MATHEMATICAL GEOGRAPHY. 19

 

SEPTEMBER
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Fig. 15. The Orbit of the Earth, showing the Change of Seasons,

As the earth proceeds in its orbit, the inclina-

tion of the axis causes it to turn the Northern .

Hemisphere more and more toward the sun. The
vertical rays, therefore, fall on portions farther
and farther north until, on the 2Zst of June, the

   

Fig. 16, The Earth at an Equinox.

vertical rays reach their farthest northern limit,
and fall directly on the Tropic of Cancer, 28° 27'
N., when the sun is said to be ‘at its summer sol-
stice.

Since the portions receiv'ag the vertical rays
of the sun are now onthe Tropic of Cancer,

 

the light and heat must extend in the Northern
Hemisphere to 23° 27’ beyond the north pole, or |
to the Arctic Circle; while in the Southern Hemi-
sphere they must fall short of the south pole by
the same number of degrees, or reach to the Ant-

 

Fig. 17, The Earth at the Summer Solstice,

arctic Circle. The Northern Hemisphere then be-
gins its summer, and the Southern tts winter.

The relative positions of the illumined and
non-illumined portions of the earth at the sum-
mer solstice are more clearly shown in Fig. 17.
Here, as is shown, the great circle of illumination