143. Let _ABCDE_ be a part or segment of the visible Heavens, in which
the Sun, Moon, Planets, and Stars appear to move at the same distance
from the Earth _E_. For there are certain limits, beyond which the eye
cannot judge of different distances; as is plain from the Moon’s
appearing to be no nearer to us than the Sun and Stars are. Let the
circle _fghiklmno_ be the Orbit in which Mercury _m_ moves round the Sun
_S_, according to the order of the letters. When Mercury is at _f_, he
disappears to the Earth at _E_, because his enlightened side is turned
from it; unless he be then in one of his Nodes § 20, 25; in which case,
he will appear like a dark spot upon the Sun. When he is at _g_ in his
Orbit, he appears at _B_ in the Heavens, westward of the Sun _S_, which
is seen at _C_: when at _h_, he appears at _A_, at his greatest western
elongation or distance from the Sun; and then seems to stand still. But,
as he moves from _h_ to _i_, he appears to go from _A_ to _B_; and seems
to be in the same place when at _i_ as when he was at _g_, only not near
so big: at _k_ he is hid from the Earth _E_ by the Sun _S_; being then
in his superiour Conjunction. In going from _k_ to _l_, he appears to
move from _C_ to _D_; and when he is at _n_, he appears stationary at
_E_; being seen as far east from the Sun then, as he was west from him
at _A_. In going from _n_ to _o_ in his Orbit, he seems to go back again
in the Heavens, from _E_ to _D_; and is seen in the same place (with
respect to the Sun) at _o_ as when he was at _l_; but of a larger
diameter at _o_, because he is then nearer the Earth _E_: and when he
comes to _f_, he again passes by the Sun, and disappears as before. In
going from _n_ to _h_ in his Orbit, he seems to go backward in the
Heavens from _E_ to _A_; and in going from _h_ to _n_, he seems to go
forward from _A_ to _E_. As he goes on from _f_ a little of his
enlightened side at _g_ is seen from _E_; at _h_ he appears half full,
because half of his enlightened side is seen; at _i_, gibbous, or more
than half full; and at _k_ he would appear quite full, were he not hid
from the Earth _E_ by the Sun _S_. At _l_ he appears gibbous again; at
_n_ half decreased, at _o_ horned, and at _f_ new like the Moon at her
Change. He goes sooner from his eastern station at _n_ to his western
station at _h_ than from _h_ to _n_ again; because he goes through less
than half his Orbit in the former case, and more in the latter.
[Sidenote: Fig. III.
The Elongations and Phases of Venus.