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A season is a part of a year. In most areas of the world there are four seasons in a year: spring, summer, autumn (British English) or fall (US English), and winter. In some areas there are a different number of seasons. for example, Australia's Northern Territory has six seasons.
Summer is a warm season because the days are longer and the Sun is high in the sky, giving direct light to the ground. Winter is a cold season because the days are shorter and the Sun is low in the sky, giving indirect light to the ground. Both the changes in the length of the day and the height of the Sun at noon are caused by the tilt of the Earth's spin axis with respect to the plane of the Earth's path around the Sun. At any time, in any season, the northern and southern hemispheres (halves of the Earth) have opposite seasons.
The date at which each season begins is different in different countries. In the United States, people say the seasons begin at the solstices and equinoxes. The summer solstice is the longest day of the year, and the winter solstice is the shortest. The equinox is the time when the day and the night are the same number of hours, assuming the sun were a point of light at its center. Since civil dawn occurs when the edge of the sun first appears over the horizon and civil dusk occurs when the edge drops over the horizon, the civil length of day is 12 hours long several days prior to the equinoxes. In the USA, summer begins at summer solstice, winter at winter solstice, spring at the spring (vernal) equinox and autumn at the autumnal equinox.
In Britain, people traditionally say that the seasons begin about seven weeks earlier: spring begins on Candlemas (February 2), summer on May Day (May 1), autumn on Lammas (August 1), and winter on All Hallows (November 1). The Irish calendar is similar, but Spring begins on February 1.
In Denmark, spring begins on March 1, summer on June 1, autumn on September 1 and winter on December 1. In Australia, summer begins on December 1, autumn on March 1, winter on June 1, and spring on September 1.
In the Chinese calendar, the solstices and equinoxes are in the middle of each season.