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The seasons result from the Earth's [[axis of rotation]] being [[axial tilt#Earth|tilted]] with respect to its [[orbital plane (astronomy)|orbital plane]] by an angle of approximately 23.4 [[degree (angle)|degrees]].<ref>{{cite web|last=Cain|first=Fraiser|title=Tilt of the Earth|url=http://www.universetoday.com/26778/tilt-of-the-earth/|access-date=2 May 2014}}</ref> (This tilt is also known as "obliquity of the [[ecliptic]]".)
Regardless of the time of year, the [[Northern Hemisphere|northern]] and [[southern hemisphere]]s always experience opposite seasons. This is because during [[summer]] or [[winter]], one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit
For approximately half of the year (from around March{{nbsp}}20 to around September{{nbsp}}22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June{{nbsp}}21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December{{nbsp}}21. The two instants when the Sun is directly overhead at the [[Equator]] are the [[equinox]]es. Also at that moment, both the [[North Pole]] and the [[South Pole]] of the Earth are just on the [[terminator (solar)|terminator]], and hence day and night are equally divided between the two hemispheres. Around the [[March equinox]], the Northern Hemisphere will be experiencing [[Spring (season)|spring]] as the hours of [[daylight]] increase, and the Southern Hemisphere is experiencing [[autumn]] as daylight hours shorten.
The effect of axial tilt is observable as the change in [[day length]] and [[altitude (astronomy)|altitude]] of the Sun at solar [[noon]] (the Sun's [[culmination]]) during the [[year]]. The low angle of Sun during the winter months means that incoming rays of solar radiation are [[effect of sun angle on climate|spread over a larger area]] of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres.▼
▲The effect of axial tilt is observable as the change in [[day length]] and the [[altitude (astronomy)|altitude]] of the Sun at solar [[noon]] (the Sun's [[culmination]]) during the [[year]]. The low angle of the Sun during the winter months means that incoming rays of solar radiation are [[effect of sun angle on climate|spread over a larger area]] of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres.
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