PLATE VII. An idea of the Earth's the Moon's. ing in a progressive circle, and the former in a curve of different degrees of concavity, which would always be the same in the same parts of the heavens, if the Moon performed a complete number of lunations in a year, without any fraction. 266. Let a nail in the end of the axle of a chariot-wheel represent the Earth, and a pin in the nave path, and the Moon; if the body of the chariot be propped up so as to keep that wheel from touching the ground, and the wheel be then turned round by hand, the pin will describe a circle both round the nail and in the space it moves through. But if the props be taken away, the horses put to, and the chariot driven over a piece of ground which is circularly convex; the nail in the axle will describe a circular curve, and the pin in the nave will still describe a circle round the progressive nail in the axle, but not in the space through which it moves. In this case the curve described by the nail, will resemble, in miniature, as much of the Earth's annual path round the Sun, as it describes while the Moon goes as often round the Earth as the pin does round the nail: and the curve described by the nail will have some resemblance to the Moon's path during so many lunations. Let us now suppose that the radius of the circular curve described by the nail in the axle is to the radius of the circle which the pin in the nave describes round the axle as 337 to 1; which is the proportion of the radius or semi-diameter of the Earth's orbit to that of the Moon's; or of the circular curve A 1 2 3 4 5 6 7 B, &c. to the little circle a; and then while the progressive nail describes the said curve from A to E, the pin will go once round the nail with regard to the centre of its path, and in so doing, will describe the curve a b c d e. The former will be a true representation of the Earth's path for one lunation, and the latter of the Moon's for that time. Here we may set aside the inequalities of the Moon's motion, and also those of the VII. Earth's moving round their common centre of gra- PLATE vity all which, if they were truly copied in this experiment, would not sensibly alter the figure of the paths described by the nail and pin, even though they should rub against a plane upright surface all the way, and leave their tracks visibly upon it. And if the chariot were driven forward on such a convex piece of ground, so as to turn the wheel several times round, the track of the pin in the nave would still be concave toward the centre of the circular curve described by the pin in the axle: as the Moon's path is always concave to the Sun in the centre of the Earth's annual orbit. In this diagram, the thickest curve-line ABCDE, with the numeral figures set to it, represents as much of the Earth's annual orbit as it describes in 32 days from west to cast; the little circles at a, b, c, d, e, shew the Moon's orbit in due proportion to the Earth's; and the smallest curve abcdef rcpresents the line of the Moon's path in the heavens for 32 days, accounted from any particular new Moon at a. The machine Fig. 5th, is for delineating the Moon's path, and shall be described, with the rest of my astronomical machinery in the last chapter. The Sun is supposed to be in the centre of the curve A1234567 B, &c. and the small dotted circles upon it, represent the Moon's orbit, of which the radius is in the same proportion to the Propor Earth's path in this scheme, that the radius of the Moon's Moon's orbit in the heavens bears to the radius of orbit to the Earth's annual path round the Sun: that is, as Earth's. 240,000, to 81,000,000*, or as 1 to 337. tion of the the When the Earth is at A, the new Moon is at a ; and in the seven days that the Earth describes the curve 1 2 3 4 5 6 7, the Moon in accompanying the Fig. II. Earth describes the curve a b; and is in her first quarter at when the Earth is at B. As the Earth * For the true distances, see p. 138. PLATE VII. The motion al describes the curve B 8 9 10 11 12 13 14, the Moon describes the curve b c; and is at c, opposite to the Sun, when the Earth is at C. While the Earth describes the curve C 15 16 17 18 19 20 21 22, the Moon describes the curve cd; and is in her third quarter at d when the Earth is at D. And lastly, while the Earth describes the curve D 23 24 25 26 27 28 29, the Moon describes the curve de; and is again in conjunction at e with the Sun when the Earth is at E, between the 29th and 30th day of the Moon's age, accounted by the numeral figures from the new Moon at A. In describing the curve abc de, the Moon goes round the progressive Earth as really as if she had kept in the dotted circle A, and the Earth continued immoveable in the centre of that circle. And thus we see that, although the Moon goes Moon's round the Earth in a circle, with respect to the ways con- Earth's centre, her real path in the heavens is not ward the very different in appearance from the Earth's path. cave to