[Sidenote: _Inferior_ and _Superior Planets_.]
We have now shewn that all the Planets turn round the Sun, and that _Mercury_ and _Venus_ are included between him and the Earth, whence they are called the _Inferior Planets_, and that the Earth is placed between the Orbits of _Mars_ and _Venus_, and therefore included within the Orbits of _Mars_, _Jupiter_, and _Saturn_, whence they are called the _Superior Planets_: And since the Earth is in the middle of these moveable bodies, and is of the same nature with them, we may conclude that she has the same sort of motions; but that she turns round the Sun is proved thus:
[Sidenote: The Earth does not stand still, but turns round the Sun.]
All the Planets seen from the Earth appear to move very unequally, as sometimes to go faster, at other times slower; sometimes to go backwards, and sometimes to be stationary, or not to move at all; which could not happen if the Earth stood still. Let S be the Sun, T the Earth, the great circle A B C D the Orbit of _Mars_, and the numbers 1, 2, 3, _&c._ its equable motion round the Sun; the correspondent numbers 1, 2, 3, _&c._ in the circle _a_, _b_, _c_, _d_, the motion of _Mars_, as it would be seen from the Earth. It is plain from this Figure, that if the Earth stood still, the motion of _Mars_, will be always progressive, (tho’ sometimes very unequal;) but since observations prove the contrary, it necessarily follows, that the Earth turns round the Sun.
[Sidenote: The Annual and Diurnal Motions of the Planets, how computed.]
The annual periods of the Planets round the Sun are determined by carefully observing the length of time since their departure from a certain point in the Heavens, (or from a fix’d Star) until they arrive to the same again. By these sort of observations the ancients determined the periodical revolutions of the Planets round the Sun, and were so exact in their computations, as to be capable of predicting Eclipses of the Sun and Moon. But since the invention of telescopes, astronomical observations are made with greater accuracy; and of consequence, our tables are far more perfect than those of the ancients. And in order to be as exact as possible, astronomers compare observations made at a great distance of time from one another, including several periods; by which means, the error that might be in the whole, is in each period subdivided into such little parts as to be inconsiderable. Thus the mean length of a Solar Year is known, even to Seconds.