Part 10

In its endless journey through space, our globe is not solitary, like some of the planets, but is attended by the Moon, our nearest celestial neighbor. Although the Moon does not attain to the dignity of a planet, and remains a secondary body in the solar system, yet, owing to its proximity to our globe, and to the great influence it exerts upon it by its powerful attraction, it is to us one of the most important celestial bodies.

While the Moon accompanies the Earth around the Sun, it also revolves around the Earth at a mean distance of 238,800 miles. For a celestial distance this is only a trifling one; the Earth in advancing on its orbit travels over such a distance in less than four hours. A cannon ball would reach our satellite in nine days; and a telegraphic dispatch would be transmitted there in 1½ seconds of time, if a wire could be stretched between us and the Moon.

Owing to the ellipticity of the Moon's orbit, its distance from the Earth varies considerably, our satellite being sometimes 38,000 miles nearer to us than it is at other times. These changes in the distance of the Moon occasion corresponding changes from 29' to 33' in its apparent diameter. The real diameter of the Moon is 2,160 miles, or a little over one-quarter the diameter of our globe; our satellite being 49 times smaller than the Earth.

The mean density of the materials composing the Moon is only ⁶⁄₁₀ that of the materials composing the Earth, and the force of gravitation at the surface of our satellite is six times less than it is at the surface of our globe. If a person weighing 150 lbs. on our Earth could be transported to the Moon, his weight there would be only 25 lbs.

The Moon revolves around the Earth in about 27⅓ days, with a mean velocity of one mile per second, the revolution constituting its sidereal period. If the Earth were motionless, the lunar month would be equal to the sidereal period; but owing to its motion in space, the Sun appears to move with the Moon, though more slowly, so that after having accomplished one complete revolution, our satellite has yet to advance 2¼ days before reaching the same apparent position in regard to the Earth and the Sun that it had at first. The interval of time comprised between two successive New Moons, which is a little over 29½ days, constitutes the synodical period of the Moon, or the lunar month.

The Moon is not a self-luminous body, but, like the Earth and the planets, it reflects the light which it receives from the Sun, and so appears luminous. That such is the case is sufficiently demonstrated by the phases exhibited by our satellite in the course of the lunar month. Every one is familiar with these phases, which are a consequence of the motion of the Moon around the Earth. When our satellite is situated between us and the Sun, it is New Moon; since we cannot see its illuminated side, which is then turned away from us towards the Sun. When, on the contrary, it reaches that point of its orbit which, in regard to us, is opposite to the Sun's place, it is Full Moon; since from the Earth we can only see the fully illuminated side of our satellite. Again, when the Moon arrives at either of the two opposite points of its orbit, the direction of which from the Earth is at right angles with that of the Sun, it is either the First or the Last Quarter; since in these positions we can only see one-half of its illuminated disk.

The curve described by the Moon around the Earth lies approximately in a plane, this plane being inclined about 5° to the ecliptic. Since our satellite, in its motion around us and the Sun, closely follows the ecliptic, which is inclined 23½° to the equator, it results that when this plane is respectively high or low in the sky, the moon is also high or low when crossing the meridian of the observer. In winter that part of the ecliptic occupied by the Sun is below the equator, and, consequently, the New Moons occurring in that season are low in the sky, since at New Moon our satellite must be on the same side of the ecliptic with the Sun. But the Full Moons in the same season are necessarily high in the sky, since a Full Moon can only occur when our satellite is on the opposite side of the ecliptic from the Sun, in which position it is, of course, as many degrees above the equator as the Sun is below. The Full Moon which happens nearest to the autumnal equinox is commonly called the Harvest Moon, from the fact that, after full, its delays in rising on successive evenings are very brief and therefore favorable for the harvest work in the evening. The same phenomenon occurs in every other lunar month, but not sufficiently near the time of Full Moon to be noticeable. When, in spring, a day or two after New Moon, our satellite begins to show its thin crescent, its position on the ecliptic is north as well as east of that occupied by the Sun; hence, its horns are nearly upright in direction, and give it a crude resemblance to a tipping bowl, from which many people who are unaware of its cause, and that this happens every year, draw conclusions as to the amount of rain to be expected.

