These different motions observed in the Planets, are easily illustrated, as followeth: The lesser circle round the Sun is the orbit of Mercury, in which he performs his revolution round the Sun, in about three months, or while the Earth is going thro’ ¼ part of her orbit, or from A to N. The numbers 1, 2, 3, &c. in the orbit of Mercury, show the spaces he describes in a week nearly, and the distance AB, BC, DC, &c. in the Earth’s orbit, do likewise show her motion in the same time. The letters A, B, C, &c. in the great orb, are the motions of Mercury in the Heavens, as they appear from the Earth. Now if the Earth be supposed in A, and Mercury in 12, near his superior conjunction with the Sun; a spectator on the Earth will see ☿, as if he were in the point of the Heavens A, and while ☿ is moving from 12 to 1, and from 1 to 2, &c. the Earth in the same time also moves from A to B, and from B to C, &c. All which time ☿ appears in the Heavens to move in a direct motion from A to B, and from B to C, &c. but gradually slower, until he arrives near the point G; near this place he appears stationary, or to stand still; and afterwards (tho’ he still continues to move uniformly in his own orbit, with a progressive motion) yet in the sphere of the fixed Stars he will appear to be retrograde, or to go backwards, as from G to H, from H to I, &c. until he has arrived near the point L, where again he will appear to be stationary; and afterwards to move in a direct motion from L to M, and from M to N, &c.
What has been here shewed concerning the motions of Mercury, is also to be understood of the motions of Venus; but the conjunctions of Venus with the Sun do not happen so often as in Mercury; for Venus moving in a larger orbit, and much slower than Mercury, does not so often overtake the Earth. But the retrogradations are much greater in Venus than they are in Mercury, for the same reasons.
The innermost circle represents the Earth’s orbit, divided into 12 parts, answering to her monthly motion; the greatest circle is in the orbit of Jupiter, which he describes in about 12 years; and therefore the ¹/₁₂ thereof, from A to N, defines his motion, in one of our years nearly; and the intermediate divisions, A, B, C, &c. his monthly motion. Let us suppose the Earth to be in the point of her orbit 12, and Jupiter in A, in his conjunction with the Sun; it is evident that from the Earth Jupiter will be seen in the great orb, or in the point of the Heavens A, and while the Earth is moving from 12 to 1, 2, &c. ♃ also moves from A to B, &c. all which time he appears in the Heavens to move with a direct motion from A to B, C, &c., until he comes in opposition to the Earth near the point of the Heavens E, where he appears to be stationary; after which ♃ again begins to move ’ (tho’ at first with a slow pace) from E through F, H, I to K, where again he appears to stand still, but afterwards he reassumes his direct motion from I thro’ K, to M, &c.
From the construction of the preceding figure it appears, that when the superior Planets are in conjunction with the Sun, their direct motion is much quicker than at other times; and that because they really move from West to East, while the Earth in the opposite part of the Heavens is carried the same way, and round the same center. This motion afterwards continually slackens until the Planet comes almost in opposition to the Sun, when the line joining the Earth and Planet, will continue for some time nearly parallel to itself, and so the Planet seems from the Earth to stand still; after which, it begins to move with a slow motion backward, until it comes into a quartile aspect with the Sun, when again it will appear to be stationary, for the above reasons; after that it will resume its direct motion, until it comes into a conjunction with the Sun, then it will proceed as above explained. Hence it also appears, that the retrogradations of the superior Planets are much slower than their direct motions, and their continuance much shorter; for the Planet, from its last quarter, until it comes in opposition to the Sun, appears to move the same way with the Earth, by whom it is then overtaken: After which it begins to go backwards, but with a slow motion, because the Earth being in the same part of the Heavens, and moving the same way that the Planet really does, the apparent motion of the Planet backwards, must thereby be lessened.
What has been here said concerning the motions of Jupiter, is also to be understood of Mars and Saturn. But the retrogradations of Saturn do oftener happen than those of Jupiter, because the Earth oftener overtakes Saturn; and for the same reason, the regressions of Jupiter do oftener happen than those of Mars. But the retrogradations of Mars are much greater than those of Jupiter, whose are also much greater than those of Saturn.
In either of the satellites of Jupiter or Saturn, these different appearances in the neighbouring Worlds are much oftener seen than they are by us in the primary Planets.
We never observe these different motions in the Moon, because she turns round the Earth as her center; neither do we observe them in the Sun, because he is the center of the Earth’s motion; whence the apparent motion of the Sun always appears the same way round the Earth.
Of the Annual and Diurnal Motion of the Earth, and of the increase and decrease of Days and Nights.
The Earth in her annual motion round the Sun, has her axis always in the same direction, or parallel to itself; that is, if a line be drawn parallel to the axis, while the Earth is in any point of her orbit, the axis in all other positions of the Earth will be parallel to the said line. This parallelism of the axis, and the simple motion of the Earth in the ecliptic, solves all the phænomena of different seasons. These things are very well illustrated by the Orrery.