149. As Venus and the Earth are superiour Planets to Mercury, they shew much the same Appearances to him that Mars and Jupiter do to us. Let Mercury _m_ be at _f_, Venus _v_ at _F_, and the Earth at _E_; in which situation Venus hides the Earth from Mercury; but, being in opposition to the Sun, she shines on Mercury with a full illumined Orb; though, with respect to the Earth, she is in conjunction with the Sun and invisible. When Mercury is at _f_, and Venus at _G_, her enlightened side not being directly towards him, she appears a little gibbous; as Mars does in a like situation to us: but, when Venus is at _I_, her enlightened side is so much towards Mercury at _f_, that she appears to him almost of a round figure. At _K_, Venus disappears to Mercury at _f_, being then hid by the Sun; as all our superiour Planets are to us, when in conjunction with the Sun. When Venus has, as it were, emerged out of the Sun beams, as at _L_, she appears almost full to Mercury at _f_; at _M_ and _N_, a little gibbous; quite full at _F_, and largest of all; being then in opposition to the Sun, and consequently nearest to Mercury at _f_; shining strongly on him in the night, because her distance from him then is somewhat less than a fifth part of her distance from the Earth, when she appears roundest to it between _I_ and _K_, or between _K_ and _L_, as seen from the Earth _E_. Consequently, when Venus is opposite to the Sun as seen from Mercury, she appears more than 25 times as large to him as she does to us when at the fullest. Our case is almost similar with respect to Mars, when he is opposite to the Sun; because he is then so near the Earth, and has his whole enlightened side towards it. But, because the Orbits of Jupiter and Saturn are very large in proportion to the Earth’s, these two Planets appear much less magnified at their Oppositions or diminished at their Conjunctions than Mars does, in proportion to their mean apparent Diameters. CHAP. VII. _The physical Causes of the Motions of the Planets. The Excentricities of their Orbits. The Times in which the Action of Gravity would bring them to the Sun._ ARCHIMEDES_’s ideal Problem for moving the Earth. The World not eternal._ [Sidenote: Gravitation and Projection. Fig. IV. PLATE II. Circular Orbits. Fig. IV.]