nodes. into 60 equal parts, called seconds: so that a second Plate I. is the 60th part of a minute; a minute the 60th part of a degree; and a degree the 360th part of a circle, or 30th part of a sign. The planes of the orbits of all the other planets likewise cut the Sun in halves; but, extended to the heavens, form circles different from one another, and from the ecliptic; one half of each being on the north side, and the other on the south side of it. Consequent- Their ly the orbit of each planet crosses the ecliptic in two opposite points, which are called the planets' nodes. These nodes are all in different parts of the ecliptic; and therefore, if the planetary tracks remained visible in the heavens, they would in some measure resemble the different ruts of waggon wheels, crossing one another in different parts, but never going far asunder. That node, or intersection of the or bit of any planet with the Earth's orbit, from which the planet ascends northward above the ecliptic, is called the ascending node of the planet: and the other, which is directly opposite thereto, is called its descending node. Saturn's ascending node* is in 21 Where sideg. 32 min. of Cancer; Jupiter's in 8 deg. 49 tuate. min. of the same sign; Mars's in 18 deg. 22 min. of Taurus 8; Venus's in 14 deg. 44 min. of Gemini п; and Mercury's in 16 deg. 2 min. of Taurus. Here we consider the Earth's orbit as the standard, and the orbits of all the other planets as oblique to it. what. 21. When we speak of the planets' orbits, all that The plan. is meant is, their paths through the open and unre- ets' orbits, sisting space in which they move, and are retained by the attractive power of the Sun, and the projectile force impressed upon them at first. Between this power and force there is so exact an adjustment, that they continue in the same tracks without any solid orbits to confine them. * In the year 1790. F Plate I. Fig. I. 22. MERCURY, the nearest planet to the Sun, goes round him, in the circle marked 8, in 87 days, 23 hours of our time, nearly; which is the length of his year. But being seldom seen, and no spots appearing on his surface or disc, the time of his rotation on his axis, or the length of his days and nights is as yet unknown. His distance from the Sun is computed to be 32 millions of miles, and his diameter 2600. In his course round the Sun, he moves at the rate of 95 thousand miles every hour. His light and heat from the Sun are almost seven times as great as ours; and the Sun appears to him May be in- almost seven times as large as to us. The great habited. heat on this planet is no argument against its being inhabited; since the Almighty could as easily suit the bodies and constitutions of its inhabitants to the heat of their dwelling, as he has done ours to the temperature of our Earth. And it is very probable that the people there have just such an opinion of us, as we have of the inhabitants of Jupiter and Saturn; namely, that we must be intolerably cold, and have very little light, at so great a distance from the Sun. Has like phases with the Moon. 23. This planet appears to us with all the various phases of the Moon, when viewed at different times by a good telescope: save only, that he never appears quite full, because his enlightened side is never turned directly toward us, but when he is so near the Sun as to be lost to our sight in its beams. And, as his enlightened side is always toward the Sun, it is plain that he shines not by any light of his own; for if he did, he would constantly appear round. That he moves about the Sun in an orbit within the Earth's orbit, is also plain (as will be more largely shewn by and by, 141, & seq.) because he is never seen opposite to the Sun, nor indeed above 56 times the Sun's breadth from his centre. and nodes. 24. His orbit is inclined seven degrees to the Plate I ecliptic. That node, § 20, from which he ascends His orbit northward above the ecliptic, is in the 16th degree of Taurus; and the opposite node, in the 16th degree of Scorpio. The Earth is in these points on the 7th of November and 5th of May; and when Mercury comes to either of his nodes at his* inferior conjunction about these times, he will appear to pass over the disc or face of the Sun, like a dark round spot. But in all other parts of his orbit his conjunctions are invisible; because he either passes above or below the Sun. upon the 25. Mr. WHISTON has given us an account of When several periods at which Mercury might be seen on seen as if the Sun's disc, viz. In the year 1782, Nov. 12th, Sun. at 3 h. 44 m. in the afternoon, 1786, May 4th, at 6 h. 57 m. in the forenoon; 1789, Nov. 5th, at 3 h. 55 m. in the afternoon; and 1799, May 7th, at 2 h. 34 m. in the afternoon. There were several intermediate transits, but none of them visible at London. Fig. 1. 