One of the most remarkable features of the Moon's motions is that our satellite rotates on its axis in exactly the same period of time occupied by its revolution around the Earth, from which it results that the Moon always presents to us the same face. To explain this peculiarity, astronomers have supposed that the figure of our satellite is not perfectly spherical, but elongated, so that the attraction of the Earth, acting more powerfully upon its nearest portions, always keeps them turned toward us, as if the Moon were united to our globe by a string. It is not exactly true, however, that the Moon always presents its same side to us, although its period of rotation exactly equals that of its revolution; since in consequence of the inclination of its axis of rotation to its orbit, combined with the irregularities of its orbital motion about us, apparent oscillations in latitude and in longitude, called librations, are created, from which it results that nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at one time or another.

The Moon is a familiar object, and every one is aware that our satellite, especially when it is fully illuminated, presents a variety of bright and dark markings, which, from their distant resemblance to a human face, are popularly known as "the man in the moon." A day or two after New Moon, when the thin crescent of our satellite is visible above the western horizon after sunset, the dark portion of its disk is plainly visible, and appears of a pale, ashy gray color, although not directly illuminated by the Sun. This phenomenon is due to the Earth-shine, or to that portion of solar light which the illuminated surface of our globe reflects to the dark side of the Moon, exactly in the same manner that the Moon-shine, on our Earth, is due to the solar light reflected to our globe by the illuminated Moon.

Seen with a telescope of moderate power, or even with a good opera-glass, the Moon presents a peculiar mottled appearance, and has a strong resemblance to a globe made of plaster of Paris, on the surface of which numerous roundish, saucer-shaped cavities of various sizes are scattered at random. This mottled structure is better seen along the boundary line called the terminator, which divides the illuminated from the dark side of the Moon. The line of the terminator always appears jagged, and it is very easy to recognize that this irregularity is due to the uneven and rugged structure of the surface of our satellite.

A glance at the Moon through a larger telescope shows that the bright spots recognized with the naked eye belong to very uneven and mountainous regions of our satellite, while the dark ones belong to comparatively smooth, low surfaces, comparable to those forming the great steppes and plains of the Earth. When examined with sufficient magnifying power, the white, rugged districts of the Moon appear covered over by numerous elevated craggy plateaus, mountain-chains, and deep ravines; by steep cliffs and ridges; by peaks of great height and cavities of great depth. This rugged formation, which is undoubtedly of volcanic origin, gives our satellite a desolate and barren appearance. The rugged tract occupies more than one-half of the visible surface of the Moon, forming several distinct masses, the principal of which occupy the south and south-western part of the disk. That this formation is elevated above the general level is proved by the fact that the mountains, peaks, and other objects which compose it, all cast a shadow opposite to the Sun; and further, that the length of these shadows diminishes with the elevation of the Sun above the lunar horizon.

Since Galileo's time the surface of the Moon has been studied by a host of astronomers, and accurate maps of its topographical configuration have been made, and names given to all features of any prominence. It may even be said that in its general features, the visible surface of our satellite is now better known to us than is the surface of our own Earth.

One of the most striking and common features of the mountainous districts of the Moon, is the circular, ring-like disposition of their elevated parts, which form numerous crater-like objects of different sizes and depths. Many thousands of crater-like objects are visible on the Moon through a good telescope, and, considering how numerous the small ones are, there is, perhaps, no great exaggeration in fixing their number at 50,000, as has been done by some astronomers. These volcanic regions of the Moon cannot be compared to anything we know, and far surpass in extent those of our globe. The number and size of the craters of our most important volcanic regions in Europe, in Asia, in North and South America, in Java, in Sumatra, and Borneo, are insignificant when compared with those of the Moon. The largest known craters on the Earth give only a faint idea of the magnitude of some of the lunar craters. The great crater Haleakala, in the Sandwich Islands, probably the largest of the terrestrial volcanoes, has a circumference of thirty miles, or a diameter of a little less than ten miles. Some of the great lunar craters, called walled plains, such as Hipparchus, Ptolemæus, etc., have a diameter more than ten times larger than that of Haleakala, that of the first being 115 miles and that of the last 100 miles. These are, of course, among the largest of the craters of the Moon, although there are on our satellite a great number of craters above ten miles in diameter.

