294. At the Poles, one half of the Ecliptic never sets, and the other
half never rises: and therefore, as the Sun is always half a year in
describing one half of the Ecliptic, and as long in going through the
other half, ’tis natural to imagine that the Sun continues half a year
together above the Horizon of each Pole in it’s turn, and as long below
it; rising to one Pole when he sets to the other. This would be exactly
the case if there were no refraction: but by the Atmosphere’s refracting
the Sun’s rays, he becomes visible some days sooner § 183, and continues
some days longer in sight than he would otherwise do: so that he appears
above the Horizon of either Pole before he has got below the Horizon of
the other. And, as he never goes more than 23-1/2 degrees below the
Horizon of the Poles, they have very little dark night: it being
twilight there as well as at all other places till the Sun be 18 degrees
below the Horizon, § 177. The Full Moon being always opposite to the
Sun, can never be seen while the Sun is above the Horizon, except when
the Moon falls in the northern half of her Orbit; for whenever any point
of the Ecliptic rises the opposite point sets. Therefore, as the Sun is
above the Horizon of the north Pole from the 20th of _March_ till the
23d of _September_, it is plain that the Moon, when Full, being opposite
to the Sun, must be below the Horizon during that half of the year. But
when the Sun is in the southern half of the Ecliptic he never rises to
the north Pole, during which half of the year, every Full Moon happens
in some part of the northern half of the Ecliptic, which never sets.
Consequently, as the polar Inhabitants never see the Full Moon in
Summer, they have her always in the Winter, before, at, and after the
Full, shining for 14 of our days and nights. And when the Sun is at his
greatest depression below the Horizon, being then in Capricorn, the Moon
is at her First Quarter in Aries, Full in Cancer, and at her Third
Quarter in Libra. And as the beginning of Aries is the rising point of
the Ecliptic, Cancer the highest, and Libra the setting point, the Moon
rises at her First Quarter in Aries, is most elevated above the Horizon,
and Full in Cancer, and sets at the beginning of Libra in her Third
Quarter, having continued visible for 14 diurnal rotations of the Earth.
Thus the Poles are supplied one half of the winter time with constant
Moon-light in the Sun’s absence; and only lose sight of the Moon from
her Third to her First Quarter, while she gives but very little light;
and could be but of little, and sometimes of no service to them. A bare
view of the Figure will make this plain; in which let _S_ be the Sun,
_e_ the Earth in Summer when it’s north Pole _n_ inclines toward the
Sun, and _E_ the Earth in Winter, when it’s north Pole declines from
him. _SEN_ and _NWS_ is the Horizon of the north Pole, which is
coincident with the Equator; and, in both these positions of the Earth,
♈♋♎♑ is the Moon’s Orbit, in which she goes round the Earth, according
to the order of the letters _abcd_, _ABCD_. When the Moon is at _a_ she
is in her Third Quarter to the Earth at _e_, and just rising to the
north Pole _n_; at _b_ she changes, and is at the greatest height above
the Horizon, as the Sun likewise is; at _c_ she is in her First Quarter,
setting below the Horizon; and is lowest of all under it at _d_, when
opposite to the Sun, and her enlightened side toward the Earth. But then
she is full in view to the south Pole _p_; which is as much turned from
the Sun as the north Pole inclines towards him. Thus in our Summer, the
Moon is above the Horizon of the north Pole whilst she describes the
northern half of the Ecliptic ♈♋♎, or from her Third Quarter to her
First; and below the Horizon during the progress through the southern
half ♎♑♈; highest at the Change, most depressed at the Full. But in
winter, when the Earth is at _E_, and it’s north Pole declines from the
Sun, the New Moon at _D_ is at her greatest depression below the Horizon
_NWS_, and the Full Moon at _B_ at her greatest height above it; rising
at her First Quarter _A_, and keeping above the Horizon till she comes
to her Third Quarter _C_. At a mean state she is 23-1/2 degrees above
the Horizon at _B_ and _b_, and as much below it at _D_ and _d_, equal
to the inclination of the Earth’s Axis _F_. _S_♋ and _S_♑ are, as it
were, a ray of light proceeding from the Sun to the Earth; and shews
that when the Earth is at _e_, the Sun is above the Horizon, vertical to
the Tropic of Cancer; and when the Earth is at _E_, he is below the
Horizon, vertical to the Tropic of Capricorn.
[Illustration: Plate VIII.
_J. Ferguson delin._ _J. Mynde Sculp._] [Illustration: Plate IX.
_J. Ferguson delin._ _J. Mynde Sculp._]