DISTRIBUTION OF INSOLATION AND THE RESULTING TEMPERATURES OF THE ATMOSPHERE, THE LAND, AND THE WATER
Source of the Earth's Heat. --It is generally held that the interior of the earth still retains an intensely high temperature, and that this hot mass is surrounded by a cool crust that is a poor conductor; so that only a small amount of heat escapes to the atmosphere. The innumerable stars, though their average temperature probably is equal to that of our sun, are too distant to have an appreciable effect in heating the exterior of the earth. It is apparent, therefore, that the sun, with an absolute temperature over twenty times as high as the absolute temperature of the surface of the earth, controls the surface temperature of our planet and its atmosphere.
Effect on Insolation of Inclination of Earth's Axis to Ecliptic, and Variation in Distance from Sun.--The quantity of heat that falls upon a horizontal area at the top of the earth's atmosphere during any consecutive twenty- four hours depends upon three conditions: (1) The altitude that the sun attains when it crosses the meridian at noon, (2) the length of the daytime, and (3) the distance of the earth from the sun; these are in a perpetual state of variation, except that near the equator the day and the night are always equal.1
Fig. 13 shows the course and the relative length of the diurnal are, at latitude 45° N., at different seasons of the year. The distance from the sun varies because the orbit of the earth is an ellipse, with the sun in one of the foci. The time that the sun remains above the horizon, and the height it reaches at noon, change from day to day and from winter to summer, because the earth's axis is inclined at a nearly constant angle to the plane of its orbit, or the ecliptic. The extent of these changes is de-____________________