_The_ Ptolemean _System refuted. The Motions and Phases of Mercury and
Venus explained._
139. The _Tychonic System_ § 97, being sufficiently refuted by the 109th
Article, we shall say nothing more about it.
140. The _Ptolemean System_ § 96, which asserts the Earth to be at rest
in the Center of the Universe, and all the Planets with the Sun and
Stars to move round it, is evidently false and absurd. For if this
hypothesis were true, Mercury and Venus could never be hid behind the
Sun, as their Orbits are included within the Sun’s: and again, these two
Planets would always move direct, and be as often in Opposition to the
Sun as in Conjunction with him. But the contrary of all this is true:
for they are just as often behind the Sun as before him, appear as often
to move backwards as forwards, and are so far from being seen at any
time in the side of the Heavens opposite to the Sun, that they were
never seen a quarter of a circle in the Heavens distant from him.
[Sidenote: Appearances of Mercury and Venus.]
141. These two Planets, when viewed with a good telescope, appear in all
the various shapes of the Moon; which is a plain proof that they are
enlightened by the Sun, and shine not by any light of their own: for if
they did, they would constantly appear round as the Sun does; and could
never be seen like dark spots upon the Sun when they pass directly
between him and us. Their regular Phases demonstrate them to be
Spherical bodies; as may be shewn by the following experiment.
[Sidenote: Experiment to prove they are round.]
Hang an ivory ball by a thread, and let any Person move it round the
flame of a candle, at two or three yards distance from your Eye: when
the ball is beyond the candle, so as to be almost hid by the flame, it’s
enlightened side will be towards you, and appear round like the Full
Moon: When the ball is between you and the candle, it’s enlightened side
will disappear, as the Moon does at the Change: When it is half way
between these two positions, it will appear half illuminated, like the
Moon in her Quarters: But in every other place between these positions,
it will appear more or less horned or gibbous. If this experiment be
made with a circular plate which has a flat surface, you may make it
appear fully enlightened, or not enlightened at all; but can never make
it seem either horned or gibbous.
[Sidenote: PLATE II.