Astronomy for Amateurs

It is the delightful hour when all Nature pauses in the tranquil calm of the silent night. The Sun has cast his farewell gleams upon the weary Earth. All sound is hushed. And soon the stars will shine out one by one in the bosom of the somber firmament. Opposite to the sunset, in the east, the Full Moon rises slowly, as it were calling our thoughts toward the mysteries of eternity, while her limpid night spreads over space like a dew from Heaven. In the odorous woods, the trees are silhouetted strangely upon the sky, seeming to stretch their knotted arms toward this celestial beauty. On the river, smooth as a mirror, wherein the pale Phœbe reflects her splendor, the maidens go to seek the floating image of their future spouse. And in response to their prayers, she rends the veil of cloud that hides her from their eyes, and pours the reflection of her gentle beams upon the sleeping waters. From all time the Moon has had the privilege of[Pg 233] charming the gaze, and attracting the particular attention of mortals. What thoughts have not been wafted to her pale, yet luminous disk? Orb of mystery and of solitude, brooding over our silent nights, this celestial luminary is at once sad and splendid in her glacial purity, and her limpid rays provoke a reverie full of charm and melancholy. Mute witness of terrestrial destinies, her nocturnal flame watches over our planet, following it in its course as a faithful satellite. The human eye first uplifted to the Heavens was struck, above all, with the brilliancy of this solitary globe, straying among the stars. The Moon first suggested an easy division of time into months and weeks, and the first astronomical observations were limited to the study of her phases. Daughter of the Earth, the Moon was born at the limits of the terrestrial nebula, when our world was still no more than a vast gaseous sphere, and was detached from her at some critical period of colossal solar tide. Separating with regret from her cradle, but attached to the Earth by indissoluble ties of attraction, she rotates round us in a month, from west to east, and this movement keeps her back a little each day in relation to the stars. If we watch, evening by evening, beginning from the new moon, we shall observe that she is each night a little farther to the left, or east, than on the preceding[Pg 234] evening. This revolution of the Moon around our planet produces the phases, and gives the measure of our months. Fig. 64.—The Full Moon slowly rises. [Pg 235] During her monthly journey she always presents the same face to us. One might think that the fear of losing us had immobilized her globe, and prevented her from turning. And so we only know of her the vague sketch of a human face that has been observed through all the ages. It seems, in fact, as though she were looking down upon us from the Heavens, the more so as the principal spots of her disk vaguely recall the aspect of a face. If we try to draw it without the aid of instruments we observe dark regions and clear regions that each interprets in his own fashion. To the author, for instance, the full Moon has the appearance represented in the following figure. The spots resemble two eyes and the sketch of a nose; resulting in a vague human figure, as indicated on the lower disk. Others see a man carrying a bundle of wood, a hare, a lion, a dog, a kangaroo, a sickle, two heads embracing, etc.[13] But generally speaking, there is a tendency to see a human figure in it. If this appearance is helped a little by drawing, it gives the profile of a man's head fairly well sketched, and furnished with an abundant crop of hair (Fig. 66).[Pg 236] Others go much more into detail, and draw a woman's head that is certainly too definite, like this of M. Jean Sardou (Fig. 67). Others, again, like M. Zamboni, see behind the man's profile the likeness of a young girl being embraced by him (Fig. 68). There is certainly some imagination about these. And yet, on the first suitable occasion, look at the Moon through an opera-glass, a few days after the first quarter, and you will not fail to see the masculine profile just described, and even to imagine the "kiss in the Moon." Fig. 65.—The Moon viewed with the unaided eye. [Pg 237] Fig. 66.—The Man's head in the Moon. These vague aspects disappear as soon as the Moon is examined with even the least powerful instruments:[Pg 238] the spots are better defined, and the illusions of indistinct vision vanish. Compare this direct photograph of the Moon, taken by the author some years ago (Fig. 69): here is neither a human figure, man, dog, hare, nor faggot; simply deep geographical configurations, and in the lower region, a luminous point whence certain light bands spread out, some being prolonged to a considerable distance. And yet, from a little way off, does it not form the man's face above indicated? Fig. 67.—Woman's head in the Moon. From the earliest astronomical observations made with the aid of instruments by Galileo, in 1609, people tried to find out what the dark spots could represent,[Pg 239] and they were called seas, because water absorbs light, and reflects it less than terra firma. The Moon of itself possesses no intrinsic light, any more than our planet, and only shines by the light of the Sun that illuminates it. As it rotates round the Earth, and constantly changes its position with respect to the Sun, we see more or less of its illuminated hemisphere, and the result is the phases that every one knows so well. Fig. 68.—The kiss in the Moon. [Pg 240] Fig. 69.