LETTER III.

OF THE SOLAR SYSTEM, AND THE FIRMAMENT OF THE FIXED STARS.

As the figure and motion of the earth are now sufficiently established, it will be proper to turn our attention to the rest of the planets; and, from describing their nature and properties, to exhibit a summary view of the whole system. In the first place, then, it is to be observed, that the planets are all opaque spherical bodies, like our earth, that have no proper light of their own, but shine by means of the borrowed light which they receive from the sun and therefore, only that side of them which is turned towards him, can receive the benefit of his light, whilst the opposite side, which the borrowed rays cannot reach, remains in obscurity, till by the rotation of the planet on its axis, it is itself turned towards the sun, and becomes equally illuminated by his beams.

The planets are also not only similar to our earth in form and structure, but they are likewise known to perform their revolutions round the sun in the same manner. For by the regular appearance and disappearance of several remarkable dark spots, which, by means of a telescope, are constantly to be seen on their bodies, we are able to ascertain that they must have such a motion about their axes, as answers to the diurnal rotation of the earth. And from their seeming at certain times to be moving forward, and at others to be station

ary, and then to go backwards, or be retrograde, we are equally certain, that they must have such a progressive motion round the sun, as answers to the annual revolution of the earth in its orbit.

Mercury, the nearest planet to the sun, goes round him in about eighty-seven days and twentythree hours, or a little less than three months; which is the length of his year. But being seldom seen, on account of his proximity to the sun, and no spots appearing on his surface, or disk, the time of his rotation upon his axis, or the length of his days and nights, is not so accurately determined as in some of the other planets; though Schroeter, a German astronomer, has lately found, from the variation of the horns of his phases, that he has such a motion; which, according to his estimation, is performed in little more than twenty-four hours, being nearly the same as that of the earth. His distance from the sun is computed to be about thirty-six millions of miles, and his diameter three thousand one hundred and twenty; and in his course round the sun, he moves at the rate of a hundred and five thousand miles an hour.

This planet, when viewed, in different positions, with a good telescope, seems to have all the phases or appearances of the moon, except that he can, at no time, be seen entirely round, or quite full; because his enlightened side is never turned directly towards us, except when he is so near the sun as to be hid in the splendour of his beams. Hence, from these phases, it is evident, that he shines not by any light of his own, as the sun does,

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as he would in that case certainly appear, at all times, round like that luminary.

Venus, the next planet above Mercury, is computed to be sixty-eight millions of miles from the sun, and by moving at the rate of seventy-six thousand miles an hour, she completes her annual revolution in 224 days and 16 hours, or about seven months and a half. Her diameter is seven thousand seven hundred miles, and her diurnal rotation on her axis, is performed in 23 hours and 21 minutes. When this planet appears to the west of the sun, she rises before him in the morning, and is called the Morning Star; and when she appears to the east of the sun, she shines in the evening after he sets, and is then called the Evening Star; being in each situation, alternately, for about 290 days: and during the whole of her revolution, she appears, through a telescope, to have all the shapes and appearances of the moon.

The next planet above Venus, in our system, is the Earth. Its distance from the sun is ninetythree millions of miles, and by moving at the rate of fifty-eight thousand miles an hour, its annual revolution is performed in 365 days, 6 hours, or the space of a year; which motion, though 120 times swifter than that of a cannon-ball, is but little more than half the velocity of Mercury in his orbit. The earth's diameter is about seven thousand nine hundred miles; and as it turns round its axis every 24 hours, from west to east, it occasions an apparent motion of all the heavenly bodies, from east to west, in the same time.

Next above the Earth's orbit, is Mars, whose distance from the sun is computed to be about one hundred and forty-two millions of miles. He moves at the rate of fifty-five thousand miles an hour, and completes his revolution round the sun in a little less than two of our years. His diameter is four thousand three hundred and ninety miles; and his diurnal rotation upon his axis is performed in about 24 hours and 39 minutes. This planet sometimes appears gibbous, but never horned, like the moon, which plainly shows, that his orbit includes that of the earth, and that he shines not by his own native light.

The next planet in our system is Vesta, for the knowledge of which we are indebted to Dr. Olbers of Bremen, being first discovered by him March 29th, 1807. Its distance from the sun is about two hundred and twenty-three millions of miles, and its annual revolution in its orbit is performed in about 3 years 7 months. But neither its diameter, nor the duration of its diurnal rotation, have yet been ascertained.

Juno, the next in order, is another new planet, discovered by Mr. Harding, at the observatory at Lilienthal, near Bremen, Sept. 1st, 1804. The mean distance of this planet from the sun is estimated at two hundred and fifty-three millions of miles, and its annual revolution is performed in 4 years, 4 months, and 6 days; but its diameter, and the time of its revolving on its axis is unknown.

The next superior planet above Juno, is Pallas, which was first observed by Dr. Olbers, March

28th, 1802: the mean distance of which, from the sun, is reckoned to be about two hundred and sixty-three millions of miles, and its revolution in its orbit is made in about 4 years 7 months and 10 days; but like the two former, its diameter and diurnal rotation have not been at present ascertained.

Ceres is the next higher planet, in our system; which was first discovered by Piazzi, of Palermo, Jan. 1st, 1801. Its mean distance is nearly the same as that of Pallas, being estimated, in round numbers, at two hundred and sixty-three millions of miles; and consequently its annual revolution is also nearly the same, being performed in 4 years, 7 months, and 11 days.

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The extreme minuteness of these planets, as well as the little time since they have been discovered, and their great distance from us, render the results of our observations upon them in some measure uncertain; we have, however, reason to conclude, that none of their diameters exceed four hundred miles, nor are less than a hundred miles. But, at present, no accurate estimate can be made of the time of their diurnal rotation.

Jupiter, is the largest of all the planets, and is reckoned to be about four hundred and eighty-five millions of miles from the sun; and by going at the rate of twenty-nine thousand miles an hour, he completes his annual revolution in something less than twelve of our years. His diameter is computed to be ninety-one thousand five hundred miles; and, by a prodigiously rapid motion upon his axis, he