Читать книгу Astronomy Explained Upon Sir Isaac Newton's Principles - James Ferguson - Страница 12

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139. The Tychonic System § 97, being sufficiently refuted by the 109th Article, we shall say nothing more about it.

140. The Ptolemean System § 96, which asserts the Earth to be at rest in the Center of the Universe, and all the Planets with the Sun and Stars to move round it, is evidently false and absurd. For if this hypothesis were true, Mercury and Venus could never be hid behind the Sun, as their Orbits are included within the Sun’s: and again, these two Planets would always move direct, and be as often in Opposition to the Sun as in Conjunction with him. But the contrary of all this is true: for they are just as often behind the Sun as before him, appear as often to move backwards as forwards, and are so far from being seen at any time in the side of the Heavens opposite to the Sun, that they were never seen a quarter of a circle in the Heavens distant from him.

Appearances of Mercury and Venus.

141. These two Planets, when viewed with a good telescope, appear in all the various shapes of the Moon; which is a plain proof that they are enlightened by the Sun, and shine not by any light of their own: for if they did, they would constantly appear round as the Sun does; and could never be seen like dark spots upon the Sun when they pass directly between him and us. Their regular Phases demonstrate them to be Spherical bodies; as may be shewn by the following experiment.

Experiment to prove they are round.

Hang an ivory ball by a thread, and let any Person move it round the flame of a candle, at two or three yards distance from your Eye: when the ball is beyond the candle, so as to be almost hid by the flame, it’s enlightened side will be towards you, and appear round like the Full Moon: When the ball is between you and the candle, it’s enlightened side will disappear, as the Moon does at the Change: When it is half way between these two positions, it will appear half illuminated, like the Moon in her Quarters: But in every other place between these positions, it will appear more or less horned or gibbous. If this experiment be made with a circular plate which has a flat surface, you may make it appear fully enlightened, or not enlightened at all; but can never make it seem either horned or gibbous.

PLATE II.

Experiment to represent the motions of Mercury and Venus.

142. If you remove about six or seven yards from the candle, and place yourself so that it’s flame may be just about the height of your eye, and then desire the other person to move the ball slowly round the candle as before, keeping it as near of an equal height with the flame as he possibly can, the ball will appear to you not to move in a circle, but rather to vibrate backward and forward like a pendulum; moving quickest when it is directly between you and the candle, and when directly beyond it; and gradually slower as it goes farther to the right or left side of the flame, until it appears at the greatest distance from the flame; and then, though it continues to move with the same velocity, it will seem to stand still for a moment. In every Revolution it will shew all the above Phases § 141; and if two balls, a smaller and a greater, be moved in this manner round the candle, the smaller ball being kept nearest the flame, and carried round almost three times as often as the greater, you will have a tolerably good representation of the apparent Motions of Mercury and Venus; especially, if the bigger ball describes a circle almost twice as large in diameter as the circle described by the lesser.

Fig. III.

The elongations or digressions of Mercury from the Sun.

PLATE II.

143. Let ABCDE be a part or segment of the visible Heavens, in which the Sun, Moon, Planets, and Stars appear to move at the same distance from the Earth E. For there are certain limits, beyond which the eye cannot judge of different distances; as is plain from the Moon’s appearing to be no nearer to us than the Sun and Stars are. Let the circle fghiklmno be the Orbit in which Mercury m moves round the Sun S, according to the order of the letters. When Mercury is at f, he disappears to the Earth at E, because his enlightened side is turned from it; unless he be then in one of his Nodes § 20, 25; in which case, he will appear like a dark spot upon the Sun. When he is at g in his Orbit, he appears at B in the Heavens, westward of the Sun S, which is seen at C: when at h, he appears at A, at his greatest western elongation or distance from the Sun; and then seems to stand still. But, as he moves from h to i, he appears to go from A to B; and seems to be in the same place when at i as when he was at g, only not near so big: at k he is hid from the Earth E by the Sun S; being then in his superiour Conjunction. In going from k to l, he appears to move from C to D; and when he is at n, he appears stationary at E; being seen as far east from the Sun then, as he was west from him at A. In going from n to o in his Orbit, he seems to go back again in the Heavens, from E to D; and is seen in the same place (with respect to the Sun) at o as when he was at l; but of a larger diameter at o, because he is then nearer the Earth E: and when he comes to f, he again passes by the Sun, and disappears as before. In going from n to h in his Orbit, he seems to go backward in the Heavens from E to A; and in going from h to n, he seems to go forward from A to E. As he goes on from f a little of his enlightened side at g is seen from E; at h he appears half full, because half of his enlightened side is seen; at i, gibbous, or more than half full; and at k he would appear quite full, were he not hid from the Earth E by the Sun S. At l he appears gibbous again; at n half decreased, at o horned, and at f new like the Moon at her Change. He goes sooner from his eastern station at n to his western station at h than from h to n again; because he goes through less than half his Orbit in the former case, and more in the latter.

Fig. III.

The Elongations and Phases of Venus.

The greatest Elongations of Mercury and Venus.

