round the sun, a circle, its radius, or in other words, the mean distance from his body would, in round numbers, be about 95 millions of miles; that the position of the earth's axis formed, in1801, an angle of about 66 deg. 32.min. with the plane of the ecliptic or of the orbit traced by the earth in its annual motion round the sun; that the period in which the earth performs one entire revolution round its axis is what we call a day; that the period elapsing between the earth's departure from any given point of her orbit, and her return to the same point, after travelling round the sun, as observed from his body, or what we call a year, or properly the tropical year, contains 365 days 5 hours 48 minutes and 49 seconds; but that the period elapsing between the earth's departure from any fixed star, as seen by an observer in the sun, and her arrival at the same star again, amounts to 365 days 6 hours 9 minutes and 12 se-. conds.But to be more particular Mercury is a globe of 3.224 Euglish miles in diameter, and the nearest to the sun of the known planets of our system, his mean distance being about 37 millions of miles. He emits a very bright white light; but it is seldom he can be observed, from his being always so near the sun as to be involved in his rays, excepting a short time before the rising or after the setting of that luminary. The angle formed at the eye of an observer on the earth, by lines drawn from Mercury and the Sun, never exceeds 27° 5′, so that this pla net never sets after, nor rises before, the sun, more than 1, hour and 50 minutes. Mercury's year contains, by the pre ceding table, very nearly 88 days; but the length of his day, or of his revolution round his axis, has not yet been ascertained. Venus. This planet is of peculiar brilliancy; and hence, according to her appearance before sun-rise or after sun-set, is commonly called the morning or the evening-star. Beng both at a greater distance from the Sun, and nearer to us than is Mercury, the angle formed at our eye, by lines drawn from her and the Sun, is considerably greater than that formed by lines from Mercury and the Sun, amounting at times to 48 degrees. Venus turns round on her axis once in 23 hours 21 minutes, which is the length of her day; and her year, or the time required to perform her revo. lution round the sun, is nearly 224 of our days 17 hours." The Earth. This planet, on which we dwell, is the next in order of distance from the Sun; Venus and Mercury revolving between him and us are therefore commonly, although inaccurately, called the inferior planets, whilst Mars, Jupiter, and the others, moving in orbits farther removed than the Earth from the Sun, are termed the superior planets. The Earth, as appears from the preceding table, moves in an orbit distant from the Sun in the centre at a medium of about 95 millions of English miles; her revolution on her axis is performed in 24 hours, or 1 day, and her revolution in her orbit in one year, or nearly 365 days; she is attended by one satellite, which we call the moon. Mars. This planet, the next on the outside of the earth, presents a dull red firy colour; his medium distance from the Sun is about 144 millions of miles: his daily revolution on his axis is performed in about 24 hours and 40 minutes of our time, and his year occupies nearly 687 of our days. Ceres and Pallas. Of these planets, but lately discovered, very little is yet known; they are both much smaller than the other planets of our system, the diameter of Ceres being only 160 miles, and that of Pallas not exceeding 80. Jupiter. This is the largest planet of the solar system, his diameter approaching to 90 thousand miles in splendour he sometimes rivals Venus herself; his daily revolution on his axis is performed with such velocity, as to require only about 10 of our hours; but his year demands upwards of 4,330 of our days, or nearly 12 of our years. Jupiter, when examined through a telescope, presents a number of spots spots and belts upon his surface; these belts are variable in position and number, from one to eight; they are in general parallel to one another, but not constantly so, nor is their breadth always the same, one gradually increasing while an other beside it proportionally decreases. The changes in the appearance of these belts and fpots, and the difference in the periods of their rotation round his axis, make it probable that they are not on the surface of Jupiter, but rather clouds transported by the winds with different velocities in his atmosphere, which must be subject to violent agitations. Jupiter is accompanied by four small planets or satellites, revolving round him as a centre, analogous in all respects to the moon which accompanies the earth. They are placed at different distances from his body, and perform their revolutions in different periods, the