The Moon is, next to the sun, the most remarkable of the celestial objects. Its form is spherical, like that of the earth round which it revolves, and by which it is carried round the sun. Its orbit is also elliptical, having the earth in one of the foci of the ellipsis. The moon always keeps the same side towards the earth, showing only at one time a little more of one side, and at another time a little more of the other side. When the moon is viewed through a good telescope, its sur face appears covered with ridgeL, moun tains, pits, and cavities of great variety. Some parts of its surface also reflect less light than the rest. It has been conjec tured that the part which reflects the least light is water, and the brightest part land. The heights of the lunar moun tains were formerly supposed to be much greater than those of our earth ; but Dr. Herschel has demonstrated that very few are more than half a mile high, and the highest little more than a mile. Several volcanos, or burning mountains, have been discovered in it. It has been doubt ed whether the moon has an atmosphere like ours, but the latest observations ap pear to proVe that it has. The moon is seen by means of the light which comes to it from the sun being reflected from it. Its changes or phases depend upon its situation relatively to the earth and the sun. When the moon is in opposition to the sun, the enlightened side is turned towards the earth, and it appears , full; when the moon is in conjunction with the sun, its dark side is turned towards us, and it is invisible. As it proceeds in its orbit, a small part of the enlightened side is seen, and then we have a new moon ; and we continue to see more and more of the enlightened side, as the moon ap proaches to the state of opposition, or full moon. The waning or decreasing of the moon takes place in the same manner, but in a contrary order. The earth must perform the same office to the moon that the moon does to us ; and it will appear to the inhabitants of the moon (ifthere be any,) like a very magnificent moon, be ing to them about thirteen times as big as the moon to us, and it will also have the same changes or phases. The moon's mo tion is subject to many irregularities, on account of the inclination of its orbit to the plane of the ecliptic, and the attrac tion of the sun and the other planets. The moon has scarcely any difference of seasons; her axis being almost perpen dicular to the ecliptic. What is very sin gular, one half of her has no darkness at all, the earth constantly affording it a strong light in the sun's absence ; while the other half has a fortnight's darkness, and a fortnight's light, by turns. Our earth, as we have already observed, is un doubtedly a moon to the moon; waxing and waning regularly, but affording her 13 times as much light as she does us. When she changes to us, the earth ap pears full to her ; and when she is in her first quarter to us, the earth is in its third quarter to her ; and vice versa. But from one half of the moon the earth is never seen at all : from the middle of the other half, it is always seen over head ; turning round almost 30 times as quick as the moon does. From the circle which limits our view of the moon, only one half of the earth's side next her is seen ; the other half being hid below the horizon of all places on that circle. To her, the earth seems to be the biggest body in the universe. As the earth turns round its axis, the several continents, seas, and islands, appear to the moon's inhabitants like so many spots of different forms and brightness moving over its surface ; but much fainter at some times than others, as our clouds cover them or leave them. By these spots the Lunarians can deter mine the time of the earth's diurnal mo tion, just as we do the motion of the sun : and perhaps they measure their time by the motion of the earth's spots ; for they cannot haye a truer dial. The moon's tads is so nearly perpendicular to the ecliptic, that the sun never removes sen. sibly from her equator ; and the obliquity of her orbit, which is next to nothing as seen from the sun, cannot cause the sun to decline sensibly from her equator. Yet her inhabitants are not destitute of means for ascertaining the length of their year, though their method and ours must differ. For we can know the length of our year by the return of our equinoxes : but the Lunarians, having always equal day and night, must have recourse to another method ; and we may suppose they measure their year, by observing when either of the poles of our earth-be gins to be enlightened, and the other to disappear, which is always at our equi noxes, they being conveniently situated for observing great tracts of land about our earth's poles, which are entirely un known to us. Hence we may conclude,
that the year is of the same absolute length both to the earth and moon, though very different as to the number of days ; we having WI natural days, and the Lunarians only every day and night in the moon being as long as 291 on the earth.
Mars is not so bright as Venus, nor even as Jupiter, though nearer to the sun. Its colour is a little reddish. Some spots have been observed upon its sur face, from which its rotation round its axis, and the inclination of its axis to the plane of its orbit, have been determined. This planet sometimes appears gibbous, but never horned, like the moon, which shews that his orbit includes that of the earth, and that he shines by a borrowed light.
Ceres Ferdinandea is a very small pla net, situated next without Mars : it was discovered on the first day of the present century, by Mr. Piazzi, an Italian astro nomer.
Pallas is another very small planet, dis covered by Dr. Olbers of Bremen, on the 28th of March, 1802. Two others have also been discovered, one by M. Harding, and the other by a pupil of Dr. Olbers. To these have been given the names of Juno and Vesta. These planets Dr. Hers chel proposes to call asteroids, because they are so much smaller than any of the other planets.
Jupiter is the brightest planet next to Venus. When viewed by a telescope, several belts are observed across its disc, parallel to its equator : these belts are va riable, and are supposed to be ranges of clouds in the atmosphere of the planet. Jupiter is surrounded by four moons, of different sizes, which move about it in different tunes. These moons are some times eclipsed by the shadow of Jupiter falling upon them ; and the eclipses have been found of great use in determining the longitudes of different places on the earth : Ex. Suppose two observers of an eclipse, one at London, the other at the Cape of Good Dope, the eclipse will ap pear at the same instant of time to both ; but being situated under different meri dians, they count different hours, accord ing to which the difference of their longi tude is found. Thus, if an emersion of a satellite is observed at London 9h 33' 12", and at another place 10h 46' 45", the dif ference of' time is lh 13' of course that other place is 18° 23' 15" east of London.
The eclipses ofJupiter's satellites have been applied also to measure the velocity of light: by comparing the times of the apparent entrance and emersion of the satellites with tables calculated for the mean distances of the earth from the sa tellite, the visible emersion at the least distance is found to happen about eight minutes sooner ; and at the greatest dis tance about eight minutes later than the tables : consequently, a ray of light is about 16 minutes in passing through the earth's orbit, or eight minutes in coming from the sun to the earth. If therefore the distance be 95,000,000 of miles, the velocity of light per second is equal to 95,000,000 — 198 thousand miles in a se 8 X 60 cond nearly.
Saturn can hardly be seen by the naked eye. When examined by a telescope, it exhibits a very remarkable appearance. It is surrounded by a thin, flat, broad, lu minous ring, which surrounds the body of the planet, but does not touch it. This ring casts a strong shadow upon the pla net, and is divided into two, by a distinct line in the middle of its breadth. The rings are circular, but appear elliptical from being viewed obliquely.
According to Dr. Herschel, the dimen sions of the rings, and the space between, are as follows.
Miles.
Inner diameter of the smaller ring 146,345 Outside diameter of ditto . 184.393 Inner diameter of the larger ring 190,248 Outside diameter of ' ditto . . 204,883 Breadth of the inner ring . . 20,000 Ditto . . . outer ring . . 7,200 Ditto of vacant space . . . . 2,839 Besides this ring, Saturn has seven moons of different sizes, and its body is surrounded also by belts, like those of Jupiter.
The Herschel planet, with its six satel lites, have been entirely discovered by Dr. Herschel. It cannot be seen without a telescope, but it does not require a powerful one. The satellites cannot be seen without the most powerful tele scopes. We shall subjoin in the opposite page a table, which will contain a number of particulars relating to the planets, that will be found of great utility to the reader