The crater-forms of the Moon have evidently appeared at different periods of time, since small craters are frequently found on the walls of larger ones; and, indeed, still smaller craters are not rarely seen on the walls of these last. The walls of the lunar craters are usually quite elevated above the surrounding surface, some of them attaining considerable elevations, especially at some points, which form peaks of great height. Newton, the loftiest of all, rises at one point to the height of 23,000 feet, while many others range from ten to twenty thousand feet in height. Several craters have their floor above the general surface--Plato, for instance. Wargentin has its floor nearly on a level with the summit of its walls, showing that at some period of its history liquid lavas, ejected from within, have filled it to the brim and then solidified. The floors of some of the craters are smooth and flat, but in general they are occupied by peaks and abrupt mountainous masses, which usually form the centre. Many of their outside walls are partly or wholly covered by numerous ravines and gullies, winding down their steep declivities, branching out and sometimes extending to great distances from their base. It would seem that these great volcanic mouths have at some time poured out torrents of lavas, which, in their descent, carved their passage by the deep gullies now visible. Sometimes, also, the crater slopes are strewn with debris, giving them a peculiar volcanic appearance.

Notwithstanding their many points of similarity with the volcanoes of the Earth, the lunar craters differ from them in many particulars, showing that volcanic forces acting on different globes may produce widely different results. For example, the floors of terrestrial craters are usually situated at considerable elevations above the general surface, while those of the lunar craters are generally much depressed, the height of their walls being only about one-half the depth of their cavities. Again, while on the Earth the mass of the volcanic cones far exceeds the capacity of their openings, on the Moon it is not rare to see the capacity of the crater cavities exceeding the mass of the surrounding walls. On the Earth, the volcanic cones and mouths are comparatively regular and smooth, and are generally due to the accumulation of the ashes and the debris of all kinds which are ejected from the volcanic mouths. On the Moon, very few craters show this character, and for the most part their walls have a very different structure, being irregular, very rugged, and composed of a succession of concentric ridges, rising at many points to great elevations, and forming peaks of stupendous height. Again, many of the larger terrestrial craters have their interior occupied by a central cone, or several such cones, having a volcanic mouth on their summits; on the Moon such central cones are very rare. Although many of the large lunar craters have their interior occupied by central masses which have been often compared to the central cones of our great volcanoes, yet these objects have a very different character and origin. For the most part, they are mountainous masses of different forms--having very rarely any craters on them--and seem to have resulted from the crowding and lifting up of the crater floor by the phenomena of subsidence, of which these craters show abundant signs. Besides, the terrestrial craters are characterized by large and important lava streams, while on the Moon the traces of such phenomena are quite rare, and when they are shown, they generally differ from those of the Earth by their numerous and complicated ramifications, and also by the fact that many of these lava streamlets take their origin at a considerable distance from the crater slopes, and are grooved and depressed as if the burning liquids which are supposed to have produced them had subsequently disappeared, by evaporation or otherwise, leaving the furrow empty.

The dark spots of the Moon, when viewed through a telescope, exhibit a totally different character, and show that they belong to a different formation from that of the brighter portions. These darker tracts do not seem to have had a direct volcanic origin like the latter, but rather appear to have resulted from the solidification of semi-fluid materials, which have overflowed vast areas at different times. The surface of this system is comparatively smooth and uniform, only some small craters and low ridges being seen upon it. The level and dark appearance of these areas led the ancient astronomers to the belief that they were produced by a liquid strongly absorbing the rays of light, and were seas like our seas. Accordingly, these dark surfaces were called Maria, or Seas, a name which it is convenient to retain, although it is well known to have originated in an error. The so-called seas of the Moon are evidently large flat surfaces similar to the deserts, steppes, pampas, and prairies of the Earth in general appearance. The great plains of the Moon are at a lower level than that of the other formation, and that which first attracts the observer's attention is the fact that they are surrounded almost on all sides by an irregular line of abrupt cliffs and mountain chains, showing phenomena of dislocation. This character of dislocation, which is general, and is visible everywhere upon the contours of the plains, seems to indicate that phenomena of subsidence, either slow or rapid, have occurred on the Moon; while, at the same time, the sunken surfaces were overflowed by a semi-fluid liquid, which solidified afterwards. The evidences of subsidence and overflowing become unmistakable when we observe that, along the borders of the gray plains, numerous craters are more or less embedded in the gray formation, only parts of the summit of their walls remaining visible, to attest that once large craters existed there. The farther from the border of the plain the vestiges of these craters are observed, the deeper they are embedded in the gray formation. That phenomena of subsidence have occurred on a grand scale on the Moon, is further indicated by the fact that the singular systems of fractures called clefts and rifts generally follow closely the outside border of the gray plains, often forming parallel lines of dislocation and fractures. In the interior regions of the gray formation, these fractures are comparatively rare.

