPENDIX. 239 Tvliioh is easy, but which borrows nothing from hypothe- sis. Tlie two hquid i^rotuberances, the successive move- ment of wliich produces tlie tides, are not in the direction of the radius vector of tlie j\Ioon, but follow a line con- stantly situated to the east of that satellite. This action of the Moon — not now on the mass of our globe, supposed to be spherical, but on these two protuber- ances, at unequal distances from that body — according to M. Delaunay produces a retardation of the Earth's rota- tion. "If we refer to the mode of lunar action which causes the phenomenon of the tides, it will lie seen that the first of these protuberances (that nearest the Moon) is, as it were, attracted by the Moon ; while the second, on the con- trary, is repulsed, as it were, by that body. From this there results a double force,* applied to the mass of the ter- restrial globe wliich tends to make it turn in a contrary direction to that in which it really turns — a force which must hence produce a retardation of the rotation of our planet." Such, in substance, is the new theory. We shall not follow M. Delaunay in the provisory calculations by means of which he shows that this action of the Moon on the tidal protuberances is far from being insensible, and may, indeed, e-xjilain the excess of secular acceleration given by ancient observations. It sufEces to admit that the mass of water formed by each protuberance is equivalent to a stratum forty inches in thickness, resting on a circular basis, with a radius of 420 miles. Such a stratum applied * Couple is the word used by M. Delaunay. Thin word is applied, la French, iu the science of Mechanics, to any system of two ecpiai forces, parallel and contrary, acting at the extremities of the same rii;ht line. 240 APPENDIX. to the surface of the Earth would have a diameter of about twelve degrees at the equatoi, In truth, the learned astronomer has, to effect this calcu- lation, assumed a hypothesis much simpler than that of reality ; hut it cannot be doubted that the actual effect produced by the Moon's action on the waters of the ocean is sufficient to cause the necessary retardation. M. Delaunay's views have undergone, of course, various more or less well-founded criticisms ; but none controvert the principle that serves as their basis. The supposition that the movement of the tides is of a nature to retard the Earth's rotation is not a new one. The originator of the dynamic theory of heat, Dr. Mayer, exju'ossed it in one of his works, and Tyndall has reproduced it since. But what constitutes the merit and originality of M. Delaunay's labor is, to use his own expressions, to have shown, 1st, that the retardation due to this cause is far from being insensible ; 2nd, that therein lies the explanation of the discrepancy existing between the observation of ancient echpses and the theory of gravitation, relative to the secular acceleration of the mean motion of the Moon. Finallj', the duration of the sidereal day is not invari- able if the theory of M. Dulauuay — so entirely sanctioned by the distinguished director of the Green^ndch obser- vatory, Mr. Airy — is adopted. This duration diminishes, in the lapse of time, about one second in a hundred thousand years. If this diminution preserved the same rate indefinitely, it would be easy to calculate the time when the motion of ro- tation would be enthely overcome, and when the length of the day would be confounded with that of the year. The APPENDIX. 241 sidereal day, cousistiag of 83,400 seconds, it would require 8,6i0,000,000 years to produce a complete stoppage of the Earth's rotation. Eighty-six raillioa four hundred thousand centuries ! Truly, between this time and then we and our gTeat-great-grandsous may rest easy. But, to tell the truth, things will not come to that pass, and for this reason : the velocity of the Earth's rotation, con- stantly diminishing, will come to be equal to that of the Moon in its orbit, so that the Earth will always present the same hemisphere to its satellite, exactly as the latter now does to the Earth. But then the tidal protuberances will no longer have a progressive movement ; they will at last, then, be constantly in the direction of the Moon, and the power of the latter will cease to act as a retarding force. M. Delaunay likewise reassures us in anotlier way. As the temperature of the Earth is constantly becoming less in consequence of the diminution of the Sun's heat, and the excess of the Earth's radiation over the heat received, the time wOl arrive when the temperature will be so low that all the seas will become frozen. " The phenomenon of the tides will no longer exist, the cause of the retardation of rotation will disappear, and the Earth Vfill then continue to move with a constant velocity." k NEW AND VALUABLE SERIES For Readers of all Ages and for the School and Family Library. ©IJp IKIusfpahb Hiftrarg OF TRAVEL, EXPLORATION. AND ADVENTURE. EDITED BY BAYA RD TAY LOR. The extraordinary popularity of the Illustrated Library of Wonders (nearly «wm and a half viillion copies having been sold in this country and in France) is considered by the publishers a sufficient guarantee of the success of an Illustrated Library of Travel, Exploration, and Adventure, embracing the same decidedly interesting and permanently valu.-ible features. 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