Man's physical universe

xanabras

CLIMATE AND WEATHER 253

orbit.

For this reason the north pole is indined toward the sun for half

of the year, and the south pole is inclined toward the sun during the

other half of the year. The

two extreme positions occur

on June 21 and December 22.

The four seasons do not

exist at the equator, and the

days and nights there are of

the same length. North of

the equator the days become

I

II

shorter from June 21 to December

22, while they become

Fig. 76. I, the earth in June; II, the earth

in December. (From the Yerkes Observatory,

longer during the same period

reprinted by permission of the Chicago University

Press.)

in the southern hemisphere.

The diagram shows that the

day is twenty-four hours long at the Arctic Circle on June 21, and

the night is twenty-four hours long on December 22.

The coldest period in the northern hemisphere at present is that time

when the earth is closest to the sun, but every 13,000 years this condition

is reversed because of precession. The distance from the earth

to the sun varies from 91,500,000 miles to

94,500,000 miles, because

the earth travels in an elliptical path. The seasons in the southern

hemisphere are more extreme; and in the northern hemisphere it is

warmer during the winter and cooler during the summer than it would

otherwise be, because the earth is closest to the sun when it is winter

in the north.

Fall Equinox fZ^^.^ Day ano Night Equal

24HourDay

Long Day

l2-HourDay

Spring Equinox

Day and Nisht Equal

Fig. 77. The seasons; a diagram of the earth in various positions in its orbit

illustrating the cause of the seasons.

The amount of heat received by the earth in the different latitudes

is determined not only by the lengths of the days and nights but also

by the angle at which the sun's rays strike the earth. Thus the winter

is not as hot as the summer, just as the evening is not as hot as the

noonday.

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