A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical cultures whose number of seasons varies.
Axial parallelism is a characteristic of the Earth (and most other orbiting bodies in space) in which the direction of the axis remains parallel to itself throughout its orbit.
Illumination of Earth at each change of astronomical season
Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m2).
Four Seasons by Alphonse Mucha (1897)
Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is enabled by Earth being a water world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its global ocean, covering 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, most of which is located in the form of continental landmasses within Earth's land hemisphere. Most of Earth's land is somewhat humid and covered by vegetation, while large sheets of ice at Earth's polar deserts retain more water than Earth's groundwater, lakes, rivers and atmospheric water combined. Earth's crust consists of slowly moving tectonic plates, which interact to produce mountain ranges, volcanoes, and earthquakes. Earth has a liquid outer core that generates a magnetosphere capable of deflecting most of the destructive solar winds and cosmic radiation.
The Blue Marble, Apollo 17, December 1972
A 2012 artistic impression of the early Solar System's protoplanetary disk from which Earth and other Solar System bodies were formed
Pale orange dot, an artist's impression of Early Earth, featuring its tinted orange methane-rich early atmosphere
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years from now
