Earth

=== Axial tilt and seasons ===
{{Main|Axial tilt#Earth}}
[[File:axial tilt vs tropical and polar circles.svg|thumb|upright=1.3|Earth's axial tilt causing different angles of seasonal illumination at different orbital positions around the Sun]]
The axial tilt of Earth is approximately 23.439281°<ref name="IERS" /> with the axis of its orbit plane, always pointing towards the [[Celestial Poles]]. Due to Earth's axial tilt, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes the seasonal change in climate, with summer in the [[Northern Hemisphere]] occurring when the [[Tropic of Cancer]] is facing the Sun, and in the [[Southern Hemisphere]] when the [[Tropic of Capricorn]] faces the Sun. In each instance, winter occurs simultaneously in the opposite hemisphere.

During the summer, the day lasts longer, and the Sun climbs higher in the sky. In winter, the climate becomes cooler and the days shorter.<ref>{{cite book|last1=Rohli|first1=Robert. V.|title=Climatology|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning|year=2018|isbn=978-1-284-12656-3|edition=fourth|pages=291–292}}</ref> Above the [[Arctic Circle]] and below the [[Antarctic Circle]] there is no daylight at all for part of the year, causing a [[polar night]], and this night extends for several months at the poles themselves. These same latitudes also experience a [[midnight sun]], where the sun remains visible all day.<ref>{{cite book|last=Burn|first=Chris|title=The Polar Night|url=http://nwtresearch.com/sites/default/files/the-polar-night.pdf|publisher=The Aurora Research Institute|date=March 1996|access-date=28 September 2015}}</ref><ref>{{cite web|url=https://www.antarctica.gov.au/about-antarctica/weather-and-climate/weather/sunlight-hours/|title=Sunlight Hours|work=Australian Antarctic Programme|date=24 June 2020|access-date=13 October 2020}}</ref>

By astronomical convention, the four seasons can be determined by the solstices—the points in the orbit of maximum axial tilt toward or away from the Sun—and the [[equinox]]es, when Earth's rotational axis is aligned with its orbital axis. In the Northern Hemisphere, [[winter solstice]] currently occurs around 21 December; [[summer solstice]] is near 21 June, spring equinox is around 20 March and [[September equinox|autumnal equinox]] is about 22 or 23 September. In the Southern Hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.<ref name="bromberg2008" />

The angle of Earth's axial tilt is relatively stable over long periods of time. Its axial tilt does undergo [[nutation]]; a slight, irregular motion with a main period of 18.6&nbsp;years.<ref name="lin2006" /> The orientation (rather than the angle) of Earth's axis also changes over time, [[axial precession|precessing]] around in a complete circle over each 25,800-year cycle; this precession is the reason for the difference between a sidereal year and a [[tropical year]]. Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth's equatorial bulge. The poles also migrate a few meters across Earth's surface. This [[polar motion]] has multiple, cyclical components, which collectively are termed [[quasiperiodic motion]]. In addition to an annual component to this motion, there is a 14-month cycle called the [[Chandler wobble]]. Earth's rotational velocity also varies in a phenomenon known as length-of-day variation.<ref name="fisher19960205" />

In modern times, Earth's [[perihelion]] occurs around 3 January, and its [[aphelion]] around 4 July. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as [[Milankovitch cycles]]. The changing Earth–Sun distance causes an increase of about 6.8% in solar energy reaching Earth at perihelion relative to aphelion.<ref>{{cite web|url=https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/|title=Milankovitch (Orbital) Cycles and Their Role in Earth's Climate|work=NASA|last1=Buis|first1=Alan|date=27 February 2020|access-date=27 October 2020}}</ref><ref group="n" name="solar_energy" /> Because the Southern Hemisphere is tilted toward the Sun at about the same time that Earth reaches the closest approach to the Sun, the Southern Hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the Southern Hemisphere.<ref>{{cite web|url=http://ocp.ldeo.columbia.edu/res/div/ocp/pub/seager/Kang_Seager_subm.pdf|title=Croll Revisited: Why is the Northern Hemisphere Warmer than the Southern Hemisphere?|work=Columbia University|last1=Kang|first1=Sarah M.|last2=Seager|first2=Richard|location=New York|access-date=27 October 2020}}</ref>