125. The Earth’s bulk is but a point, as that at _C_, compared to the
Heavens; and therefore every inhabitant upon it, let him be where he
will, as at _n_, _e_, _m_, _s_, &c. sees one half of the Heavens. The
inhabitant _n_, on the North Pole of the Earth, constantly sees the
Hemisphere _ENQ_; and having the North Pole _N_ of the Heavens just over
his head, his [25]Horizon coincides with the Celestial Equator _ECQ_.
Therefore all the Stars in the Northern Hemisphere _ENC_, between the
Equator and North Pole, appear to turn round the line _NC_, moving
parallel to the Horizon. The Equatoreal Stars keep in the Horizon, and
all those in the Southern Hemisphere _ESQ_ are invisible. The like
Phenomena are seen by the observer _s_ on the South Pole, with respect
to the Hemisphere _ESQ_; and to him the opposite Hemisphere is always
invisible. Hence, under either Pole, only one half of the Heavens is
seen; for those parts which are once visible never set, and those which
are once invisible never rise. But the Ecliptic _YCX_ or Orbit which the
Sun appears to describe once a year by the Earth’s annual motion, has
the half _YC_ constantly above the Horizon _ECQ_ of the North Pole _n_;
and the other half _CX_ always below it. Therefore whilst the Sun
describes the northern half _YC_ of the Ecliptic he neither sets to the
North Pole nor rises to the South; and whilst he describes the southern
half _CX_ he neither sets to the South Pole nor rises to the North. The
same things are true with respect to the Moon; only with this
difference, that as the Sun describes the Ecliptic but once a year, he
is for half that time visible to each Pole in it’s turn, and as long
invisible; but as the Moon goes round the Ecliptic in 27 days 8 hours,
she is only visible for 13 days 16 hours, and as long invisible to each
Pole by turns. All the Planets likewise rise and set to the Poles,
because their Orbits are cut obliquely in halves by the Horizon of the
Poles. When the Sun (in his apparent way from _X_) arrives at _C_, which
is on the 20th of _March_, he is just rising to an observer at _n_ on
the North Pole, and setting to another at _s_ on the South Pole. From
_C_ he rises higher and higher in every apparent Diurnal revolution
’till he comes to the highest point of the Ecliptic _y_, on the 21st of
_June_, and then he is at his greatest Altitude, which is 23-1/2
degrees, or the Arc _Ey_, equal to his greatest North declination; and
from thence he seems to descend gradually in every apparent
Circumvolution, ’till he sets at _C_ on the 23d of _September_; and then
he goes to exhibit the like Appearances at the South Pole for the other
half of the year. Hence the Sun’s apparent motion round the Earth is not
in parallel Circles, but in Spirals; such as might be represented by a
thread wound round a Globe from Tropic to Tropic; the Spirals being at
some distance from one another about the Equator, but gradually nearer
to each other as they approach nearer to the Tropics.
[Sidenote: Phenomena at the Equator.