In studying Fig. 15 let us begin with the spring equinox, March 21. On that date, as appears in the uppermost of the little globes, the sunlight barely reaches the North Pole. In other words from there the sun would be seen on the horizon. There it remains through out the twenty-four hours, swinging around the horizon through but not seeming to rise higher or sink lower. Except at the poles all parts of the earth at this date have a day and night of equal length. Therefore this date is called the spring equinox, for the name means " equal night." There is also an autumn equinox about September 22.
Look now at the diagrams for April, May, and June. At the pole the sun is now considerably above the horizon. In spite of the earth's rotation, it remains visible at all times, so that there is no night. It stands at a slowly increasing height day after day. If its path were traced in the heavens it would form a fiat spiral mounting slowly upward until it reaches its highest point about June 21. Then the sun ceases to rise in the heavens, and from this point of view seems to stand still before it begins to descend again. Hence June 21 is called the solstice, or standing still of the sun.
Let us work out the length of the days at different latitudes and at different seasons. For instance, on July 21 five-sixths of the Arctic Circle is in the sunlight. Therefore a miner at the great bend of the Yukon would see the sun five-sixths of the time, or about twenty hours. During the night of four hours the sun would be so little below the horizon that he could see all the time. Let us see how day and night would compare about July 21 in St. Paul and Minne apolis in latitude 45°. In the July diagram in Fig. 15 approxi mately four and a half out of the twelve divisions into which the meridians divide the 45th parallel are in the darkness. As each division represents 30° of longitude the dark part of the circle contains about and the light part 225°. As 15° of longitude equal one hour of time, the night lasts nine nours, and the day fifteen.
The Cause of the Seasons.—(1) The Relative Length of Day and Night.—The seasons play so overwhelming a part in our lives that it is interesting to understand their causes. The difference between summer and winter is due to three chief causes, each of which is dependent upon the inclination of the earth's axis. The first, but not the most important cause, is the relative length of day and night.
We have already seen that when the period of sunlight is short in winter, the amount of heat given to the earth by the sun is necessarily small, 'but it increases as the days grow longer.
(2) The Relative Distance Traversed by the Sun's Rays in the Atmosphere.—The second cause of the seasons is the degree to which the sun's heat is absorbed by the atmosphere. At sunrise or sunset, even on the hottest day, one can look directly at the sun without difficulty. At noon, however, this is impossible. The reason for the contrast is that the air itself intercepts much light and heat, while the dust and moisture contained in the air intercept still more. At sunrise or sunset the rays of light reach the eye only after passing through much more air than at noon, as may be seen in Fig. 16.
Hence much less heat reaches the earth's surface when the sun is low. Since the sun never rises high in polar latitudes, such regions are always cold. Since the sun is low during part of the year in middle latitudes, and high at other times such places have pronounced seasons of warm and cool weather. Where the sun is always high in equatorial latitudes, the weather is warm at all times and the seasons are not pronounced.
(3) The Varying Slant of the Sun's Noonday Rays.—A third important reason for the difference of the seasons is illustrated in Fig. 17. The middle globe shows the earth at the equinoxes, March 21 and SepteMber 23. The sun, which is far away to the right, is so placed that its rays are vertical at the equator. Between the sun and the earth has been placed a screen with two rectangular holes of the same size. The same amount of sunlight falls through each and warms a spot on the earth's surface. The spot at the equator, however, is much smaller than the one between 50° and 60° farther north. There is a difference in size because at the equator the rays fall vertically and hence cover the smallest pos sible amount of space, while toward the poles they fall aslant and in this particular latitUde are spread over an area twice as large as at the equator. Since the amount of heat is the same in both cases, a square niile, for instance, would receive twice as much heat at the equator as a square mile in the other position. This simple illustration shows that the sun gives most heat where its rays are vertical and least where they are most slanting.