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arising from this difference, would in itself scarcely be sensible, and it is completely overpowered by other causes which produce the variations of the seasons; but the explanation of these must be deferred, till we have made some further observations on the heavenly bodies..
The planets are celestial bodies which revolve round the sun, on the same principle as the earth. They are divided into primary and secondary. Those, which revolve immediately round the sun, are called primary. Many of these are attended in their course by smaller planets, which revolve round them: these are called secondary planets, satellites, or moons; such is our moon, which accompanies the earth, and is carried with it round the sun. The sun is the general centre of attraction to our system of planets; but the satellites revolve round the primary planets, on account of their greater proximity. The force of attraction is not only proportional to the quantity of matter, but to the
degree of proximity of the attracting body. The power of attraction diminishes as the squares of the distance increase; so that a planet, situated at twice the distance at which we are from the sun, would gravitate four times less than we do. The more distant planets, therefore, move slower in their orbits, for their projectile force must be proportioned to that of attraction. This diminution of attraction, by the increase of distance, also accounts for the motion of the secondary round the primary planets, in preference to the sun; for the vicinity of the primary planets renders their attraction stronger than that of the sun. But since attraction between bodies is mutual, the primary planets are also attracted by their satellites. The moon attracts the earth, as well as the earth the moon; but as the latter is the smaller body, her attraction is proportionally less. The result is, that neither does the earth revolve round the moon, nor the moon round the earth; but they both revolve round a point, which is their common centre of gravity, and which is as much nearer the earth's centre of gravity than that of the moon, as the weight of the former exceeds that of the latter.
The earth then has three different motions; it revolves round the sun,-it revolves upon its axis,—and it revolves round the point towards which the moon attracts it; and this is the case with every planet which is attended by satellites. The planets act on the sun in the same manner as they are themselves acted on by their satellites; but the gravity of the planets (even when taken collectively) is so trifling, compared with that of the sun, that they do not cause it to move so much as one-half of its diameter. The planets therefore, do not revolve round the centre of the sun, but round a point at a small distance from its centre, about which the sun also revolves. The sun likewise revolves on its axis. This motion is ascertained by observing certain spots which disappear and reappear regularly at stated times.
Mercury is the planet nearest the sun; his orbit is consequently contained within ours; but his vicinity to the sun occasions his being nearly lost in the brilliancy of his rays; and when we do see this planet, the sun is so dazzling, that very accurate observations cannot be made upon him. He performs his revolution round the sun in about eighty-seven days, which is consequently the length of his year; the time of his rotation on his axis is not accurately known; his distance from the sun is computed to be 37 millions of miles, and his diameter 3,224 miles.
Venus, the next in the order of the planets, is 68 millions of miles from the sun; she revolves about her axis in 23 hours and 21 minutes, and goes round the sun in 224 days 17 hours. The diameter of Venus is 7,687 miles. The orbit of Venus is within ours; during nearly one-half of her course we see her before sun-rise, when she is called the morning star; in the corresponding part of her orbit, on the other side, she rises later than the sun. We cannot then see her rising, as she rises in the day time; but she also sets later; so that we perceive her approaching the horizon after sun-set; she is then called Hesperus or the evening star.
The Earth is next to Venus. At present we shall only observe that we are 95 millions of miles distant from the sun-that we perform our annual revolution in 365 days, 5 hours, and 49 minutes, and are attended in our course by a single moon.
Mars comes next. He can never be between us and the sun, like Mercury and Venus. His distance
from the sun is 144 millions of miles, he turns on his axis in 24 hours and 39 minutes; and he performs his annual revolution in about 687 of our days: his diameter is 4,189 miles. Then follow four very small planets-Juno, Ceres, Pallas and Vesta, which have been recently discovered, but whose dimensions and distances from the sun have not been very accurately ascertained.
Jupiter is next in order. This is the largest of all the planets; he is about 490 millions of miles distant from the sun, and completes his annual period in nearly twelve of our years, he revolves on his axis in about ten hours; he is above 1,400 times as large as our earth, his diameter being 89,170 miles. He is attended by four moons.
Saturn comes next, whose distance from the sun is about 900 millions of miles. His diurnal rotation is performed in ten hours and a quarter; his annual revolution in nearly thirty of our years; his diameter is 79,000 miles. This planet is surrounded by a luminous ring, the nature of which astronomers are much at a loss to conjecture; he has seven moons.
Georgium Sidus, or Uranus, or Herschel (for all these names have been given to this planet) is the last. It was discovered by Dr. Herschel in 1791. It is attended by six moons. It is the most distant planet from the sun that has yet been discovered; being at a distance of no less than 1800 millions of miles from that luminary. Its diameter is about 35,000 miles. It takes about 83 years and a half to complete its revolution round the sun.
Comets are supposed to be planets. The reappearance of some of them at stated times proves that they revolve round the sun; but in orbits so eccentric, and running to such a distance from the sun, that they disappear for a great number of years. They are distinguished from the other celestial bodies, by their ruddy appearance, and by a long train of light called the tail. The length of these tails is often many millions of miles. Some comets have been ascertained to move in long narrow ellipses or ovals, round the
sun, from which it has been inferred, perhaps hastily, that they all do so. The number of comets which have occasioally been seen within the limits of our system, since the commencement of the Christian era, is about 500 of which the paths of 98 have been calculated
The ancients, in order to recognize the fixed stars, formed them into groups, to which they gave particular names. In order to show their proper situations in the heavens, they should be painted on the internal surface of a hollow sphere, from the centre of which they might be viewed. We should then see them as they appear to be situated in the heavens. The twelve constellations, called the Signs of the Zodiac, are those which are so situated, that the earth, in its annual revolution, passes directly between them and the sun. They occupy a complete circle, or broad belt, in the heavens. Hence, a right line, drawn from the earth, and passing through the sun, would reach one of these constellations; and the sun is said to be in that constellation in which such a line would terminate. The circle in which the sun appears to move, and which passes through the middle of the Zodiac, is called the. ecliptic.
We have no means of ascertaining the distance of the fixed stars. When therefore they are said to be in the Zodiac, it is merely implied that they are situated in that direction, and that they shine upon us through that portion of the heavens which we call the Zodiac. Whether the apparent difference of the size and brilliancy of the stars proceeds from various degrees of remoteness or of dimension, is a point which astronomers are not able to ascertain. Considering them as suns, we know no reason why they should not vary in size, as well as the planets belonging to them.