Why the Seasons Change

            Light energy from the sun does not change to heat energy as it passes through the air. But when sunlight strikes the surface of our earth, heat energy is produced. The surface of the earth becomes warm, and this in turn warms the air that is close to it. When light covers a small space, the heat increases. When the same amount of light spreads out over a wide area, the heat is spread out also, and there is not as much heat energy at any one spot.
            In December the sunlight produces less heat energy in Omaha, Nebraska than it does in Buenos Aires, Argentina. The tilt of the earth has something to do with this difference in temperature. The seasons are different as different parts of the earth changes their positions in relation to the sun. This happens because our earth is tilted.
            The number of hours of daylight each day has some effect on temperatures. When the days are long, there are more hours for the surface of the earth to be warmed by the sunlight. There is less time at night for that part of the earth to cool. Winter days are short, and summer days are long. It takes the earth one day, or about 24 hours, to spin around once. To make one complete journey about the sun takes our spinning earth one year, or about 365¼ days. Another way to say this is that the earth rotates 365¼ times while it revolves about the sun once.
         The north pole and the south pole of the earth are opposite ends of an imaginary line called the earth's axis. As the earth spins, it turns about this imaginary line. If you started at the north pole and continued the line of the earth's axis out into space, it would point toward the North Star, or polestar. All year long, as the earth journeys about the sun, the line of the earth's axis tilts toward the North Star.  
            As the sun shines on the earth, half the earth is always having day. All year long, part of the earth is turned toward the sun, and part is in the shadow we call night. Half the earth is always lighted, but the lighted half is constantly changing.
            As the earth moves onward in its journey around the sun, some parts of the earth receive different amounts of heat energy than they did in September. Days and nights do not continue to be of equal length. In September neither pole tilts toward the sun, but in December this is not so. The sun shines bright on the Southern Hemisphere in December. The Northern Hemisphere tilts away from the sun in December. The light spreads out more. This means that the sunlight does not produce as much heat energy here. The days are becoming cold and wintry. There are no leaves on most of the trees. Autumn is over and winter has begun.
            Because the earth is tilted, there are more hours of daylight south of the equator in December than there are in September. At the same time there are more hours of night and fewer hours of day in the Northern Hemisphere.
            Days and nights are about equal everywhere on the globe in March, as they were in September. In March also neither hemisphere is tilted more than the other toward the sun. Places north of the equator receive about as much light and heat energy in March as those the same distance south of the equator. But again the seasons are different. You remember that December was a time of higher temperatures in the Southern Hemisphere and of lower temperatures in the Northern. As the months change from December to March, the temperatures rise in the Northern Hemisphere while they are dropping south of the equator. Even though in March the temperatures of both hemispheres are becoming somewhat the same, the seasons are different. This is because the weather in one hemisphere is moving from a period of warmth to a cooler time while just the opposite is happening in the other hemisphere.
            The seasons change as first one pole and then the other is tilted toward the sun. Autumn, winter, spring, and summer – one season follows another because the earth is tilted as it makes its yearly trip around the sun.