If while you turn the handle you observe the Planets, you will see them perform their motions in the same relative times as they really do in the heavens, each making its period in the times mentioned in the Tables, Page, 28, 27¼ turns of the handle will bring the Moon round the Earth, which is called a Periodic Month; and all the while she keeps the same face towards the Earth; for the Moon’s annual and diurnal motion are performed both in the same time nearly, so that we always see the same face or side of the Moon.
If before the instrument is put into motion, the satellites of Jupiter and Saturn be brought into the same right line from their respective primaries, you will see them, as you turn the handle, immediately dispersed from one another, according to their different celerities. Thus one turn of the handle will bring the first of Jupiter’s Moons about ⁴/₇ part round Jupiter, while the second has described but ²/₇ part, the third but above ¹/₇, and the fourth not quite ¹/₁₆ part, each of its respective orbits. If you turn the handle until the hour index has moved 18½ hours more, the first satellite will then be brought into its former position, and so has made one entire revolution; the second at the same time will be almost diametrically opposite to the first, and so has made a little more than half of one revolution; the others will be in different aspects, according to the length of their periods, as will be plainly exhibited by the instrument. The same observations may be made with respect to the satellites of Saturn.
The machine is so contrived, that the handle may be turned either way; and, if before you put it into motion, you observe the aspect (or situation with respect to each other) of the Planets, and then turn the handle round any number of times; the same number of revolutions being made backwards, will bring all the Planets to their former situations. I shall next proceed to particulars.
Of the Stations and Retrogradations of the Planets.
Retrograde Motion of the Planets.
The primary Planets, as they all turn round the Sun, at different distances, and in different times, appear to us from the Earth to have different motions; as sometimes they appear to move from West to East, according to the order of the signs, which is called their Direct Motion; then by degrees they slacken their pace, until at last they lose all their motion, and become Stationary, or not to move at all; that is, they appear in the same place with respect to the fixed Stars for some time together; after which they again begin to move, but with a contrary direction, as from East to West, which is called their Retrograde Motion; then again they become stationary, and afterwards reassume their direct motion. The reason of all these appearances is very evidently shewn by the Orrery.
Of the Stations, &c. of the Inferior Planets.
We shall instance in the Planet Mercury, because his motion round the Sun differs more from the Earth’s than that of Venus does.
When Mercury is in his superior conjunction (or when he is in a direct line from the Earth beyond the Sun) fasten a string about the axis of the Earth, and extend it over Mercury to the ecliptic; then turning the handle, keep the thread all the while extended over ☿, and you will find it move with a direct motion in the ecliptic, but continually slower, until Mercury has the greatest elongation from the Earth. Near this position, the thread for some time will lay over Mercury without being moved in the ecliptic, tho’ the Earth and Mercury both continue their progressive motion in their respective orbits. When Mercury has got a little past this place, you will find the thread must be moved backward in the ecliptic, beginning first with a slow motion, and then faster by degrees, until Mercury is in his inferior conjunction, or directly between the Earth and the Sun. Next this position of ☿, his retrograde motion will be the swiftest; but he still moves the same way, tho’ continually slower, ’till he has again come to his greatest elongation, where he will appear the second time to be stationary; after which he begins to move forward, and that faster by degrees, until he is come to the same position with respect to the Earth, that he was in at first. The same observations may be made relating to the motions of Venus. In like manner the different motions observed in the superior Planets may be also explained by the Orrery. If you extend the thread over Jupiter, and proceed after the same manner as before we did in regard to Mercury, you will find that from the time Jupiter is in conjunction with the Sun, his motion is direct, but continually slower, until the Earth is nearly in a quadrate aspect with Jupiter, near which position Jupiter seems to be stationary: After which he begins to move, and continually mends his pace, until he comes in opposition to the Sun, at which time his retrograde motion is swiftest. He still seems to go backward, but with a slower pace, ’till the Earth and he are again in a quadrate aspect, where Jupiter seems to have lost all his motion; after which he again resumes his direct motion, and so proceeds faster by degrees, ’till the Earth and he are again in opposition to each another.