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The seasons of the year are characterized by the ever-changing patterns of stars and weather that 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.
The lengths of the seasons are in part due to the elliptical path of the earth. As the earth gets closer to the sun, its movement speeds up, while the further it gets, it slows down. The earth’s closest point to the sun is called perihelion, and its most distant point is called aphelion. The exact lengths of the seasons in the Northern Hemisphere are 92.76 days in the spring, 93.65 days in the summer, 89.84 days in the autumn, and 88.99 days in the winter. These seasonal lengths total an annual year of 356.24 days. Climatic seasons are measured by temperature, while astronomical seasons are measured by the relationship of the earth to the sun.
As the earth makes its annual journey, the amount of sunshine builds and recedes as the days lengthen and shorten. The amount of heat on the surface of the earth increases as the phenomenon of heat lag occurs, which is the warming and cooling of the earth. The time of maximum insolation in the Northern Hemisphere peaks on June 21 and is thereafter decreasing because the days are shortening again. However, the heating of the earth increases as the land masses and air masses continue to increase in temperature. As a result, the astronomical and meteorological seasons do not exactly match each other. The temperature of the summer season is the hottest time of the year. But its temperature usually rises with the heating of the earth during the summer even though sunshine is actually diminishing.
The North and South Poles have two seasons: a light season (summer) and a dark season (winter). In the dark season, there is no light available from the sun, while in the light season, it is light for 24 hours a day at the peak of the summer. In tropical zones where the heating of the earth by the sun is fairly constant, there are usually two seasons. In the wet season, rains come regularly, while a dry season elicits no rain. During the wet seasons, tropical storms such as hurricanes or typhoons occur.
- Charles Laird Calia, The Stargazing Year: A Backyard Astronomer’s Journey Through the Seasons of the Night Sky (Penguin Group, 2006);
- Jerry Dennis, It’s Raining Frogs and Fishes: Four Seasons of Natural Phenomena and Oddities in the Sky (HarperCollins, 1992);
- Robert Gendler, A Year in the Life of the Universe: A Seasonal Guide to Viewing the Cosmos (MBI Publishing Company, 2006);
- Jeff Kanipe, A Skywatcher’s Year (Cambridge, 1999);
- Stanley Lilijaszek and S.S. Strickland, eds., Seasonality and Human Ecology (Cambridge University Press, 1993).