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.