Since earliest times, man has been curious about the changing of the seasons. Why is it warm in summer and cold in winter? Why do the days gradually grow longer in the spring? Why are the nights so long in winter?
We all know the earth revolves around the sun, and at the same time, it revolves on its own axis. As it moves around the sun, it’s also spinning like a top.
Now if the axis of the earth (the line from the North Pole through the South Pole) were at right angles to the path of the earth around the sun, we would have no such thing as different seasons, and all the days of the year would be of equal length.
But the axis of the earth is tilted. The reason for this is that a combination of forces is at work on the earth. One is the pull of the sun, the other is the pull of the moon, the third is the spinning action of the earth itself.
The result is that the earth goes around the sun in a tilted position. It keeps that same position all year, so that the earth’s axis always points in the same direction, toward the North Star.
This means that during part of the year the North Pole tilts toward the sun and part of the year away from it. Because of this tilt, the direct rays of the sun sometimes fall on the earth north of the Equator, sometimes directly on the Equator, and sometimes south of the Equator.
These differences in the way the direct rays of the sun strike the earth cause the different seasons in different parts of the world. When the Northern Hemisphere is turned toward the sun, the countries north of the Equator have their summer season, and the countries south of the Equator have their winter season.
When the direct rays of the sun fall on the Southern Hemisphere, it is their summer and it is winter in the Northern Hemisphere. The longest and shortest days of each year are called the summer solstice and winter solstice.
There are two days in the year when night and day are equal all over the world. They come in the spring and fall, just halfway between the solstices. One is the autumnal equinox, which occurs about September 23, and the other is the spring equinox, which occurs about March 21.