144. In the same Figure, let FGHIKLMN be the Orbit in which Venus v moves round the Sun S, according to the order of the letters: and let E be the Earth as before. When Venus is at F she is in her inferiour Conjunction; and disappears like the New Moon because her dark side is toward the Earth. At G she appears half enlightened to the Earth, like the Moon in her first quarter: at h she appears gibbous; at I, almost full; her enlightened side being then nearly towards the Earth: at K, she would appear quite full to the Earth E; but is hid from it by the Sun S: at L, she appears upon the decrease, or gibbous; at M, more so; at N, only half enlightened; and at F she disappears again. In moving from N to G, she seems to go backward in the Heavens; and from G to N, forward: but, as she describes a much greater portion of her Orbit in going from G to N than from N to G, she appears much longer direct than retrograde in her motion. At N and G she appears stationary; as Mercury does at n and h. Mercury, when stationary seems to be only 28 degrees from the Sun; and Venus when so, 47; which is a demonstration that Mercury’s Orbit is included within Venus’s, and Venus’s within the Earth’s.

Morning and Evening Star, what.

145. Venus, from her superiour Conjunction at K to her inferiour Conjunction at F is seen on the east side of the Sun S from the Earth. E; and therefore she shines in the Evening after the Sun sets, and is called the Evening Star: for, the Sun being then to the westward of Venus, he must set first. From her inferiour Conjunction to her superiour, she appears on the west side of the Sun; and therefore rises before him, for which reason she is called the Morning Star. When she is about N or G, she shines so bright, that bodies cast shadows in the night-time.

PLATE II.

The stationary places of the Planets variable.

146. If the Earth kept always at E, it is evident that the Stationary places of Mercury and Venus would always be in the same points of the Heavens where they were before. For example; whilst Mercury m goes from h to n, according to the order of the letters, he appears to describe the arc ABCDE in the Heavens, direct: and whilst he goes from n to h, he seems to describe the same arc back again, from E to A, retrograde: always at n and h he appears stationary at the same points E and A as before. But Mercury goes round his Orbit, from f to f again, in 88 days; and yet there are 116 days from any one of his Conjunctions, or apparent Stations, to the same again: and the places of these Conjunctions and Stations are found to be about 114 degrees eastward from the points of the Heavens where they were last before; which proves, that the Earth has not kept all that time at E, but has had a progressive motion in it’s Orbit from E to t. Venus also differs every time in the places of her Conjunctions and Stations; but much more than Mercury; because, as Venus describes a much larger Orbit than Mercury does, the Earth advances so much the farther in it’s annual path before Venus comes round again.

The Elongations of all Saturn’s inferiour Planets as seen from him.

147. As Mercury and Venus, seen from the Earth, have their respective Elongations from the Sun, and Stationary places; so has the Earth, seen from Mars; and Mars, seen from Jupiter; and Jupiter, seen from Saturn. That is, to every superiour Planet, all the inferiour ones have their Stations and Elongations; as Venus and Mercury have to the Earth. As seen from Saturn, Mercury never goes above 2¹⁄₂ degrees from the Sun; Venus 4¹⁄₃; the Earth 6; Mars 9¹⁄₂; and Jupiter 33¹⁄₄: so that Mercury, as seen from the Earth, has almost as great a Digression or Elongation from the Sun, as Jupiter seen from Saturn.

A proof of the Earth’s annual motion.

148. Because the Earth’s Orbit is included within the Orbits of Mars, Jupiter, and Saturn, they are seen on all sides of the Heavens; and are as often in Opposition to the Sun as in Conjunction with him. If the Earth stood still, they would always appear direct in their motions, never retrograde nor stationary. But they seem to go just as often backward as forward; which, if gravity be allowed to exist, affords a sufficient proof of the Earth’s annual motion.

Fig. III.

PLATE II.

General Phenomena of a superiour Planet to an inferiour.

149. As Venus and the Earth are superiour Planets to Mercury, they shew much the same Appearances to him that Mars and Jupiter do to us. Let Mercury m be at f, Venus v at F, and the Earth at E; in which situation Venus hides the Earth from Mercury; but, being in opposition to the Sun, she shines on Mercury with a full illumined Orb; though, with respect to the Earth, she is in conjunction with the Sun and invisible. When Mercury is at f, and Venus at G, her enlightened side not being directly towards him, she appears a little gibbous; as Mars does in a like situation to us: but, when Venus is at I, her enlightened side is so much towards Mercury at f, that she appears to him almost of a round figure. At K, Venus disappears to Mercury at f, being then hid by the Sun; as all our superiour Planets are to us, when in conjunction with the Sun. When Venus has, as it were, emerged out of the Sun beams, as at L, she appears almost full to Mercury at f; at M and N, a little gibbous; quite full at F, and largest of all; being then in opposition to the Sun, and consequently nearest to Mercury at f; shining strongly on him in the night, because her distance from him then is somewhat less than a fifth part of her distance from the Earth, when she appears roundest to it between I and K, or between K and L, as seen from the Earth E. Consequently, when Venus is opposite to the Sun as seen from Mercury, she appears more than 25 times as large to him as she does to us when at the fullest. Our case is almost similar with respect to Mars, when he is opposite to the Sun; because he is then so near the Earth, and has his whole enlightened side towards it. But, because the Orbits of Jupiter and Saturn are very large in proportion to the Earth’s, these two Planets appear much less magnified at their Oppositions or diminished at their Conjunctions than Mars does, in proportion to their mean apparent Diameters.