26. VENUS, the next planet in order, is com- Venus, puted to be 59 millions of miles from the Sun; and by moving at the rate of 69 thousand miles every hour, in her orbit in the circle marked ?, she goes round the Sun in 224 days, 17 hours of our time, nearly; in which, though it be the full length of her year, she has only 9 days, according to BIANCHINI's observationst; so that, to her, * When he is between the Earth and the Sun in the nearest part of his orbit. t The elder Cassini had concluded from observations made by himself in 1667, that Venus revolved on her axis m a little more than 23 h. because in 24 h. he found that a spot on her surface was about 15° more advanced than it was at the day before; and it ap peared to him that the spot was very sensibly advanced in a quarter of an hour. In 1728, Bianchini published a splendid work, in foli at Rome, entitled Hesperi et Phosphori nova phænomena; in which are the observations here referred to. Bianchini agrees Plate I. Her orbit lies be tween the every day and night together is as long as 244 days and nights with us. This odd quarter of a day in every year makes every fourth a leap-year to Venus; as the like does to our Earth. Her diameter is 7906 miles; and by her diurnal motion the inhabitants about her equator are carried 43 miles every hour, beside the 69,000 above-mentioned. 27. Her orbit includes that of Mercury within it; for at her greatest elongation, or apparent disEarth and tance from the Sun, she is 96 times the breadth of Mercury. that luminary from his centre; which is almost She is our and even turns. double of Mercury's greatest elongation. Her orbit is included by the Earth's; for if it were not, she might be seen as often in opposition to the Sun, as she is in conjunction with him; but she has ne ver been seen 90 degrees, or a fourth part of a circle from the Sun. 28. When Venus appears west of the Sun, she morning rises before him in the morning, and is called the ing star by morning star: when she appears east of the Sun, she shines in the evening after he sets, and is then called the evening star: being each in its turn for 290 days. It may perhaps be surprising at first view, that Venus should keep longer on the east or west of the Sun, than the whole time of her period round him. But the difficulty vanishes when we consider that the Earth is all the while going round the Sun the same way, though not so quick as Venus and therefore her relative motion to the perfectly with Cassini that the spots, which are seen on the surface of Venus, advance about 15° in 24 h. but he asserts that he could not perceive they had made any advance in 3 h. and therefore concludes that instead of making one complete revolution and 15° of another, as Cassini conjectured, in 24 h. those spots advance but the odd 15° in that time, and that the time of a revolution is somewhat more than 24 days. The arguments in favour of the two hypotheses are very equal; but almost every astronomer, except Mr. Ferguson, has adopted Cassini's. Earth must in every period be as much slower than her absolute motion in her orbit, as the Earth during that time advances forward in the ecliptic; which is 220 degrees. To us she appears, through a telescope, in all the various shapes of the moon. 29. The axis of Venus is inclined 75 degrees to the axis of her orbit; which is 51 degrees more than our Earth's axis is inclined to the axis of the ecliptic and therefore her seasons vary much more than ours do. The north pole of her axis inclines toward the 20th degree of Aquarius; our Earth's to the beginning of Cancer; consequently the northern parts of Venus have summer in the signs where those of our Earth have winter, and vice versa. 30. The artificial day at each pole of Venus is Remarkas long as 112† natural days on our Earth. able ap pearances. polar cir 31. The Sun's greatest declination on each side Her troof her equator amounts to 75 degrees; therefore pics and her tropics are only 15 degrees from her poles; cles how and her polar circles are as far from her equator, situate. Consequently the tropics of Venus are between her polar circles and her poles; contrary to what those of our Earth are. course, 32. As her annual revolution contains only 91 The Sun's of her days, the Sun will always appear to go daily through a whole sign, or twelfth part of her orbit, in a little more than three quarters of her *The time between the Sun's rising and setting. † One entire revolution, or 24 hours. These are lesser circles parallel to the équator, and as many degrees from it, toward the poles, as the axis of the planet is inclined to the axis of its orbit. When the Sun is advanced so far north or south of the equator, as to be directly over either tropic, he goes no farther; but returns toward the other. These are lesser circles round the poles, and as far from them as the tropics are from the equator. The poles are the very north and south points of the planet. |