Seasons

The seasons of the year are characterized by the ever-changing patterns of stars and weather that [Page 1576]come and go year after year. There are astronomic seasons and climatic (also called meteorological) seasons. Seasons vary in different parts of the earth both astronomically and meteorologically. Each of the four seasons of spring, summer, autumn, and winter are three months in duration, but they vary in the amount of daylight and darkness, temperatures, and weather conditions.

The astronomical seasons of the year are measured by the rotation of the earth around the sun. The axis of the earth is an imaginary line running through the earth from the North Pole to the South Pole. The earth is not perpendicular to the plane of its orbit around the sun, but permanently tilted on its axis at 23 degrees and 26 minutes away from the sun. If the earth were not tilted on its axis, there would be no seasons and the weather would be about the same every day of the year.

The astronomical seasons are caused by changes in the earth's relationship to the sun in its annual orbit. The earth's orbit around the sun is elliptical, but it is not the varying distance from the sun that causes seasonal changes. Seasonal changes are rather due to the way the earth's tilt on its axis causes sunlight to strike the earth at different angles throughout all the days of the year. The equator receives the most sunshine in an almost uniform manner, but the amount of sunshine on the North and South poles varies enormously over the days of each year.

For half of each year, the sun's light falls almost directly overhead in the Northern Hemisphere. For the other half of the year, it falls almost directly overhead in the Southern Hemisphere. For this reason, the temperate zones of the Southern and Northern Hemispheres occur in reverse; when it is winter in the Southern Hemisphere, it is summer in the Northern Hemispheres. The two hemispheres are seasonal opposites of each other.

When viewed from earth, the daily track of the sun across the sky reaches different positions above the horizon. The angle of the sun relative to the earth's surface reaches its peak about noon on June 20 or June 21 each year in the Northern Hemisphere when the angle is at 23 degrees and 26 minutes north latitude. In contrast, in the Southern Hemisphere this is the time when the sun's angle is diminished to its lowest position.

Some annually varying hour occurring between June 20 and June 21 is the hour that marks the longest day of the year, or the shortest night, in the Northern Hemisphere. From June 20 or June 21 the angle of the sun declines day by day in the Northern Hemisphere. Meanwhile, it increases day by day in the Southern Hemisphere until September 21 arrives, when the length of the day and the night are exactly the same.

As the earth continues its journey around the sun, the days shorten in the Northern Hemisphere and lengthen in the Southern Hemisphere until December 21. This is the shortest day and the longest night in the Northern Hemisphere and the opposite in the Southern Hemisphere, making it winter in the Northern Hemisphere and summer in the Southern Hemisphere. On about March 20 and September 22 each year, the length of the days and night are exactly the same. March 20 or March 21 marks the vernal equinox and September 22 or September 23 marks the autumnal equinox. June 20 or June 21 marks the summer solstice and December 21 or December 22 marks the winter solstice. The exact day for the beginning of each season varies from year to year because the amount of time it takes the earth to travel around the sun each year varies. Besides the influence of gravity—which can speed or slow the journey of the earth—its orbit is not exactly 365 days in length. There is a fraction of a day that can cause the day that begins each season to shift forward or backward each year.

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