Part of a natural cyclical change in the prevailing environmental conditions on the surface of a planet, over the course of a complete orbit round the Sun. A planet experiences seasons if its rotation axis is not at 90í to the plane of the ecliptic. Seasonal effects, particularly on the polar ice caps, are quite marked on the Earth and Mars.
Conventionally, four seasons are identified - spring, summer, autumn and winter - but there are no strict divisions between them and seasonal conditions may vary considerably from year to year.
