Disregarding the very slight amount of heat radiated from the earth's interior to the surface, and also that received from other heavenly bodies, the sun must be regarded as the source of the heat received at the earth’s surface. The greatest intensity of heat is received in equatorial regions where the sun’s rays are practically vertical; the least intensity is in polar regions where the rays fall obliquely.
The inclination of the earth’s axis to the plane of the
Redway’s Physical Geography.
Relative length of day and night.
ecliptic, 23° 27', is an important factor in the distribution of warmth. The direction of the axis, minor oscillations excepted, is constant; it ranges very nearly toward the north star. One result of the constant parallelism of the earth’s axis is a movement of the belt of vertical rays—the “heat belt”—back and forth, an angular distance of nearly 47 degrees—from the Tropic of Cancer to the Tropic of Capricorn. The polar circles, 23° 27' from each pole, mark the farthest point beyond each pole to which the sun’s rays extend when vertical at a tropic. Another result of the inclination of the earth’s axis is the increasing length of summer days and winter nights as the latitude increases. At either tropical circle the longest day is a little more than 13.5 hours; the shortest, about 10.5 hours. Within the temperate zones the longest days vary from
