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Sun, being a bright moon to its inhabitants. The other hemisphere has alternate light and darkness in succession, each continuing about 14 days.

The Moon, like the other planets, is opaque, shining only by the reflected light of the Sun) The side of the Moon which is next to the Sun is enlightened, the other half dark and invisible. Hence, when she comes between us and the Sun, she is not seen, her dark side being then towards us.

When she is advanced a little way in her orbit, a small part of her illumined side becomes visible in the form of a beautiful luminous crescent. This is called

the new Moon. When she has performed one fourth of a lunation, her illuminated side becomes dichotomized, or one half of the bright side becomes visible. She is then said to be in her first quarter. From this time to her opposition, she is said to be gibbous, presenting still more of her illuminated side, as she moves forward, or becoming more protuberant. When she becomes opposite to the Sun, nearly the whole of her enlightened hemisphere is presented to the Earth. She is then said to be full; and is called the full Moon. It must be remembered, however, that the bright side of the Moon is never exactly towards us, as she is never directly opposite to the Sun; except in her nodes, when she falls into the Earth's shadow, and is eclipsed. From the full to the change, the Moon passes in a retrograde order through the same phases; first gibbous, then dichotomized in her last quarter, then horned, till, coming between the Earth and the Sun, she again becomes invisible.

Let S be the Sun, (Plate v. Fig. 3) E the Earth; ABCDEFGH the Moon's orbit; the small circle at these letters, the Moon in different parts of a lunation. The varied appearances of the Moon at the Earth are represented in the external circle at a b c d e f g h. To understand these requires but a slight inspection.

The axis of the Moon being almost perpendicular to

the ecliptic, difference of seasons must be nearly unknown to the lunarians.

It is confidently asserted by some astronomers, that there is no lunar atmosphere of any visible density. If so, it must be uninhabitable to beings constituted like the inhabitants of this Earth. It can have neither winds nor clouds, dews nor rains. But all are not agreed in this opinion. "It is not determined," says Enfield, "whether there is an atmosphere belonging to the Moon." But it can hardly be supposed that so large a field remains untenanted. There seems now a strong preponderance in favor of the opinion, that the Moon is surrounded by an atmosphere. "No large seas or tracts of water have been observed in the Moon by Dr. Herschel, or any other astronomer; nor did he notice any indications of a lunar atmosphere. Recent observations, however, on the occultations of Jupiter and Venus by the Moon, render it highly probable, that the Moon, as well as the Earth, is surrounded by an atmosphere. On April 5th, 1824, Mr. Ramage of Aberdeen, Capt. Ross of the navy, and Mr. Camfield, at Northampton, observed, with excellent telescopes, the occultation of Jupiter, and to all of them the disk of the planet appeared distorted, when it approached the limb of the Moon; and Mr. Camfield, at Clapham, on October 30th, 1825, observed on the emersion of Saturn from behind the dark limb of the Moon, first the disk of the planet, and then the eastern extremity of the ring decidedly flattened; a phenomenon perfectly analogous to what would be produced by refraction, and therefore rendering it highly probable, that the Moon is surrounded by an atmosphere." Guy's Astronomy.

The dark parts of the Moon attract the attention of the most careless observer. Hence "the man in the Moon," is familiar to boyhood, and common to the unlearned. These dark parts were formerly thought to be seas, but are now considered dark cavities, not reflecting the light of the Sun. Many pits and caverns are dark

on the side next to the Sun, which shows that they are hollow. But deep cavities are not all: the surface of the Moon is found to be diversified with large tracts of mountains, (Plate iv. Fig. 2). Dr. Brewster, in his supplement to Ferguson, has well described the irregularities in the lunar surface.

