But as the Globe turns round it’s Axis, move your hand slowly forward so
as to carry the Globe from _H_ towards _E_, and the boundary of light
and darkness will approach towards the north Pole, and recede towards
the south Pole; the northern places will go through less and less of the
light, and the southern places through more and more of it; shewing how
the northern days decrease in length, and the southern days increase,
whilst the Globe proceeds from _H_ to _F_. When the Globe is at _E_, it
is at a mean state between the lowest and highest parts of it’s Orbit;
the candle is directly over the Equator, the boundary of light and
darkness just reaches to both the Poles, and all places on the Globe go
equally through the light and dark Hemispheres, shewing that the days
and nights are then equal at all places of the Earth, the Poles only
excepted; for the Sun is then setting to the north Pole, and rising to
the south Pole.
[Sidenote: Winter Solstice.]
Continue moving the Globe forward, and as it goes through the quarter
_A_, the north Pole recedes still farther into the dark Hemisphere, and
the south Pole advances more into the light, as the Globe comes nearer
to ♋; and when it comes there at _F_, the candle is directly over the
Tropic of Capricorn, the days are at the shortest, and nights at the
longest, in the northern Hemisphere, all the way from the Equator to the
arctic Circle; and the reverse in the southern Hemisphere from the
antarctic Circle; within which Circles it is dark to the north frigid
Zone and light to the south.
[Sidenote: Vernal Equinox.]
Continue both motions, and as the Globe moves through the quarter _B_,
the north Pole advances toward the light, and the south Pole recedes as
fast from it; the days lengthen in the northern Hemisphere, and shorten
in the southern; and when the Globe comes to _G_ the candle will be
again over the Equator (as when the Globe was at _E_) and the days and
nights will again be equal as formerly: and the north Pole will be just
coming into the light, the south Pole going out of it.
Thus we see the reason why the days lengthen and shorten from the
Equator to the polar Circles every year; why there is no day or night
for several turnings of the Earth, within the polar Circles; why there
is but one day and one night in the whole year at the Poles; and why the
days and nights are equally long all the year round at the Equator,
which is always equally cut by the circle bounding light and darkness.
[Sidenote: Remark.
Fig. III.