203. From this oblique view of the Earth’s Orbit, let us suppose
ourselves to be raised far above it, and placed just over it’s center
_S_, looking down upon it from it’s north pole; and as the Earth’s Orbit
differs but very little from a Circle, we shall have it’s figure in such
a view represented by the Circle _ABCDEFGH_. Let us suppose this Circle
to be divided into 12 equal parts called _Signs_, having their names
affixed to them; and each Sign into 30 equal parts called _Degrees_,
numbered 10, 20, 30, as in the outermost Circle of the Figure, which
represents the great Ecliptic in the Heavens. The Earth is shewn in
eight different positions in this Circle, and in each position _Æ_ is
the Equator, _T_ the Tropic of Cancer, the _dotted Circle_ the parallel
of _London_, _U_ the arctic or north polar Circle, and _P_ the north
Pole where all the Meridians or hour Circles meet § 198. As the Earth
goes round the Sun the north Pole keeps constantly towards one part of
the Heavens, as it keeps in the Figure towards the right hand side of
the Plate.
[Sidenote: Vernal Equinox.]
When the Earth is at the beginning of Libra, namely on the 20th of
_March_, in this Figure (as at _g_ in Fig. I.) the Sun _S_ as seen from
the Earth appears at the beginning of Aries in the opposite part of the
Heavens[53], the north Pole is just coming into the light, the Sun is
vertical to the Equator; which, together with the Tropic of Cancer,
parallel of _London_, and arctic Circle, are all equally cut by the
Circle bounding light and darkness, coinciding with the six o’clock hour
Circle, and therefore the days and nights are equally long at all
places: for every part of the Meridian _ÆTLa_ comes into the light at
six in the morning, and revolving with the Earth according to the order
of the hour-letters, goes into the dark at six in the evening. There are
24 Meridians or hour-Circles drawn on the Earth in this Figure, to shew
the time of Sun rising and setting at different Seasons of the Year.
[Sidenote: Fig. II.]
As the Earth moves in the Ecliptic according to the order of the letters
_ABCD_, &c. through the Signs Libra, Scorpio, and Sagittarius, the north
Pole comes more and more into the light; the days increase as the nights
decrease in length, at all places north of the Equator _Æ_; which is
plain by viewing the Earth at _b_ on the 5th of _May_, when it is in the
15th degree of Scorpio[54], and the Sun as seen from the Earth appears
in the 15th degree of Taurus. For then, the Tropic of Cancer _T_ is in
the light from a little after five in the morning till almost seven in
the evening; the parallel of _London_ from half an hour past four till
half an hour past seven; the polar Circle _U_ from three till nine; and
a large track round the north Pole _P_ has day all the 24 hours, for
many rotations of the Earth on it’s Axis.
[Sidenote: Summer Solstice.]
When the Earth comes to _c_, at the beginning of Capricorn, and the Sun
as seen from the Earth appears at the beginning of Cancer, on the 21st
of _June_, as in this Figure, it is in the position _a_ in Fig. I; and
it’s north Pole inclines toward the Sun, so as to bring all the north
frigid Zone into the light, and the northern parallels of Latitude more
into the light than the dark from the Equator to the polar Circles; and
the more so as they are farther from the Equator. The Tropic of Cancer
is in the light from five in the morning till seven at night, the
parallel of _London_ from a quarter before four till a quarter after
eight; and the polar Circle just touches the dark, so that the Sun has
only the lower half of his Disc hid from the inhabitants on that Circle
for a few minutes about midnight, supposing no inequalities in the
Horizon and no Refractions.
[Sidenote: Autumnal Equinox.