\20 PHYSICAL GEOGRAPHY.

 

extends in the Northern Hemisphere as far over
the pole as the Arctic Circle.

After the 21st of June the Northern Hemi-
sphere is turned less toward the sun, and the
vertical rays continually approach the equator,
all the movements of the preceding season being
reversed, until on the 22d of September, the time
of the September equinox, the equator again receives
the vertical rays, the great circle of illumination
again coinciding with the meridian circles. The
earth has now moved from one equinox to an-
other, and has traversed one-half of its orbit.
The Southern Hemisphere then begins its spring,
the Northern its autumn.

From the 22d of September until the 20th of
March, while the earth moves through the other
half of its orbit, the same phenomena occur in
the Southern Hemisphere that’ have already. been
noticed in the Northern. Immediately after the
22d of September the inclination of the axis
causes the earth to be so turned toward the sun
_ that its rays begin to fall south of the equator ;
and, as the earth proceeds in its orbit, the South-

ern Hemisphere is turned more and more toward |

the sun, and the vertical rays fall farther and
farther toward. the pole. This continues until
the 21st of December, when the rays fall vertically
on the Tropic of Capricorn, and the December sol-
stice is reached. The great circle of illumination
now extends beyond the south pole as far as the
Antarctic Circle, but falls short of the north pole
93° 27’, reaching only the Arctic Circle. Sum-
mer then commences in the Southern Hemisphere,
and winter in the Northern.

After the 21st of December the Southern
Hemisphere is turned less and less toward the

   

—'S.FRIGID
Fig. 18, Mathematical Climatic Zones,

gun, and the part receiving the vertical rays
approaches the equator, until on the 20th of

 

March the equator again receives the vertical
rays, and, with the March equinox, spring com-
mences in the Northern Hemisphere, and with
it a new astronomical year.

The equinoxes and solstices as a rule occur on the dates
named. Occasionally. they occur immediately before or
after said dates. s

28. Mathematical Zones.—The Torrid Zone.—
That belt of the earth’s surface which lies be-
tween the tropics is called the Torrid Zone.
During one time or another throughout the
year every part of its surface receives the ver-
tical rays of the sun.

The Temperate Zones are included between the
tropics and the polar circles. The northern zone
is called the North Temperate Zone, and the south-
ern zone, the South Temperate Zone.

The Polar Zones are included’ between the
polar circles and the poles. The northern zone
is called the North Frigid Zone, and the southern .

’ gone, the South Frigid Zone.

These zones, which are separated by the parallels of lati-
tude, are generally termed the astronomical or mathematical
zones to distinguish them from others called physical zones,
which are bounded by the lines of mean annual temper-

- ature.

It will be noticed that the distance of the tropics from
the equator and of the polar circles from the poles is 23°
27’, or the value of the’inclination of the plane of the
ecliptic to the plane of the equator.

29, Length of Day and Night.— Whenever
more than half of either the Northern or South-
ern Hemisphere is illumined, the great circle of
illumination will divide the parallels unequally,
and the length of the daylight in that hemisphere
will exceed that of the night in proportion as the
length of the illumined part, measured along any
of the parallels, exceeds that of the dark part.

The length of daylight or darkness may exceed
that of one complete rotation of the earth. The
great circle of illumination may at times pass
over the poles as far beyond them as 23° 27’;
and places situated within this limit may remain
during many rotations exposed to the rays of the
sun.

A little consideration will show that the longest day
must occur at the poles, since the poles must continue
to receive the sun’s rays from the time they are first illu.
mined at one equinox until the sun passes through a sol-
stice and returns to the other equinox. Nowhere, outside
the polar circles, will the length of daylight exceed one
entire rotation of the earth.

The length of the longest day at the equator, latitude
0°, is 12 hours. ;

Of the longest day a3; the poles, latitude 90°, is six
months. .