places of their orbits varying a little the one from the other, and all from that of the orbit of Jupiter himself. By employing half the diameter (the semi-diameter) of Jupiter, as a standard by which to measure the distance of his satellites or moons from his centre, it has been observed, that the nearest to to him, or the first satellite, is distant 5.697 semi-diameters, subtending at the eye of an observer on the earth an angle of 1 minute 51 seconds; and that it performs its revolution round Jupiter in 1 day 18 hours 27 minutes 34 seconds of our time. The second satellite revolves at the distance of 9.017 semi-diameters, subtending an angle of 2 minutes 56 seconds, in 3 days 13 hours 13 minutes 43 seconds. The third satellite, at the distance of 1.4354 semi-diameters, or 4 minutes 42 seconds in seven days 3 hours 42 minutes 36 seconds. The fourth revolves at the distance of 25.266 semi-diameters, subtending an angle of 8 minutes 16 seconds in 16 days 16 hours 32 minutes 9 seconds. The semi-diameter of Jupiter being about 44,585 English miles, his first satellite revolves at a distance of only about 250,000 miles from his centre; his second, at the distance of about 402,000 miles; his third, at 640,000 miles; and his fourth, at the distance of 1,113,600 miles. Saturn. This planet is very bright, but inferior in brilliancy, as well as in magnitude, to Jupiter; his diameter being about eight-ninths of that of the latter. His daily revolution on his axis is equal to 10 hours 16 minutes of our time; but his year or revolution round the sun extends to 10,756 days, or nearly 29 of our years. Saturn is attended by no fewer than seven moons, or satellites. The first revolves at the distance of 4.893 semi-diameters of Saturn from his centre, in 1 day 21 hours 18 minutes 57 seconds of our time; the second at the distance of 6 268 semi-diameters, in 2 days 17 hours, 41 minutes 22 seconds; the third at the distance of 8,754 semi-diameters, in 4 days 12 hours 25 minutes 12 seconds; the fourth at 20.295 semi-diameters, in 15 days 22 hours 41 minutes 12 seconds; the fifth at 59,154 semi-diameters, in 79 days 7 hours 47 minutes. The two remaining satellites of Saturn were discovered in the years 1787, and 1788, and are placed nearer to the body of Saturn than any of the preceding five; but, in order to prevent confusion in speaking of them, they are numbered the sixth and seventh satellites: the 6th revolves round Saturn within the orbit of the first, at the distance of 3 semi-diameters, in 1 day 8 hours 53 miaules, 9 seconds; the seventh, which is the nearest to Saturn's body, at the distance of only 2 semi-diameters, in the short space of 22 hours 40 minutes 46 seconds. These moons, therefore, arranged in the order of their proximity to the body of the planet, are the seventh, the sixth, the first, the second, the third, the fourth, and the fifth, Saturn, observed through a telescope, presents an appearance different from that of every other heavenly body hitherto discovered. It appears that his globe is surrounded in the plane of his equator by a broad thin ring, at a distance from his body equal to the breadth of the ring; so that if the the diameter of his globe be divided into three equal parts, one of these parts would be equal to the breadth of the ring and one to the interval between his body and the ring, making the exterior diameter of the ring equal to seven of such third parts. As we never come to be in a position perpendicular to the plane of the ring, we never can observe it completely round, or even in all parts detached from the body of the planet; and when, by the motions of Saturn' and the Earth, we come to be in the plane of the ring, it then ceases to be visible, in all probability from its being too thin to reflect light to us: in this state, however, the ring has been perceived, with good glasses, as a dark line crossing the planet's surface. The ring revolves round the same axis with the planet itself, but in a longer time, being of a much greater diameter; for Saturn performs his diurnal revolution in 10 hours 16 minutes, while the ring requires 10 hours 32 minutes 15.4 seconds. By more accurate observations, it appears that this ring is probably composed of two concentric rings; for a dark belt, or broad line, runs round the surface of the ring, dividing it into two rings, the inner much broader than the outer. The following are the proportional breadths of these rings, and their distances the one from the other, and from the centre of Saturn. If the distance from his centre to the inner edge of the inner ring be divided into any number of equal parts, as The radius of the outer edge of the same ring is Consequently the breadth of the inner ring is The radius of the outer edge of the inner ring Breadth of belt or space between the rings Parts. 590 751 161 751 774 23 |