The Earth revolves constantly around the Sun, and at the same time
rotates upon an axis inclined twenty-three and one-half degrees towards
the plane of its orbit. In consequence of the inclination of the axis,
the declination of the Sun, or its angular distance from the Equator,
varies with the advance of the Earth in its orbit, causing periodical
variations in the length of day and night, and, consequently, in
temperature.
VERNAL EQUINOX. On the twentieth of March, at mid-day, the Sun is
vertical at the Equator. Rising directly in the east it ascends the
heavens to the zenith, and, descending, sets directly in the west.
The illuminated hemisphere extends from pole to pole, and embraces half
of every parallel of latitude; hence every point on the Earth’s surface
is under the rays of the Sun during half of the diurnal rotation; the
days and nights are equal all over the globe; and the heating power of
the Sun is the same in both the northern and the southern hemisphere.
SUMMER SOLSTICE. As the Earth advances in its orbit the vertical Sun
declines northward; and on the twenty-first of June, at the Summer
Solstice, it is over the northern Tropic, twenty-three and one-half
degrees from the Equator.
The illuminated hemisphere, extending ninety degrees on each side of the
parallel of the vertical Sun, reaches twenty-three and one-half degrees
beyond the North Pole; but, at the south, it barely touches the
Antarctic circle. It embraces more than half of each parallel north of
the Equator, hence throughout the northern hemisphere the day is longer
than the night, the difference in their duration increasing with the
latitude; and all points within the Arctic circle are in the light
during the entire rotation.
In the southern hemisphere, less than half of each parallel being
illuminated, the night is longer than the day, and within the Antarctic
circle there is constant night. The heating power of the Sun is now at
the maximum in the northern hemisphere, while in the southern it is at
the minimum.
AUTUMNAL EQUINOX. On the twenty-second of September, the distribution of
light and heat upon the two hemispheres is the same as at the Vernal,
and at the _Winter Solstice_, on the twenty-second of December, it is
the reverse of that at the Summer Solstice.
=WHAT CAUSES THE SEASONS AND DAY AND NIGHT=
[Illustration: =FIGURE ILLUSTRATING THE CHANGE OF SEASONS THROUGHOUT THE
YEAR=
The change of seasons is caused by the _revolution_ of the earth around
the sun, and the inclinations of the planes of the equator and ecliptic.
These causes also account for the difference in the length of the days
and nights and the difference in the height of the midday sun. The exact
duration of the seasons we get by observing the dates of equinoxes and
solstices.]
[Illustration: =FIGURE SHOWING THE CAUSE OF DAY AND NIGHT=
The _revolution_ of the earth gives us the length of the year; its
_rotation_ on its axis, the length of the day and night, by causing the
risings and settings and daily apparent motion of the sun and stars.]