from C to D, half its orbit, the earth, instead of passing through the small portion AF, has passed through nearly

of her orbit; through which, and considerably more, (because the earth's motion is constant,) Venus must pass before she is seen below the sun. It is found that Venus is morning and evening star(alternately, during about 290 days); a period, considerably exceeding a complete revolution of that planet in her orbit.

SECT. 5.

ART. 1. Of the Earth.

23. The planet next to Venus in the solar system, is the earth, which we inhabit. It revolves about the sun at the mean distance of 93 millions of miles. It completes this revolution in a year, and turns on its axis in a day, or twenty-four hours. The consideration of the figure of the earth will be resumed when we come to treat of physical Astronomy; and the other phenomena relating to this planet will be continued in CHAP. II. and III.

ART. 2. Of the Moon.

24. The moon is a secondary planet, revolving round the earth in about 293 days, and is carried with the earth round the sun once a year. Its distance from the earth is about 240,000 miles. It turns on its axis in precisely the same time that it performs its revolution round the earth.

25. The most obvious fact relating to the moon, is, that her disk is constantly changing its appearance; sometimes only a semicircular edge is illuminated, while the rest is dark; and at another time, the whole surface appears resplendent. The first appearance is

called the new moon, and is exhibited when the sun and moon appear near each other; that is, in the same region of the heavens. The second is called the full moon, and is exhibited when the sun and moon appear most distant; that is, in opposite regions of the heavens. When the moon is in conjunction with the sun, that is, passes by him, it is said to change; and when it is in opposition to the sun, that is, when the sun is in one part of the heavens, as west, and the moon in the opposite part, as east, the moon is said to full.

26. The different phases of the moon are easily accounted for. In Pl. II, fig. 3, let S be the sun, Ě the earth, and ABCD the moon in different parts of her orbit. When the moon changes, as at A, its dark side will be towards the earth, its illuminated part being always towards the sun. Hence the moon will appear to us as represented at a, if it be seen at all. But when

she has advanced in her orbit, and come to B, a small part of her illuminated side comes in sight, and she appears as represented at b, a new moon, and is said to be horned. When she has come to C, one half her illuminated side is visible, and she appears as at c.

At C and in the opposite point of her orbit, the moon is said to be in quadrature. At D her appearance is as represented at d, and she is said to be gibbous. At E all her illuminated side is towards us, and we have a full moon. During the other half of her revolution, less and less of her illuminated side is seen till it again becomes invisible at A.

The following signs are used in our common almanacs to denote the different positions and phases of the moon. > or D denotes the moon in the first quadrature, that is, the quadrature between change and full. (or denotes the moon in the last quadrature, that is, the quadrature between full and change. denotes new moon. notes full moon.

de

27. The earth, seen from the moon, exhibits precisely the same phases that the moon does to us; only

in an opposite order. When the moon is full to us, the earth will be dark to the inhabitants of the moon; and when the moon to us is dark, the earth to them is full. The earth appears to them about 13 times larger than the moon does to us. But as the moon turns on its axis in the same time that it goes round the earth, she always exhibits the same side to us (consequently we never see one half of the moon's surface, and the earth is never seen by that portion of the moon's inhabitants who dwell there.)

28. When viewed through a telescope, the surface of the moon appears wonderfully diversified. Large dark spots, which are excavations or valleys, are visible to the eye; also some, which are even more lucid than the general surface. These are ascertained to be mountains, by the shadows which they cast. Maps of the moon's surface have been drawn; on which most of these valleys and mountains are delineated, and names are given to them. Some of these excavations are thought to be 4 miles deep and 40 wide. A high ridge generally surrounds them, and often a mountain rises in the centre. These immense depressions probably very much resemble what would be the appearance of the earth at the moon, were all the seas and lakes dried up. the mountains(are supposed to be volcanic.

Some of

Dr. Brewster, speaking of the Moon, says, "Her mountainous scenery bears a stronger resemblance to the towering sublimity, and the terrific ruggedness of 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 over-hanging cliffs, we expect every moment that they are to be torn from their base, and that the process of destructive separation which we had only contemplated in its effects, is about to be exhibited before us in tremendous real

ity, The mountains, called the Apennines, which traverse a portion of the moon's disk from north-east to south-west, rise with a precipitous and craggy front from the level of the Mare Imbrium. In 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 north-east, while on the south-west they sink in gentle declivity to the plains.

"The analogy between the surface of the earth and the moon fails in a still more remarkable degree, when we examine the circular cavities which appear on every part of her disk. Some of these immense caverns are nearly four miles deep and forty miles in diameter. A high annular 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 it is from this cause, that 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 the full moon, like a number of brilliant radiations, issuing from the small 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 either intended for the reception of water, or that they are the beds of lakes and seas which have formerly existed in the moon. The circumstance of there being no water in the moon is a strong confir mation of this theory."

SECT. 6. Of Mars.

29. Next to the earth is the planet Mars. It revolves in its orbit in little less than two years, at the distance of 144 millions of miles from the sun; and turns on its axis in little less than 25 hours. The light reflected by Mars is remarkably red. Spots and sometimes belts have been seen on the disk of this planet, some of which are permanent, and others variable. Some of the most remarkable appearances of this kind, as they are seen through a telescope, are represented in the annexed wood-cut. These variations are supposed to arise from