ASTRONOMY. 3 6« 
at reft, and the Earth revolves in the fame orbit, a fpec- 
tator on the Earth will fee the Sun defcribe the fame cir¬ 
cle EFGH, in the concave fphereof the heavens. Hence, 
if the plane of the Earth’s orbit be imagined to be extend¬ 
ed to the heavens, it would cut the Harry firmament in 
that very circle, in which a fpeiSlator in the Sun would 
fee the Earth revolve every year: while an inhabitant of 
the Earth would obferve the Sun to go through the fame 
circle, and in the fame fpace of time, that the folarfpec- 
tator would fee the Earth defcribe it. 
The inhabitants of all the other planets will obferve juft 
fuch motions in the Sun as we do, and for the very fame 
rcafons ; and the Sun will be fecn from every planet to de¬ 
fcribe the fame circle, and in the fame fpace of time, that 
a fpeclator in the Sun would obferve the planet to do. 
For example, an inhabitant of Jupiter would think that 
the Sun revolved round him, defcribing a circle in the 
heavens in the fpace of twelve years; this circle would 
not be the fame with our ecliptic, nor would the Sun ap¬ 
pear to pafs through the fame ftars which he does to us. 
'On the fame account, the Sun, feen from Saturn, will ap¬ 
pear to move in another circle, diftinft from either of the 
former; and will not feem to finifti his period in lefs time 
than thirty years. Now, as it is impolfible that the Sun 
can have all thefe motions really in itfelf, we may fafely 
affirm, that none of them are real, but that they are all ap¬ 
parent, and arife from the motions of the refpedtive planets. 
One phenomenon arifing from the annual motion of the 
Earth, which has already been (lightly touched upon, may 
now be more fully explained; for, as from this motion 
the Sun appears to move from weft to eaft in the heavens, 
if a (far riles or lets along with the Sun at any time, it will 
in the courfe of a few days rife or fet before it, becaufe the 
Sun’s apparent place in the heavens will be removed to file 
eaftward of that (far. Hence thofe ftars, which at one 
time of the year fet with the Sun, and therefore do not 
appear at all, (hall at another time of the year rife when 
the Sun fets, and fliine all the night. And as any one (tar 
ftiifts its place with refpeft to the Sun, and in confequence 
of that with refpedt to the hour of the night, fo do all the 
reft. Hence it is that all thofe ftars, which at one time of 
the year appear on any one fide of the pole ftar in the even¬ 
ing, (hall in half a year appear on the contrary fide thereof. 
The phenomena occalioned by the annual and diurnal 
motions of the Earth, we (hall now explain ; and firft, thofe 
that arife from the diurnal motion. As the Earth is of a fphe- 
rical figure, that part of it which comes at any time under 
the confined view of an obferver, will feem to be extended 
like a plane ; and the heavens will appear as a concave fphe- 
rical fuperficies, divided by the aforefaid plane into two 
equal parts, one of which is vifible, the other concealed 
from us by the opacity of the Earth. Now the Earth, by 
•its revolution round its axis, carries the fpeblator and the 
aforefaid plane from weft to eaft; therefore all thofe bodies 
to the eaft, which could not be feen becaufe they were be¬ 
low the plane of the horizon, will become vifible, or rife 
above it, when, by the rotation of the Earth, the horizon 
links as it were below them. On the other hand, the op- 
pofite part of the plane, towards the weft, riling above 
the ftars on that fide, will hide them from the fpeblator, 
and they will appear to fet, or go below the horizon. 
As the Earth, together with the horizon of the fpebta- 
tor, continues moving to the eaft, and about the fame axis, 
all fuch bodies as are (eparated from the Earth, and which 
do not partake of that motion, will feem to move uniform¬ 
ly in the fame time, but in an oppofite direction, that is, 
from eaft to weft ; excepting theceleftial poles, which will 
appear to be at reft. Therefore, when we fay, that the 
w hole concave fphere of the heavens appears to turn round 
upon the axis of the world, whilft the Earth is performing 
one rotation round its own axis, we mull be underftood to 
except the two poles of the world, for thefe do not par¬ 
take of this apparent motion. It is, therefore, on account 
of the revolution of the Earth round its axis, that the 
fpeclator imagines the whole (tarry firmament, and every 
Vol. II. No. 77. 
point of the heaven (excepting the tw’o celeffial poles), to 
revolve about the Earth from eaft to weft every twenty- 
four hours, each point defcribing a greater or fmaller circle, 
as it is more or lefs remote from one of the celeftia! poles. 
