Jill Welch, age 12, of Shreveport, Louisiana, for her question:
Why do the seasons change?
The experts agree that our seasons change because the earth's axis is tilted 23 1/2 degrees to the plane of its orbit. However, this bit of moving geometry is not that easy for ordinary folk to grasp. Besides, various extras tend to interfere with events that happen on a global scale. For example, our changing seasons are modified by the shape of the earth, by its lands and seas and its circulating atmosphere.
We know from experience that summer and winter are separated by the milder seasons of spring and fall. Global travelers know that the seasons alternate north and south of the equator. This is why Australia gets midsummer weather around Christmas time. These seasonal changes depend on how the earth moves around the sun. You can solve the secret by walking through a small scale demonstration.
Place a small table in the middle of a room and stack it to your eye level with books or boxes. Balance a medium sized ball on top of the pile to play the role of the sun. You play the role of the earth. Make a trial orbit as you would normally walk around a table. If you turn your head to keep an eye on the ball, obviously your circular stroll will give you a view from all sides of it. This is an oversimplified demonstration of the earth traveling around its 600 million mile orbit at an average speed of 66,000 miles per hour.
If this were the entire performance, all our days and nights would be equal and the seasons would never change. Complications occur because the earth's axis is tilted 23 1/2 degrees to its orbital path. Use a ruler to play the axis. Grasp it around the waist, tip it 23 1/2 degrees forward and hold it at arm's length. Now complete a similar orbit, watching where the two ends of the ruler point as you circle the table.
It helps to section your orbit in four equal parts. Through sections one and three, the ruler is parallel with the table and neither end points to the ball. But, one end points to the bail during section two and the opposite end points to the ball during section four. Such is the magic of moving geometry.
The two ends of your ruler, of course, represent the two poles of the earth's axis. When the North Pole bows to the sun, a larger portion of our hemisphere is exposed to daylight. Summer's heat accumulates because the days are longer and the noon sun climbs higher overhead. Six months later, things are reversed. The South Pole bows to the summer sun and we get the short cool days of winter. Spring and fall occur between bows, when neither pole is tipped toward the sun.
Almost everywhere the basic seasonal pattern is modified by geography. Exposed deserts get extra hot during the day and cool fast after sundown. Summers are cooler on high mountains. Beaches are refreshed by sea breezes. And changing weather spells arrive with ail seasons, as warm and cool air pockets circulate around the globe.