The gray, lava-like formation is obviously of later origin than the mountainous system to which belong the embedded craters above described. Its comparatively recent origin might also be inferred from the smallness of its craters and its low ridges. The few large craters observed on this formation evidently belong to the earlier system.

The color of this system of gray plains is far from being uniform. In general appearance it is of a bluish gray, but when observed attentively, large areas appear tinted with a dusky olive-green, while others are slightly tinged with yellow. Some patches appear brownish, and even purplish. A remarkable example of the first case is seen on the surface, which encloses within a large parallelogram the two conspicuous craters, Aristarchus and Herodotus. This surface evidently belongs to a different system from that of the Oceanus Procellarum surrounding it, as, besides its color, which totally differs from that of the gray formation, its surface shows the rugged structure of the volcanic formation.

When the Moon is full, some very curious white, luminous streaks are seen radiating from different centres, which, for the most part, are important craters, occupied by interior mountains. The great crater Tycho is the centre of the most imposing of the systems of white streaks. Some of the diverging rays of this great centre extend to a distance equal to one-quarter of the Moon's circumference, or about 1,700 miles. The true nature of these luminous streaks is unknown, but it seems certain that they have their origin in the crater from which they diverge. They do not form any relief on the surface, and are seen going up over the mountains and steep walls of the crater, as well as down the ravines and on the floors of craters.

The Moon seems to be deprived of an atmosphere; or, if it has any, it must be so excessively rare that its density is less than of the density of the Earth's atmosphere, since delicate tests afforded by the occultation of stars have failed to reveal its presence. Although no atmosphere of any consequence exists on the Moon, yet phenomena which I have observed seem to indicate the occasional presence there of vapors of some sort. On several occasions, I have seen a purplish light over some parts of the Moon, which prevented well-known objects being as distinctly seen as they were at other times, causing them to appear as if seen through a fog. One of the most striking of these observations was made on January 4th, 1873, on the crater Kant and its vicinity, which then appeared as if seen through luminous purplish vapors. On one occasion, the great crater Godin, which was entirely involved in the shadow of its western wall, appeared illuminated in its interior by a faint purplish light, which enabled me to recognize the structure of this interior. The phenomenon could not be attributed in this case to reflection, since the Sun, then just rising on the western wall of the crater, had not yet grazed the eastern wall, which was invisible. It is not impossible that a very rare atmosphere composed of such vapors exists in the lower parts of the Moon.

If the Moon has no air, and no liquids of any sort, it seems impossible that its surface can maintain any form of life, either vegetable or animal, analogous to those on the Earth. In fact, nothing indicating life has been detected on the Moon--our satellite looking like a barren, lifeless desert. If life is to be found there at all, it must be of a very elementary nature. Aside from the want of air and water to sustain it, the climatic conditions of our satellite are very unfavorable for the development of life. The nights and days of the Moon are each equal to nearly fifteen of our days and nights. For fifteen consecutive terrestrial days the Sun's light is absent from one hemisphere of the Moon; while for the same number of days the Sun pours down on the other hemisphere its light and heat, the effects of which are not in any way mitigated by an atmosphere. During the long lunar nights the temperature must at least fall to that of our polar regions, while during its long days it must be far above that of our tropical zone. It has been calculated that during the lunar nights the temperature descends to 23° below zero, while during the days it rises to 468°, or 256° above the boiling point.

It has been a question among astronomers whether changes are still taking place at the surface of the Moon. Aside from the fact that change, not constancy, is the law of nature, it does not seem doubtful that changes occur on the Moon, especially in view of the powerful influences of contraction and dilatation to which its materials are submitted by its severe alternations of temperature. From the distance at which we view our satellite, we cannot expect, of course, to be able to see changes, unless they are produced on a large scale. Theoretically speaking, the largest telescopes ever constructed ought to show us the Moon as it would appear to the naked eye from a distance of 40 miles; but in practice it is very different. The difficulty is in the fact that, while we magnify the surface of a telescopic image, we are unable to increase its light; so that, practically, in magnifying an object, we weaken its light proportionally to the magnifying power employed. The light of the Moon, especially near the terminator, where we almost always make our observations, is not sufficiently bright to bear a very high magnifying power, and only moderate ones can be applied to its study. What we gain by enlarging an object, we more than lose by the weakening of its light. Besides, a high magnifying power, by increasing the disturbances generally present in our atmosphere, renders the telescopic image unsteady and very indistinct. On the whole, the largest telescopes now in existence do not show us our satellite better than if we could see it with the naked eye from a distance of 300 miles or more. At such a distance only considerable changes would be visible.

