128. Let us now suppose that the observer has gone from the Equator e
towards the North Pole _n_, and that he stops at _i_, from which place
he then sees the Hemisphere _MElNL_; his Horizon _MCL_ having shifted as
many [27]Degrees from the Celestial poles _N_ and _S_ as he has
travelled from under the Equinoctial _E_. And as the Heavens seem
constantly to turn round the line _NCS_ as an Axis, all those Stars
which are as far from the North Pole _N_ as the observer is from under,
the Equinoctial, namely the Stars north of the dotted parallel _DL_,
never set below the Horizon; and those which are south of the dotted
parallel _MO_ never rise above it. Hence, the former of these two
parallel Circles is called _the Circle of perpetual Apparition_, and the
latter _the Circle of perpetual Occultation_: but all the Stars between
these two Circles rise and set every day. Let us imagine many Circles to
be drawn between these two, and parallel to them; those which are on the
north side of the Equinoctial will be unequally cut by the Horizon
_MCL_, having larger portions above the Horizon than below it; and the
more so, as they are nearer to the Circle of perpetual Apparition; but
the reverse happens to those on the south side of the Equinoctial,
whilst the Equinoctial is divided in two equal parts by the Horizon.
Hence, by the apparent turning of the Heavens, the northern Stars
describe greater Arcs or Portions of Circles above the Horizon than
below it; and the greater as they are farther from the Equinoctial
towards the Circle of perpetual Apparition; whilst the contrary happens
to all Stars south of the Equinoctial: but those upon it describe equal
Arcs both above and below the Horizon, and therefore they are just as
long above as below it.
[Sidenote: PLATE II.]
129. An observer on the Equator has no Circle of perpetual Apparition or
Occultation, because all the Stars, together with the Sun and Moon, rise
and set to him every day. But, as a bare view of the Figure is
sufficient to shew that these two Circles _DL_ and _MO_ are just as far
from the Poles _N_ and _S_ as the observer at _i_ (or one opposite to
him at _o_) is from the Equator _ECQ_; it is plain, that if an observer
begins to travel from the Equator towards either Pole, his Circle of
perpetual Apparition rises from that Pole as from a Point, and his
Circle of perpetual Occultation from the other. As the observer advances
toward the nearer Pole, these two Circles enlarge their diameters, and
come nearer one another, until he comes to the Pole; and then they meet
and coincide in the Equator. On different sides of the Equator, to
observers at equal distances from it, the Circle of perpetual Apparition
to one is the Circle of perpetual Occultation to the other.
[Sidenote: Why the Stars always describe the same parallel of motion,
and the Sun a different.]
130. Because the Stars never vary their distances from the Equinoctial,
so as to be sensible in an age, the lengths of their diurnal and
nocturnal Arcs are always the same to the same places on the Earth. But
as the Earth goes round the Sun every year in the Ecliptic, one half of
which is on the north side of the Equinoctial and the other half on it’s
south side, the Sun appears to change his place every day, so as to go
once round the Circle _YCX_ every year § 114. Therefore whilst the Sun
appears to advance northward, from having described the Parallel _abX_
touching the Ecliptic in _X_ the days continually lengthen and the
nights shorten, until he comes to _y_ and describes the Parallel _yzx_,
when the days are at the longest and the nights at the shortest: for
then, as the Sun goes no farther northward, the greatest portion that is
possible of the diurnal Arc _yz_ is above the Horizon of the inhabitant
_i_; and the smallest portion _zx_ below it. As the Sun declines
southward from _y_ he describes smaller diurnal and greater nocturnal
Arcs, or Portions of Circles, every day; which causeth the days to
shorten and nights to lengthen, until he arrives again at the Parallel
_abX_; which having only the small part _ab_ above the Horizon _MCL_,
and the great part _bX_ below it, the days are at the shortest and the
nights at the longest; because the Sun recedes no farther south, but
returns northward as before. It is easy to see that the Sun must be in
the Equinoctial _ECQ_ twice every year, and then the days and nights are
equally long; that is, 12 hours each. These hints serve at present to
give an idea of some of the Appearances resulting from the motions of
the Earth; which will be more particularly described in the tenth
Chapter.
[Sidenote: Fig. I.