3 6§ ASTRO 
enough to be meafured by obfervation. But, if we re- 
preient the fp ere of the heaven by a larger circle ZHNO, 
the fern diam uer of the Earth A I', mealured in this cir¬ 
cle, will amount to fewer degrees; for the arc EO is lefs 
than the arc ro; ar.d the larger the fphere of the heaven 
is, in proportion to the globe of the Earth, the lefs fenfi- 
ble is'the difference between the two horizons. Now, as 
the fphere of the Earth is but as a point when compared 
to the ftarry heaven, the difference between the fenfible 
and rational horizon will be infenfible. Front what has 
been laid, it appears, that the only dillindtion between the 
fenfible and rational horizon arifes front the diftanceof the 
object we are looking at. 
As the rational horizon divides the apparent celeftial 
fphere into two equal hemifpheres, and ferves as a bound¬ 
ary, from which to meafure the elevation or deprelfion ot 
celeftiul objedts ; thofe in the upper or vilible hentifphere 
are faid to be high, or elevated above the horizon ; and 
thofe in the other hentifphere are called low, or below the 
horizon. The Earth being a fpherical body, the horizon, 
or limits of our view, mult change as we change our place ; 
and therefore every place upon the Earth has a different 
horizon. Thus, if a man lives at A, in the above figure, 
his horizon is HO ; if he lives at b, his horizon is IK; if 
at D, it is ZN; if at c, it is LM, &c. From hence we 
obtain another proof of the fphericity of the Earth ; for, 
if it were flat, all the inhabitants thereof would have the 
fame horizon. 
Again, as that point in the heavens, which is diredtly 
over the head of a fpedlator, is the zenith ; and that point 
directly under his feet, is the nadir; therefore, if a man 
lives at A, his zenith is Z, his nadir N ; if he lives at b, 
his zenith is L, his nadir M ; and fo on. Confequently 
the zenith and horizon of an obferver remains fixed in the 
heavens, fo long'as he continues in the fame place; but 
be no Jooner changes his polition, than the horizon touches 
the Earth in another point, and his zenith anfwers to a dif¬ 
ferent } oint in the heavens. 
Of the Annual Motion of the Earth. 
It is.owing to the induftry of modern aftronomers, that 
the annual motion of the Earth has been fully evinced ; 
for, though this motion had been known to and adopted by 
many among the ancient philofophers, yet they were not 
able to give their opinions that degree of probability which 
is attainable from modern difeoveries, much lefs the evi¬ 
dence arifing from thofe demonftrative proofs of which we 
are now in potTeflion. 
The apparent motion of the Sun, arifing from the Earth’s 
N O M Y. 
annual motion round it, is the fame as when a perfon fails 
along the fea-coaft, the fhore, villages, and other remarkable 
places on land, appear to change their lituation, and to pafs 
by him. To a fpedlator upon the Earth, as its moves along 
its orbit, or fails as it were through celeftial fpace, the 
Sun, the planets, and the fixed ftars, appear to change 
their places. Apparent change of place is of two forts : 
the one^ is that of bodies at reft, the change of whole place 
pends folely on that of the fpedlator; the other is tliatof 
bodies in motion, whofe apparent change of place depends 
as well on their own motion as on that of the fpedlator. 
We lhall firft conftder only that apparent change which 
takes place in thofe which are at reft, and which is owing 
wholly to the motion of the Earth, and ftsew that the Sun, 
when feen from the Earth, will appear to move in the fame 
manner, whether it revolves round the Earth, or whether 
the Earth revolves round the Sun. Let us fuppofe the 
Earth at reft, without any motion of its own, and let the 
Sun be fuppofed to revolve round it in the orbit ABCD, 
in the annexed figure ; and let EFGH be a circle in the 
concave fphere of the ftarry heavens; as the Sun moves 
in the order of the letters ABCD in its orbit, it will ap¬ 
pear to a fpedlator on the Earth to have deferibed the cir¬ 
cle EFGH. When the Sun is at A, it will appear as if it 
was among the fixed ftars that are at E; when it is at B, 
it will appear among the fixed ftars at F ; when at C, among 
thofe at G ; and, when it is at D, it will appear among the 
fixed ftars at H. Indeed, the fixed ftars and the Sun are 
not feen at the fame time ; but we may tell in what part 
of the heavens the Sun is, or what fixed ftars it is near, by 
knowing thofe which are oppofite to it, or come to the 
fouth at midnight. Therefore, if we find that any fet of 
ftars, as thofe at H for inftance, come to the fouth at mid¬ 
night, we may be fare that they are oppofite to the Sun; 
and confequently, if we could fee the ftars in that part of 
the heaven where the Sun is, we ftiould find them to be 
thofe at F. 
25 
in which we (hall now fuppofe the Earth to move, in the 
order of the letters ABCD. Upon this fuppofition, when 
the Earth is at A, the Sun will appear in that part of the 
heavens where the ftars G are; when the Earth is at B, 
the Sun w ill appear in that part of the heavens where the 
ftars H are; when the Earth is at C, the Sun will appear 
in that part of the heavens where the ftars E are ; and, as 
the Earth revolves round the Sun in the orbit ABCD, 
the Sun will appear to a fpedlator on the Earth todeferibe 
the circle GHEF. Thus, whether the Earth be at reft, 
and the Sun revolves in the orbit ABCD ; or the Sun be 
at 