—Photograph of the Moon. At the commencement of each lunation, the Moon is between the Sun and the Earth, and its non-illuminated hemisphere is turned toward us. This is the New[Pg 241] Moon, invisible to us; but two days later, the slim crescent of Diana sheds a gentle radiance upon the Earth. Gradually the crescent enlarges. When the Moon arrives at right angles with ourselves and with the Sun, half the illuminated hemisphere is presented to us. This is the first quarter. At the time of Full Moon, it is opposite the Sun, and we see the whole of the hemisphere illuminated. Then comes the decline: the brilliant disk is slightly corroded at first; it diminishes from day to day, and about a week before the New Moon our fair friend only shows her profile before she once more passes in front of the Sun: this is the last quarter. Fig. 70.—The Moon's Phases. When the Moon is crescent, in the first evenings of the lunation, and after the last quarter, the rest of the[Pg 242] disk is visible, illuminated feebly by a pale luminosity. This is known as the ashy light. It is due to the shine of the Earth, reflecting the light received from the Sun into space. Accordingly the ashy light is the reflection of our own sent back to us by the Moon. It is the reflection of a reflection. This rotation of the Moon round the Earth is accomplished in twenty-seven days, seven hours, forty-three minutes, eleven seconds; but as the Earth is simultaneously revolving round the Sun, when the Moon returns to the same point (the Earth having become displaced relatively to the Sun), the Moon has to travel two days longer to recover its position between the Sun and the Earth, so that the lunar month is longer than the sidereal revolution of the Moon, and takes twenty-nine days, twelve hours, forty-four minutes, three seconds. This is the duration of the sequence of phases. This revolution is accomplished at a distance of 384,000 kilometers (238,000 miles). The velocity of the Moon in its orbit is more than 1 kilometer (0.6214 mile) per second. But our planet sweeps it through space at a velocity almost thirty times greater. The diameter of the Moon represents 273⁄1,000 that of the Earth, i.e., 3,480 kilometers (2,157 miles). Its surface = 38,000,000 square kilometers (15,000,000 square miles), a little more than the [Pg 243]thirteenth part of the terrestrial surface, which = 510,000,000 (200,000,000 square miles). In volume, the Moon is fifty times less than the Earth. Its mass or weight is only 1⁄81 that of the terrestrial globe. Its density = 0.615, relatively to that of the Earth, i.e., a little more than three times that of water. Weight at its surface is very little: 0.174. A kilogram transported thither would only weigh 174 grams. At the meager distance of 384,000 kilometers (238,000 miles) that separates us from it (about thirty times the diameter of the Earth), the Moon is a suburb of our terrestrial habitation. What does this small distance amount to? It is a mere step in the universe. A telegraphic message would get there in one and a half second; a projectile fired from a gun would arrive in eight days, five hours; an express-train would be due in eight months, twenty-two days. It is only the 1⁄388 part of the distance that separates us from the Sun, and only the 100⁄1,000,000 part of the distance of the stars nearest to us. Many men have tramped the distance that separates us from the Moon. A bridge of thirty terrestrial globes would suffice to unite the two worlds. Owing to this great proximity, the Moon is the best known of all the celestial spheres. Its geographical[Pg 244] (or more correctly, selenographical, Selene, moon) map was drawn out more than two centuries ago, at first in a vague sketch, and afterward with more details, until to-day it is as precise and accurate as any of our terrestrial maps of geography. Before the invention of the telescope, from antiquity to the seventeenth century, people lost themselves in conjectures as to the nature of this strange lunar figure. It was held to be a mysterious world, the more extraordinary in that it always presented the same face to us. Some compared it to an immense mirror reflecting the image of the Earth. Others pictured it as a silver star, an enchanted abode where all was wealth and happiness. For many a long day it was the fashion to think, quite irrationally, that the inhabitants of the Moon were fifteen times bigger than ourselves. The invention of telescopes, however, brought a little order and a grain of truth into these fantastic assumptions. The first observations of Galileo revolutionized science, and his discoveries filled the best-ordered minds with enthusiasm. Thenceforward, the Moon became our property, a terrestrial suburb, where the whole world would gladly have installed itself, had the means of getting there been as swift as the wings of the imagination. It became easy enough to invent a thousand enchanting descriptions of the charms of our[Pg 245] fair sister, and no one scrupled to do so. Soon, it was observed that the Moon closely resembled the Earth in its geological features; its surface bristles with sharp mountain peaks that light up in so many luminous points beneath the rays of the Sun. Alongside, dark and shaded parts indicate the plains; moreover, there are large gray patches that were supposed to be seas because they reflect the solar light less perfectly than the adjacent countries. At that epoch hardly anything was known of the physical constitution of the Moon, and it was figured as enveloped with an atmospheric layer, analogous to that at the bottom of which we carry on our respiration. To-day we know that these "seas" are destitute of water, and that if the lunar globe possesses an atmosphere, it must be excessively light. The Moon became the favorite object of astronomers, and the numerous observations made of it authorized the delineation of very interesting selenographic charts. In order to find one's way among the seas, plains, and mountains that make up the lunar territory, it was necessary to