Sun. motion is alter To shew that the Moon's path is concave to the Sun, even at the time of change, it is carried on a little farther into a second lunation, as to f 267. The Moon's absolute motion from her change to her first quarter, or from a to b, is so much slower than the Earth's, that she falls 240 thousand miles (equal to the semi-diameter of her orbit) behind the Earth at her first quarter in b, when the Earth is at B; that is, she falls back a space equal How her to her distance from the Earth. From that time her motion is gradually accelerated to her opposition or nately re- full at c, and then she is come up as far as the Earth, tarded and having regained what she lost in her first quarter from a to b. a to b. From the full to the last quarter at d, her motion continues accelerated, so as to be just as far before the Earth at d, as she was behind it at her first quarter in b. But from d to e her motion is retarded, so that she loses as much with respect to the Earth as is equal to her distance from it, or to the semi-diameter of her orbit; and by that means accelerat ed. VII. she comes to e, and is then in conjunction with the PLATE Sun as seen from the Earth at E. Hence we find, that the Moon's absolute motion is slower than the Earth's from her third quarter to her first; and swifter than the Earth's from her first quarter to her third; her path being less curved than the Earth's in the former case, and more in the latter. Yet it is still bent the same way toward the Sun; for if we imagine the concavity of the Earth's orbit to be measured by the length of a perpendicular line Cg, let down from the Earth's place upon the straight line bgd at the full of the Moon, and connecting the places of the Earth at the end of the Moon's first and third quarters, that length will be about 640 thousand miles; and the Moon when new only approaching nearer to the Sun by 240 thousand miles than the Earth, is the length of the perpendicular let down from her place at that time upon the same straight line, all which shews that the concavity of that part of her path, will be about 400 thousand miles. ed. 268. The Moon's path being concave to the Sun A difficul throughout, demonstrates that her gravity toward ty remov the Sun at her conjunction, exceeds her gravity toward the Earth. And if we consider that the quantity of matter in the Sun is almost 230 thousand times as great as the quantity of matter in the Earth, and that the attraction of each body diminishes as the square of the distance from it increases, we shall soon find, that the point of equal attraction between the Earth and the Sun, is about 70 thousand miles nearer the Earth than the Moon is at her change. It may then appear surprising that the Moon does not abandon the Earth, when she is between it and the Sun, because she is considerably more attracted by the Sun than by the Earth at that time. But this difficulty vanishes when we consider, that a common impulse on any system of bodies affects PLATE VII. Fig. III. The absolute path delineat ed. not their relative motions; but that they will continue to attract, impel, or circulate round one another, in the same manner as if there were no such impulse. The Moon is so near the Earth, and both of them so far from the Sun, that the attractive power of the Sun may be considered as equal on both: and therefore the Moon will continue to circulate round the Earth nearly in the same manner as if the Sun did not attract them at all. For bodies in the cabin of a ship, may move round, or impel one another in the same manner when the ship is under sail, as when it is at rest; because they are all equally affected by the common motion of the ship. If by any other cause, such as the near approach of a comet, the Moon's distance from the Earth should happen to be so much increased, that the difference of their gravitating forces toward the Sun should exceed that of the Moon toward the Earth; in that case the Moon when in conjunction, would abandon the Earth, and be either drawn into the Sun or comet, or circulate round about it. 269. The curves which Jupiter's satellites de-. scribe, are all of different sorts from the path describ. ed by our Moon, although these satellites go round Jupiter as the Moon goes round the Earth. Let ABCDE, &c. be as much of Jupiter's orbit as he describes in 18 days from A to T; and the curves' a, b, c, d, will be the paths of his four moons going round him in his progressive motion. Now let us suppose all these moons to set out of Jupiter from a conjunction with the Sun, as seen from Jupiand his ter at A; then his first or nearest moon will be at a, satellites his second at b, his third at c, and his fourth at d. At the end of 24 terrestrial hours after this conjunction, Jupiter has moved to B, his first moon or satellite has described the curve a 1, his second the curve b 1, his third c 1, and his fourth d 1. The next day, when Jupiter is at C, his first satellite has |
