The earth's axisthe imaginary line around which the earth turns in its daily rotationis not at right angles to the path followed by the earth in its orbit around the sun; instead, it is slightly tilted. The seasons as we know them are caused by this tilt and by the fact that the earth's axis always holds the same orientation in space.
The tilt of the earth's axis is 2327', or almost 231/2. If the axis were not tilted, the sun would always be directly over the Equator and the Northern and Southern hemispheres would both have constant, similar weather the year round.
As the earth travels around the sun, the North Pole is sometimes directed toward the sun and sometimes directed away from it. When the North Pole is directed toward the sun, the sun's rays strike most directly on the Northern Hemisphere. The more directly the rays strike, the closer to vertical they are. Vertical rays of sunlight are more effective in producing light and heat than are slanting rays. There are two reasons: (1) slanting rays must pass through a greater thickness of atmosphere than vertical rays and thus lose more of their heat; and (2) slanting rays are spread out over a greater area than vertical rays and are therefore less concentrated. Thus when the North Pole is directed toward the sun, summer is produced in the Northern Hemisphere; the Southern Hemisphere has winter. When the South Pole is directed toward the sun, the Southern Hemisphere receives the most heat and has summer.
The greater efficiency of the sun's rays is one reason summer is warmer than winter. A second reason is that the days are longer. While the sun is shining, the land and air heat up. When the sun goes down, they begin to lose heat. From the vernal equinox to the autumnal equinox, each day in the Northern Hemisphere is longer than the night and more heat is gained from the sun during the day than is lost at night.
The reverse is true as winter approaches and the northern half of the earth begins to cool. Not only does the sun move lower in the sky and become less effective in providing heat, but it shines for a shorter time each day, and heat is lost for a longer time than it is gained.
The earth's orbit is slightly elliptical, or elongated, so the earth is farther from the sun at some times than at other times. The earth is closest to the sun and moving most rapidly in January. In early July, when the earth is farthest from the sun, it is moving more slowly. The earth takes 186 days to go from the vernal to the autumnal equinox, but only 179 days to return from the autumnal to the vernal equinox.
There is also a small difference in the amount of heat received by the earthless heat in July than in January. The total effect of this variation in time and heat is that the seasons in the Northern Hemisphere are a little milder than those in the Southern Hemisphere. The northern winter is shorter and warmer than the southern winter. The northern summer is longer and cooler than the southern summer.