17. CHAP. II. A brief Description of the SOLAR SYSTEM. THE The Solar HE Sun, with the planets and comets Plate 1. Fig. I.. which move round him as their centre, constitute the solar system. Those planets which are near the Sun not only finish their circuits sooner, but likewise move faster in their respective orbits, than those which are more remote from him. Their motions are all performed from west to east, in orbits nearly circular. Their names, distances, magnitudes, and periodical revolutions, are as follows: System. 18. The Sun, an immense globe of fire, is The Sun. placed near the common centre, or rather in the lower* focus of the orbits of all the planets and comets; and turns round his axis in 25 days 6 hours, as is evident by the motion of spots seen on his surface. His diameter is computed to be 763,000 Fig. 1; miles; and by the various attractions of the circumvolving planets, he is agitated by a small motion * If the two ends of a thread be tied together, and the thread be then thrown loosely round two pins stuck in a table, and moderately stretched by the point of a black-lead pencil carried round by an even motion, and light pressure of the hand, and oval or ellipsis will be described; and the points where the pins are fixed are called the foci or focuses of the ellipsis. The orbits of all the planets are elliptical, and the Sun is placed in or near one of the foci of each of them: and that in which he is placed, is called the lower focus. † Astronomers are not far from the truth when they reckon the Sun's centre to be in the lower focus of all the planetary orbits. Though, strictly speaking, if we consider the focus of Mercury's orbit to be in the Sun's centre, the focus of Venus's orbit will be in the common centre of gravity of the Sun and Mercury; the focus of the Earth's orbit in the common centre of gravity of the Sun, Mercury, and Venus; the focus of the orbit of Mars in the common centre of gravity of the Sun, Mercury, Venus, and the Earth; and so of the rest. Yet the focuses of the orbits of all the planets, except Saturn, will not be sensibly removed from the centre of the Sun; nor will the focus of Saturn's orbit recede sensibly from the common centre of gravity of the Sun and Jupiter. Plate I. round the centre of gravity of the system. All the planets, as seen from him, move the same way, and according to the order of the signs in the graduated circle 8, &c. which represents the great ecliptic in the heavens: but, as seen from any one planet, the rest appear sometimes to go backward, sometimes forward, and sometimes to stand still. These apparent motions are not in circles nor in ellipses, but* in looped curves, which never return into themselves. The comets come from all parts of the heavens, and move in all directions. 19. Having mentioned the Sun's turning round his axis, and as there will be frequent occasion to speak of the like motion of the Earth and other planets, it is proper here to inform the young Tyro in astronomy, that neither the Sun nor planets have material axes to turn upon, and support them, as The axes in the little imperfect machines contrived to repreof the pla-sent them. For the axis of a planet is an imginary nets, what. Their or bits are line, conceived to be drawn through its centre, about which it revolves as if on a real axis. The extremities of this axis, terminating in opposite points of the planet's surface, are called its poles. That which points toward the northern part of the heavens, is called the north pole; and the other, pointing toward the southern part, is called the south pole. A bowl whirled from one's hand into the open air, turns round such a line within itself, while it moves forward; and such are the lines we mean, when we speak of the axes of the heavenly bodies. 20. Let us suppose the Earth's orbit to be a thin, not in the even, solid plane; cutting the Sun through the censame tre, and extended out as far as the starry heavens, plane with where it will mark the great circle called the ecliptic. This circle we suppose to be divided into 12 equal parts, called signs; each sign into 30 equal parts, called degrees; each degree into 60 equal parts, called minutes; and each minute into 60 the eclip tic. * As represented in Plate III. Fig. I. and described § 138, 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 nodes. 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 orbit 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 ; 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 planis 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. 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 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. habited. Has like phases Moon. 23. This planet appears to us with all the variwith the ous 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. 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 Fig. I. 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 91 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. †The elder Cassini had concluded from observations made by himself in 1667, that Venus revolved on her axis in 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 folio, at Rome, entitled Hesperi et Phosphori nova phænomena; in which are the observations here referred to. Bianchini agrees |