Seasons

Seasons are divisions of the year distinguished by changes in climate, day length, and solar radiation reaching each part of the Earth. The common names in Western languages are spring (dry-to-wet transition), summer, autumn (wet-to-dry transition), and winter (cold).

The fundamental cause of seasons is not the Earth's distance from or proximity to the sun, but rather the tilt of the Earth's rotation axis, approximately 23.5 degrees relative to the orbital plane. Consequently, the northern and southern hemispheres each receive direct sunlight for varying periods, causing seasons to shift alternately between the two—when the north has summer, the south has winter.

Astronomical definition of seasons

Astronomically, seasons are defined by four points on the orbit: two equinoxes (day-night equal) and two solstices (the points where the sun's path turns most steeply). Equinoxes occur around 20–21 March and 22–23 September, while solstices occur around 20–21 June and 21–22 December. The exact dates shift by up to one day from year to year.

Regional variation

The four-season model based on temperature is clearly evident in temperate zones. In tropical regions near the equator, where temperature remains fairly constant year-round, seasons are often divided instead by rainfall: rainy season and dry season. In polar regions near the Arctic, the most significant marker is day length, with periods of continuous daylight followed by extended darkness.

Somalia and the Horn of Africa

Somali peoples have historically employed four seasons tied to monsoons and rainfall: gu' (first rains, typically April–June), xagaa (heat and coastal winds, July–September), deyr (second rains, October–November), and jiilaal (drought and cold, December–March). This division forms the basis for pastoral and agricultural practices, such as pastoral migration and planting times.