Seasons, the four divisions of the year, called spring, summer, autumn (or fall), and winter. North and south of the tropics, summer is the warmest season, winter the coolest Spring and autumn are transitions between the two extremes. In the tropics, little temperature variation occurs with the seasons.
Like the day and the year, the seasons are based on astronomical occurrences. The seasons, however, are related only to the year itself and not to any other units of time.
In the Northern Hemisphere, spring begins about March 21, summer about June 21, autumn about September 23, and winter about December 22. In the Southern Hemisphere, it is autumn that begins in March and spring that begins in September. Winter in the Southern Hemisphere begins about June 21 and summer about December 22, so Christmas comes in summer.
|First day of the astronomical seasons in the Northern Hemisphere|
|2003||March 20||June 21||Sept. 23||Dec. 22|
|2004||March 20||June 20||Sept. 22||Dec. 21|
|2005||March 20||June 21||Sept. 22||Dec. 21|
|2006||March 20||June 21||Sept. 22||Dec. 21|
|2007||March 20||June 21||Sept. 23||Dec. 22|
|2008||March 19||June 20||Sept. 22||Dec. 21|
|2009||March 20||June 20||Sept. 22||Dec. 21|
|2010||March 20||June 21||Sept. 22||Dec. 21|
|2011||March 20||June 21||Sept. 23||Dec. 21|
|2012||March 19||June 20||Sept. 22||Dec. 21|
|2013||March 20||June 20||Sept. 23||Dec. 21|
|2014||March 20||June 21||Sept. 22||Dec. 21|
|2015||March 20||June 21||Sept. 23||Dec. 21|
|2016||March 19||June 20||Sept. 22||Dec. 21|
|2017||March 20||June 20||Sept. 22||Dec. 21|
Year after year, the seasons form a never-ending cycle. The sun is farthest south (lowest) in the sky and the period of daylight shortest in the Northern Hemisphere about December 22, the date of the winter solstice. From then on, each day the sun reaches a little higher (farther to the north) in the noon sky and each day the period of daylight is a little longer. About March 21, on the vernal equinox, the sun reaches halfway on its northward journey in the sky; it crosses the Equator. On that day, daylight and darkness are the same length.
The days continue to get longer north of the tropics and the sun farther north in the noon sky until June 21, the date of the northern summer solstice. The sun is then directly overhead at 2327' north of the Equatorthe Tropic of Cancer. After the summer solstice, the sun begins to move southward again. Each day in the area north of the tropics, the period of daylight is a little shorter and the night is a little longer. Each day the sun is a little farther south in the sky at noon. In September comes the autumnal equinox, when day and night are again the same length. The sun continues southward until the winter solstice is reached, when the sun is directly over the Tropic of Capricorn (2327' south of the Equator).