Fig. 23.—Winter Solstice, December 21. North Pole is dark now instead of light, as at Summer Solstice. Pole leans in same direction but Earth being on opposite side of its orbit rays come from opposite direction. Refer to [Figure 24].
If the earth’s axis were vertical to the plane of its orbit all places on its surface always would have days of twelve hours each and the nights would be of the same length; sunshine would just touch both poles ([Figure 21]) throughout the entire course of the earth around the sun and there would be no seasons. One would need to change one’s location on the earth in order to get a change of weather, which would be monotonous and quite different from the active conditions of the atmosphere that we now enjoy. The whole conditions of life would be altered for the worse. You have seen a top tilt over to one side as it spun on the floor. In the same way the earth spins on its axis as it pursues its course around the sun without changing the direction towards which its axis points, as shown by [Figure 24].
Fig. 24.—Note that direction of axis does not change as Earth moves around Sun. This causes variation in area of surface illuminated. If axis were perpendicular to plane of orbit there would be no seasons.
Fig. 25.—As angle of incidence decreases from 90° to 10° the heat received on upper end of blocks is spread over greater area at bottom, and its temperature diminished. (Abbe.)
The intensity of the sun’s rays at sunrise and at sunset is less than at midday because the quantity of heat received at the outer limits of the atmosphere on a given area, as for instance at the area of the upper ends of the blocks in [Figure 25], passes through a deeper stratum of air the lower the angle of incidence, and because it is distributed over a larger area when it reaches the surface of the earth.
As the heat of day increases from morning until midday and then decreases, so does the heat of the year increase from midwinter to midsummer and then decrease, and for the same reason: change in obliquity of the sun’s rays, to which must be added change in distance from the central luminary. [Figure 26] shows that the sun reaches its greatest midday altitude on June 21st and its least on December 21st.