The Earth is made to revolve on its axis, in order to 
give alternate night and day to every part of its furface. 
But, although every place on the furface is illuminated by 
all the ftars which are above the horizon of that place ; 
yet, when the Sun is above the horizon, his light is fo 
ftrong, that it quite extinguifties the faint light of the ftars, 
and produces day. When the Sun goes below the horizon, 
or, more properly, when our horizon gets above the Sun, 
the ftars give their light, and we are in that (late called 
night. Now, as the Earth is an opaque fpherical body, at 
a great diftance from the Sun, one half of it will always 
be illuminated thereby, while the other half will remain 
in darknefs. The circle which diftinguiflies or divides the 
illuminated face of the Earth from the dark fide, and is 
the boundary between light and darknefs, is generally call¬ 
ed the terminator. A line drawn from the centre of the 
Sun to the centre of the Earth, is perpendicular to the 
plane of this circle. Hence it is plain, that, when any gi¬ 
ven place on the globe firft gets into the enlightened he- 
mifphere, the Sun is juft rifen to that part; when it gets 
half-way, or to its greateft diftance from the terminator, it 
is then noon; and, when it leaves the enlightened hemif- 
phere, it is then fun-fet. 
Here it will be neceffary to premife a few confiderations: 
Firft, that, on account of the immenfe diftance of the Sun 
from the Earth, the rays which proceed from it may be 
confidered as parallel to each other. Secondly, that only 
one-half of a globe can be illuminated by parallel rays, 
and therefore only one-half of the Earth will be enlight¬ 
ened by the Sun at one time. Thefe confiderations will 
be rendered more obvious, by referring to the Aftrono- 
mical-Plate III. where, at Jig. r, S reprefents the Sun, 
from which we fuppole parallel rays to flow in all direc¬ 
tions. At A, B, C, are reprefented three different pofi- 
tions of the globe of the Earth, the bright part being that 
which is illuminated by the rays proceeding from the Sun, 
and the (haded part the portion of the globe which is in 
darknefs; of courfe the line TT is the terminator, or 
boundary of light and darknefs. In the globe at C, the 
poles coincide with the terminator. In the globe at A, 
the north pole is in the enlightened portion, and the fouth 
pole in the dark hemifphere; while in the oppofite globe 
at B, the fouthern pole is in the illuminated part, and the 
north pole in obfeurity. It is evident, that it is day in any 
given place on the globe, fo long as that place continues 
in the enlightened hemifphere; but when, by the diurnal 
rotation of the Earth on its axis, it is carried into the dark 
hemifphere, it becomes night to that place. 
The length of the day and the night depend therefore oit 
the pofition of the terminator, with refpecl to the axis of 
the Earth. If the poles of the Earth be lituated in the 
terminator, as at C, every parallel will be divided into 
two equal parts; and, as the uniform motion of the Earth 
caufes any given place to defcribe equal parts of its paral¬ 
lel in equal times, the day and night would be equal on 
every parallel of latitude, that is, all over the globe, ex¬ 
cept at the poles, where the Sun would neither rife nor 
fet, but continue in the horizon. But if, as at A and B, 
the axis be not in the plane of the terminator, the termi¬ 
nator will divide the equator into two equal parts, but all 
the circles parallel to it into unequal parts; thofe circles 
that are (ituated towards the enlightened pole will have a 
greater part of their circumference in the enlightened than 
in the dark hemifphere, while (imilar parallels towards 
the other pole will have a greater part of their circumfe¬ 
rence in the dark hemifphere. Whence it follows, that 
the firft-mentioned parallels will enjoy longer days titan 
nights; and the contrary will happen to the latter, where 
the days will be the (horteft, and the nights the longed ; 
while, at the equator, the days will be always of the fame 
length. Having thus (hewn that the viciliitudes in the 
5 B length 
