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The Phenomena of the Heavens as seen from diffe
rent Parts of the Solar System.
O vastly great is the distance of the starry 132.
heavens, that if viewed from any part of the sular system, or even many millions of miles beyond it, the appearance would be the very same as it is to us. The Sun and stars would all seem to be fixed on one concave surface, of which the spectator's eye would be the centre.
But the planets, being much nearer than the stars, their appearances will vary considerably with the place froin which they are viewed.
. 133. If the spectator be at rest without the orbits of the planets, they will seem to be at the same distance as the stars; but continually changing their places with respect to the stars, and to one another; assuming various phases of increase and decrease like the Moon; and, notwithstanding their regular motions about the Sun, will sometimes appear to move quicker, sometimes slower, be as often to the west as to the east of the Sun, and at their greatest distances seem quite stationary. The duration, extent, and distance, of those points in the heavens where these digressions begin and end, would be more or less, according to the respective distances of the several planets from the Sun : but in the same planet, they would continue invariably the same at all times; —like pendulums of unequal lengths oscillating together, the shorter would move quick, and go over a small space; the longer would move slow, and go over a large space. If the observer be at rest within the orbits of the planets, but not near the common centre,their apparent motions will be irregular; but less so than in the former case. Each of the several planets will appear larger and less by turns, as they approach
nearer to, or recede farther from, the observer; the nearest varying most in their size. They will also move quicker or slower with regard to the fixed stars, but will never be either retrograde or stationary.
134. If an observer in motion view the heavens, the same apparent irregularities will be observed, but with some variation resulting from his own motion. If he be on a planet which has a rotation on its axis, not being sensible of his own motion, he will imagine the whole heavens, Sun, planets, and stars, to revolve about him in the same time that his planet turns round, but the contrary way; and will not be easily convinced of the deception. If his planet move round the Sun, the same irregularities and aspects as above mentioned will appear in the motions of the other planets; and the Sun will seem to move among the fixed stars or signs, in an opposite direction to that in which his planet moves, changing its place every day as he does. In a word, whether our observer be in motion or at rest, whether within or without the orbits of the planets, their motions will seem irregular, intricate, and perplexed, unless he be placed in the centre of the system; and from thence, the most beautiful order and har. mony will be seen by him.
135. The Sun being the centre of all the planets' The Sun's motions, the only place from which their motions only point could be truly seen, is the Sun's centre; where the from observer being supposed not to turn round with the which the Sun (which, in this case, we must imagine to be a tions and transparent body) would see all the stars at rest, places of
the plaand seemingly equidistant from him. To such an nets could observer, the planets would appear to move among be seen. the fixed stars; in a simple, regular, and uniform manner : only, that as in equal times they describe equal areas, they would describe spaces somewhat unequal, because they move in elliptic orbits, $ 155. Their motions would also appear to be what they are in fact, the same way round the heavens ; in
paths which cross at small angles in different parts of the heavens, and then separate a little from one another, $ 20. So that, if the solar astronomer should make the path or orbit of any planet a standard, and consider it as having no obliquity, | 201, he would judge the paths of all the rest to be inclined to it; each planet having one half of its path on one side, and the other half on the opposite side of the standard-path or orbit. And if he should ever see all the planets start from a conjunction with each other*, Mercury would move so much faster than Venus, as to overtake her again (though not in the same point of the heavens) in a space of time about equal to 145 of our days and nights, or, as we commonly call them, natural days, which include both the days and nights : Venus would move so much faster than the Earth, as to overtake it again in 585 natural days: the Earth so much faster than Mars, as to overtake him again in 778 such days: Mars so much faster than Jupiter, as to overtake him again in 817 such days: and Jupiter so much faster than Saturn, as to overtake him again in 7236 days, all
of our time. The judg- 136. But as our solar astronomer could have no a a solar as. idea of measuring the courses of the planets by our tronomer days, he wouid probably take the period of Merprobably cury, which is the quickest-moving planet, for a make con- measure to compare the periods of the others with. cerning As all the stars would appear quiescent to him, he tances and would never think that they had any dependance magni
the Sun; but would naturally imagine that tudes of
upon the pla. the planets have, because they move round the
Sun. And it is by no means improbable, that he
* Here we do not mean such a conjunction, as that the nearest planet should hide all the rest from the observer's sight; (for that would be impossible, unless the intersections of all their orbits were coincident, which they are not. See § 21.) but when they were all in a line crossing the standard-orbit at right angles.
would conclude those planets, whose periods are quickest, to move in orbits proportionably less than those do which make slower circuits. But being destitute of a method for finding their parallaxes, or, more properly speaking, as they would have no parallax to him, he could never know any thing of their real distances or magnitudes. Their relative distances he might perhaps guess at by their periods, and from thence infer something of truth concerning their relative magnitudes, by comparing their appa. rent magnitudes with one another. For example, Jupiter appearing larger to him than Mars, he would conclude it to be so in fact; and that it must be farther from him, on account of its longer period. Mercury and the Earth would appear to be nearly of the same magnitude; but by comparing the pe. riod of Mercury with that of the Earth, he would conclude that the Earth is much farther from him than Mercury, and consequently that it must be really larger though apparently of the same magnitude; and so of the rest. And as each planet would appear somewhat larger in one part of its orbit than in the opposite, and to move quickest when it seems. largest, the observer would be at no loss to conclude that all the planets move in orbits, of which the Sun is not precisely the centre.
137. The apparent magnitudes of the planets The placontinually change as seen from the Earth, which metarymodemonstrates that they approach nearer to it, and irregular recede arther from it by turns. From these phe- from the nomena, and their apparent motions among the Earth. stars, they seem to describe looped curves, which never return into themselves ---Venus's path excepted. And if we were to trace out all their apparent paths, and put the figures of them together in one diagram, they would appear so anomalous and confused, that no man in his senses could be. lieve them to be representations of their real paths; but would immediately conclude, that such appa.
Plate III. rent irregularities must be owing to some optic illu
sions. And after a good deal of enquiry, he might perhaps be at a loss to find out the true causes of these irregularities; especially if he were one of those who would rather, with the greatest justice, charge frail man with ignorance, than the Almighty with being the author of such confusion.
138. Dr. Long, in his first volume of Astronomy, and Venus has given us figures of the apparent paths of all the represent- planets, separately from CASSINI; and on seeing
them I first thought of attempting to trace some of them by a machine* that shews the motions of the Sun, Mercury, and Venus, the Earth, and Moon, according to the Copernican System. Having taken off the Sun, Mercury, and Venus, I put black-lead pencils in their places, with the points turned upward ; and fixed a circular sheet of paste-board so, that the Earth kept constantly under its centre in going round the Sun; and the paste-board kept its parallelism. Then, pressing gently with one hand upon the paste-board, to make it touch the three pencils; with the other hand I turned the winch that moves the whole machinery : and as the Earth,
together with the pencils in the places of Mercury Fig. 1.
and Venus, had their proper motions round the Sun's pencil, which kept at rest in the centre of the machine, all the three pencils described a diagram, from which the first figure of the third plate is truly copied in a smaller size. As the Earth moved round the Sun, the Sun's pencil described the dotted circle of months, whilst Mercury's pencil drew the curve with the greatest number of loops, and Venus's that with the fewest. In their inferior conjunctions they come as much nearer to the Earth, or within the circle of the Sun's apparent motion round the heavens, as they go beyond it in their superior conjunctions. On each side of the loops they appear stationary: in that part of
* The ORRERY fronting the Title-page.