Notwithstanding these difficulties, it is believed that changes have been detected in Linné, Marius, Messier, and several other craters. An observation of mine seems to indicate that changes have recently taken place in the great crater Eudoxus. On February 20th, 1877, between 9h. 30m. and 10h. 30m., I observed a straight, narrow wall crossing this crater from east to west, a little to the south of its centre. This wall had a considerable elevation, as was proved by the shadow it cast on its northern side. Towards its western end this wall appeared as a brilliant thread of light on the black shadow cast by the western wall of the crater. The first time I had occasion to observe this crater again, after this observation, was a year later, on February 17th, 1878; no traces of the wall were then detected. Many times since I have tried to find this narrow wall again, when the Moon presented the same phase and the same illumination, but always with negative results. It seems probable that this structure has crumbled down, yet it is very singular that so prominent a feature should not have been noticed before.

From a study made in 1875]

The "Mare Humorum," or sea of moisture, as it is called, which is represented on Plate VI., is one of the smaller gray lunar plains. Its diameter, which is very nearly the same in all directions, is about 270 miles, the total area of this plain being about 50,000 square miles. It is one of the most distinct plains of the Moon, and is easily seen with the naked eye on the left-hand side of the disk. The floor of the plain is, like that of the other gray plains, traversed by several systems of very extended but low hills and ridges, while small craters are disseminated upon its surface. The color of this formation is of a dusky greenish gray along the border, while in the interior it is of a lighter shade, and is of brownish olivaceous tint. This plain, which is surrounded by high clefts and rifts, well illustrates the phenomena of dislocation and subsidence. The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is seen in the upper left-hand corner of the gray plain. Close to Vitello, at the east, is the large broken ring-plain Lee, and farther east, and a little below, is a similarly broken crater called Doppelmayer. Both of these open craters have mountainous masses and peaks on their floor, which is on a level with that of the Mare Humorum. A little below, and to the left of these objects, is seen a deeply embedded oval crater, whose walls barely rise above the level of the plain. On the right-hand side of the great plain, is a long fault, with a system of fracture running along its border. On this right-hand side, may be seen a part of the line of the terminator, which separates the light from the darkness. Towards the lower right-hand corner, is the great ring-plain Gassendi, 55 miles in diameter, with its system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its floor. This crater slopes southward towards the plain, showing the subsidence to which it has been submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on the plain is only 500 feet high, and is even wholly demolished at one place where the floor of the crater is in direct communication with the plain. In the lower part of the mare, and a little to the west of the middle line, is found the crater Agatharchides, which shows below its north wall the marks of rills impressed by a flood of lava, which once issued from the side of the crater. On the left-hand side of the plain, is seen the half-demolished crater Hippalus, resembling a large bay, which has its interior strewn with peaks and mountains. On this same side can be seen one of the most important systems of clefts and fractures visible on the Moon, these clefts varying in length from 150 to 200 miles.

ECLIPSES OF THE MOON

PLATE VII

Since the Moon is not a self-luminous body, but shines by the light which it borrows from the Sun, it follows that when the Sun's light is prevented from reaching its surface, our satellite becomes obscured. The Earth, like all opaque bodies exposed to sunlight, casts a shadow in space, the direction of which is always opposite to the Sun's place. The form of the Earth's shadow is that of a long, sharply-pointed cone, which has our globe for its base. Its length, varying with the distance of the Earth from the Sun, is, on an average, 855,000 miles, or 108 times the terrestrial diameter. This conical shadow of the Earth, divided longitudinally by the plane of the ecliptic, lies half above and half below that plane, on which the summit of the shadow describes a whole circumference in the course of a year. If the Moon's orbit were not inclined to the ecliptic, our satellite would pass at every Full Moon directly through the Earth's shadow; but, owing to that inclination, it usually passes above or below the shadow. Twice, however, during each of its revolutions, it must cross the plane of the ecliptic, the points of its orbit where this happens being called nodes. Accordingly, if it is near a node at the time of Full Moon, it will enter the shadow of the Earth, and become either partly or wholly obscured, according to the distance of its centre from the plane of the ecliptic. The partial or total obscuration of the Moon's disk thus produced constitutes a partial or total eclipse of the Moon. The essential conditions for an eclipse of the Moon are, therefore, that our satellite must not only be full, but must also be at or very near one of its nodes.