ASTRONOMY. 
change tvitt Be made in its geocentric place, than if it was 
to move in the tangent. But the geocentric place of the 
planet would not be altered, if the planet was to move in 
the tangent. For, if it was to move from T towards A, 
or-from A to V, he appaient place of it in the heavens 
would in one cafe be at T, in the other cafe at V. There¬ 
fore, while the planet is tit its greatefl elongation, and is 
defcribing a fmall arc in its orbit, that nearly coincides 
with the tangent, its geocentric place does not feem to al¬ 
ter, but it appears to continue for fome time in the fame 
part of rhe heavens, or is ftationary. 
We have hitherto fuppofed the Earth to be at reft, and 
upon that fuppolition have explained the progrefs and re- 
refs, the conjunctions and ftations, of the inferior planets, 
f this luppofition was true, VT, or the arc which the 
planet at any time defcribes in its progrefs, and TV, the 
arc which it defcribes in its regrefs, would always be in 
the fame part of the heavens. The planet, when-in con- 
j-nndtion, would always appear at Q^> among the fame fix¬ 
ed ftars; and at its elongation, or when it is ftationary, it 
would alw ays appear among the fame fixed ftars at T on 
one fide of the Sun, and at V on the other fide. But 
this fuppofirion is not true : for the Earth revolves in its 
crbit ABO round the Sun. Now if the Earth is at A, at 
the time of either conjunction, the planet at this conjunc¬ 
tion would appear among the fixed ftars at Q^> and the arcs 
of the greateft elongation QV and QT would be on each 
fide of thofe ftars. But if the Earth is at B, at the time 
of either of the conjunctions, then at the time of this 
conjunction the planet will appear in the- line BST, and 
be feen among the fixed ftars at T, and the arcs of tile 
greateft elongation will be on each fide of thefe ftars ; that 
is, the conjunctions and elongations will happen in a dif¬ 
ferent part of the heavens, when the Earth is at B, from 
what they would when the Earth is at A. In other re- 
fpeCts, the foregoing phenomenawill be much the fame, 
notwithftanding the motion of the Earth, only the planet 
will be more direCf in the fartheft part of the orbit, and 
lefs retrograde in the neareft. 
The inferior planets, Venus and Mercury, always, ap¬ 
pear very near the Sun; but, by the motion of the Earth 
in its orbit, the Sun appears in different parts of the hea¬ 
vens, in different times of the year. Therefore the infe¬ 
rior planets, as they are always very near the Sun, will 
alfo appear in different parts of the heavens at different 
times of the year. And confequently their conjunctions and 
greateft elongations will fometimes happen when they are 
in one part of the heavens, and fometimes when they are 
in another part. Venus, feen from the Earth, will appear 
to vibrate in an arc VT, half of which is on one fide of the 
Sun's apparent place, and half on the other fide. Hence, 
when an inferior planet, view ed from a fuperior, moves 
apparently retrograde, the fuperior planet has alfo an ap¬ 
parently retrograde motion. And when a fuperior planet, 
viewed from an inferior, appears ftationary, the inferior 
planet, viewed at the fame time from the fuperior, is alfo 
ftationary. 
Of the Phases of Venus and Mercury. 
That the planets are opaque bodies, and confequently 
ftiine only by the light they receive from the Sun, is plain, 
becaufe they are not vifible when they are in fuch parts of 
their orbits as are between the Sun and Earth, that is, 
when their illuminated fide is turned from us. The Sun 
enlightens only half, a planet at once ; the illuminated he- 
Hiifphere is always that which is turned towards the Sun, 
the other hemifphore of the planet is dark. To fpeak with 
accuracy, the Sun, being larger than any of the planets, 
will illuminate rather more than half; but this difference, 
on account of the great diftance of the Sun from any of 
the planets, is fo fmall, that its light may be confidered 
as coming to them in lines phyfically parallel. Like other 
opaque bodies, they caft a fliadow behind them, which is 
always oppofite to the Sun. The line in the planet's bo¬ 
dy, which diftinguifties the lucid from the obfeure part, 
appears fometimes ftraight, fometimes crooked. The 
convex part of the curve is fometimes towards the fplen- 
did, and the concave towards that which is obfeure, and 
vice verfa, according to the fituation of the eye with refpect 
to the planet, and of the Sun which enlightens it. Hence 
the inferior planets going round the Sun in lefs orbits than 
the Earth does, will fometimes harve more, fometimes lefs, 
ot their illuminated fide towards us; and, as it is the illu¬ 
minated part only which is vifible fo us, Mercury and 
Venus will, through a good telefcope, exhibit the feveral 
appearances of the Moon, from a fine thin crefcent to an 
enlightened hemifphere. 
If we view Venus through a telefcope, when (lie fol¬ 
lows the Sun’s rays on the eaftern fide, and appears above _ 
the horizon after fun-fet, we (hall fee her appear nearly 
round, and but fmall; (he is at that time beyond the Sun, 
and prefents to us an enlightened hemifphere. As (he de¬ 
parts from the Sun towards the eaft, (he augments in her 
apparent fize ; and, on viewing her through a telefcope, 
is feen to alter her figure, abating of her apparent round- 
nefs, and appearing fucceflively like the Moon, in the dif¬ 
ferent ftages of her decreafe. At length, when (he is at 
her greateft elongation, fhe is like the Moon in her firft 
quarter, and appears as fhe does when from a full fhe has 
decreafed to a half moon. 
After this, as fhe approaches to the Sun, fne appears 
concave in her illuminated part, as the Moon when fire 
forms a crefcent; thus fhe continues till (lie is hid entirely, 
in the Sun’s rays, and prefents to us her whole dark he¬ 
mifphere, as the Moon does in her conjunftion, no part 
of the planet being then vifible. When fhe departs out 
of the Sun’s rays on the weftern fide, we fee her in the 
morning, juft before day-break. It is in this fituation that 
Venus is called the morningJlar, as in-the other fhe is called 
the evening Jlar. She at tins time appears very beautiful, 
like a fine thin crefcent; a fmall verge of filver light is 
juft feen on her edge. From this period the grows more 
and more enlightened every day, till (lie is arrived at her 
greateft digreflion or-elongation, when file again appears 
as a half-moon, or as tile Moon in her firft quarter; fro in 
this time, if continued to be viewed with a telefcope, (lie 
is found to be more and more enlightened, though the is 
all the while decreafing in magnitude, and thus continues, 
growing fmaller and rounder till (he is'again hid'orMdft. 
in the Sun’s rays. 
T 
The m e edfig ne reprefents the orb’t of Venus, with 
the Suu m the ceiV.re; and the planet being drawn in the 
eight 