"The strata of mountains and the insolated hills, which mark the disk of this luminary, have evidently no analogy with those in our own globe. Her mountainous scenery, however, bears a stronger resemblance to the towering sublimity and terrific ruggedness of the Alpine regions, than to the tamer inequalities of less elevated countries. Huge masses of rock rise at once from the plains, and raise their peaked summits to an immense height in the air, while projecting crags spring from their rugged flanks, and, threatening the valleys below, seem to bid defiance to the laws of gravitation. Around the base of these frightful eminences are strewed numerous loose and unconnected fragments, which time seems to have detached from their parent mass; and when we examine the rents and ravines, which accompany the ever-changing cliffs, we expect every moment that they are to be torn from their bases, and that the process of destructive separation, which we had only contemplated in its effects, is about to be exhibited before us in tremendous reality. The strata of lunar mountains, called the Apennines, which traverse a portion of her disk from north-east to south-west, rise, with a precipitous and craggy front, from the level of the Mare Imbrium. some places, their perpendicular elevation is above four miles; and, though they often descend to a much lower level, they present an inaccessible barrier to the northeast, while in the south-west, they sink in gentle declivity to the plains.

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The analogy between the surface of the Earth and Moon fails in a still more remarkable degree, when we examine the circular cavities which appear in every part of her disk. Some of these immense caverns are nearly

four miles deep, and forty in diameter. A high, regular ridge, marked with lofty peaks and little cavities, generally encircles them; an insulated mountain frequently rises in their centre, and sometimes they contain smaller cavities of the same nature with themselves. These hollows are most numerous in the south-west part of the Moon; and from this cause that portion of this luminary is more brilliant than any other part of her disk. The mountainous ridges which encircle the cavities, reflect the greatest quantity of light; and, from their lying in every possible direction, they appear, near the time of full Moon, like a number of brilliant radiations, issuing from the large spot called Tycho.

It is difficult to explain, with any degree of probability, the formation of these immense cavities; but we cannot help thinking, that our Earth would assume the same figure, if all the seas and lakes were removed; and it is therefore probable, that the lunar cavities are intended for the reception of water; or that they are the beds of lakes and seas, which have formerly existed in that luminary. The circumstance of there being no water in the Moon is a strong confirmation of this theory. The deep caverns, and the broken, irregular ground, which appear in almost every part of the Moon's surface, have induced several astronomers to believe that these inequalities are of volcanic origin."

The irregularities of the Moon's surface are of great use to us, by reflecting the rays of the Sun in different directions. If the surface of the Moon were covered with water, or smooth and polished like a mirror, it would reflect the rays of the Sun, but not in the copious manner they are now diffused. In the direction of a reflected ray, she would show the Sun's image not larger than a point, but with a lustre hurtful to the organs of vision.

*The learned author has not told us what has become of the water. Chemists say, bodies are never annihilated.

The light of the Moon is exceedingly soft and cheering; but is little in the extreme compared with that of the Sun. In this authors are agreed. But, from their different modes of computation, they have come to different results, and made considerable difference in the disproportion between the lunar and solar light. Dr. Hooke, accounting for the reason why the Moon's light affords no perceptible heat, observes, "that the quantity of light which falls on the hemisphere of the full Moon, is rarified into a sphere 288 times greater in diameter than the Moon, before it arrives at us; and, consequently, that the Moon's light. is 104,368 times weaker than that of the Sun. It would therefore require 104,368 full Moons to give a light and heat equal to that of the Sun at noon. The light of the Moon, condensed by the best mirror, produces no sensible heat upon the ther

mometer.

Dr. Smith has endeavored to show, in his book on optics, that the light of the full Moon is but equal to a 90,900 part of the common light of the day, when the Sun is hidden by a cloud." Rees's Cyclopædia.

To a spectator at the Sun, the Moon never appears more than 10' distant from the Earth.

The Moon's orbit is an ellipse, the Earth being in one of the foci. (The point of the orbit nearest the Earth is called the perigee that farthest distant, the apogee.) Syzygy is a common name for the conjunction and opposition, or change and full, of the Moon.

Dr. Herschel saw, or thought he saw three volcanoes> at the same time in the Moon; one showing an eruption of fire, or luminous matter; the other two about to break out, or nearly extinct.

When the Moon is about three or four days old, the part of her disk not enlightened by the Sun becomes visible, faintly illuminated by light reflected from the Earth. The horns of the enlightened part, appearing to project beyond the old Moon, seem part of a sphere larger than the faintly enlightened part. This phenomenon is call

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