performs his diurnal rotation in 9 hours and 55 minutes. Saturn, the next planet in the system above Jupiter, is about eight hundred and ninety millions of miles from the sun; and by moving at the rate of twenty-two thousand miles an hour, he performs his annual circuit round that luminary in a little less than 291 of our years. His diameter is computed to be about seventy-six thousand miles; but, on account of his immense distance, and the deficiency of light occasioned by such a remote situation, the time of his diurnal rotation upon his axis was formerly unknown. It is now however ascertained to be about 10 hours 16 minutes. The next and highest planet in our system at present known, is Uranus, or the Georgium Sidus; which was first discovered by Dr. Herschel, March 13th, 1781. The elements of this planet have been now accurately determined; from which it appears, that its mean distance from the sun is about one thousand eight hundred millions of miles, and its diameter thirty-five thousand. Its annual revolution is performed in about 84 years; but the time of its revolving on its axis has not been discovered by observation; although, from analogy, Laplace conceives that it must be performed in about the same time, or rather less, than that of Saturn. The various particulars of this and the other new planets, being highly interesting, I shall speak of them more at large in some future letter. At present it is only necessary to observe, that beside the primary planets here mentioned, there are eighteen others, called secondary planets, or satellites, which regard the primaries as the centres of their motions, and revolve about them in the same manner, as those primaries themselves revolve about the sun. One of the most conspicuous of these satellites is the Moon, who is a constant attendant on our Earth; and, whilst she accompanies it in its annual progress through the heavens, keeps revolving round it continually, by a different motion, in the space of a month. The Moon's diameter is about two thousand one hundred and sixty miles; her distance from the Earth two hundred and thirty seven thousand miles; and in bulk she is about one sixteenth part of that of the Earth. Jupiter has four such moons, Saturn seven, and Uranus six; and from the continual change of their phases, or appearances, it is evident that these also are opaque bodies, like the planets, and shine only by means of the borrowed light which they receive from the sun. It may also be observed, that our Earth is a moon to the Moon, waxing and waning in exactly the same manner; but appearing always stationary, and presenting a diameter near four times greater than hers appears to us; the whole disc being thirteen times larger, and, of course, affording a proportional quantity of light. When she changes to us, the Earth will appear full to her, and when she is in her first quarter to us, the Earth will be in its third quarter to her. And, as her axis is almost perpendicular to the plane of the ecliptic, one half of her orb will be constantly illuminated by the reflected light afforded by the Earth in the sun's absence, whilst the other half will have a fortnight's darkness, and a fortnight's light, alternately. The rotation of the Moon upon her axis, is also performed in exactly the same time that she goes once round the Earth, as is evident from her always presenting the same face to us during the whole of her monthly revolution; and, on this account, it is plain that the inhabitants of one half of the lunar world, are totally deprived of a sight of the Earth, and must for ever remain ignorant of its existence, unless business, or pleasure, leads them to explore the opposite hemisphere; where they may have a full view of our globe, appearing to them like a newly-created planet, with a disc near 13 times larger than that of the sun. A number of other circumstances relating to this subject, will be mentioned in their proper places. But a general idea of the solar system, together with the periods, distances, bulks, &c. of the planets, will be best acquired from the following table, which is formed from the latest observations of the best modern astronomers. A minute exactness in these matters, cannot be easily obtained; and as the nearest approximate numbers are better retained in the memory than those which are more accurate, I have preferred this method of express |