| Fig. 1 This is a diagram of the seasons. Note that, regardless of the time of day (i.e. the Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle. |
| Fig. 2 As the Earth revolves around the Sun, the seasons in the northern and southern hemispheres are opposite. |
A season is one of the major divisions of the year. Typically, the year is divided into four seasons: spring, summer, autumn (fall), and winter. Some cultures may have a different number of seasons; for instance, some indigenous peoples in Australia's Northern Territory use six seasons.
In tropical regions it is common to speak of the rainy (or wet, or monsoon) season and the dry season, as the amount of precipitation may vary more drastically than the average temperature.
The seasons are caused ultimately by the fact that the Earth's axis is not perpendicular to its orbital plane; it deviates by an angle of approximately 23.5 degrees of arc. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1). This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons (see Fig. 2 and Seasonality table).
Climatic effects
In most parts of the world, seasons are marked by changes in the amount of sunlight, which in turn often cause cycles of dormancy in plants and hibernation in animals. In the tropics, there is no noticeable change in the amount of sunlight, but there are still fluctuations in rainfall, producing a similar alternation between fertile and infertile times of the year. The concept of seasonality originated from these changes long before its celestial cause became known.
These fluctuations are more pronounced at higher latitudes. The Equator does not have any noticeable fluctuation at all, while the North Pole and South Pole have extreme fluctuations. There, the sun rises once in the spring and sets once in the fall; thus, the day and night last uninterrupted for 183 calendar days each. Seasonal weather fluctuations also depend on factors such as proximity to oceans or other large bodies of water, currentss in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds.
The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. Thus, when it is day in the North Pole it is night in the South pole and vice-versa. Moreover, when it is summer in the Northern hemisphere, it is winter in the Southern hemisphere, and vice versa, and when it is spring in the Northern hemisphere it is autumn in the Southern hemisphere, and vice versa.
The date at which each season begins depends on how it is defined. Astronomically, summer begins at summer solstice, winter at winter solstice, spring at the vernal equinox and autumn at the autumnal equinox.
In the Chinese calendar, the seasons are defined so that the solstice or equinox, occurs in the Middle of the season.
Reckoning
| Seasonality table | ||||
|---|---|---|---|---|
| Northern Hemisphere | Northern Hemisphere alternate | Month | Southern Hemisphere | Southern Hemisphere alternate |
| Winter | Winter | January | Summer | Summer |
| February | ||||
| Spring | March | Autumn | ||
| Spring | April | Autumn | ||
| May | ||||
| Summer | June | Winter | ||
| Summer | July | Winter | ||
| August | ||||
| Autumn | September | Spring | ||
| Autumn | October | Spring | ||
| November | ||||
| Winter | December | Summer | ||