A surprisingly large number of people misunderstand what causes the
seasons. Some, of course, think that the earth is closer to the sun during the
summer. In fact, the earth is slightly closer to the sun in the northern
winter.Return to my Astronomy/Space pages
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© Copyright 1997, Jim Loy
A surprisingly large number of people misunderstand what causes the
seasons. Some, of course, think that the earth is closer to the sun during the
summer. In fact, the earth is slightly closer to the sun in the northern
winter.
Most people seem to know that the seasons are caused by the tilt of the earth (23 degrees 27 minutes). But many are hazy about the details. The books tell us that the sunlight is "indirect" in the winter. What does that mean? Is the sunlight somehow reflected or bent? No, it goes straight to the earth. Then why is it called indirect?
In the winter, the sun is lower in the sky.
The light is indirect because the light strikes the earth from a lower,
shallower angle, a glancing blow. Because of this lower angle, the same amount
of sunlight is spread out. Any particular area on the earth receives less
sunlight, because the light is spread out over more earth, because of this
lower angle.
Another result, of the sun being lower in the sky, is that the days are shorter. Sunrise is later, and sunset is earlier. This gives the sun less time to warm the earth.
Winter in the southern hemisphere happens during the northern summer. See the first diagram. The earth is slightly farther away from the sun, during the southern winter. Is their winter colder than ours? Yes, more or less. It depends a lot on geography (mountains, ocean currents, air currents, and properties of earth, water, and ice). But, in general, southern winters are colder, and southern summers are warmer.
The use of the word "indirect" is accurate, but confusing. A better word could be chosen. Or the whole situation could be explained better, as I have tried to do here.
The first day of winter (Winter Solstice) is the shortest day of the year. Shouldn't this day be the coldest day of the year? And, shouldn't it be in the middle of winter? Well, you may have noticed that January is generally colder than December. It takes a while for the earth and air to lose its heat. We get less sunlight at the end of December. But, the earth and air continue to lose heat through most of January. It causes a sort of seasonal inertia.
Likewise, the Summer Solstice, the longest day of the year is the beginning of summer. The Spring Equinox and the Autumn Equinox are midway between the two solstices.