Editing Planet (section)

==== Axial tilt ====
{{Main|Axial tilt}}
[[File:AxialTiltObliquity.png|thumb|Earth's [[axial tilt]] is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing.]]

Planets have varying degrees of axial tilt; they spin at an angle to the [[reference plane|plane]] of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the [[Northern Hemisphere]] points away from its star, the [[Southern Hemisphere]] points towards it, and vice versa. Each planet therefore has [[season]]s, resulting in changes to the [[climate]] over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its [[solstice]]. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of [[Climate of Uranus|its solstices]].<ref name="Weather">{{cite web | last=Harvey |first=Samantha |date=1 May 2006 |url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-url=https://web.archive.org/web/20060831201346/http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725 |archive-date=31 August 2006 |title=Weather, Weather, Everywhere? |publisher=NASA |access-date=23 August 2008}}</ref> In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones.<ref name="factsheets">[https://web.archive.org/web/20160304052405/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html Planetary Fact Sheets], NASA</ref><ref name="Schorghofer2016">{{Cite journal |last1=Schorghofer |first1=N. |last2=Mazarico |first2=E. |last3=Platz |first3=T. |last4=Preusker |first4=F. |last5=Schröder |first5=S. E. |last6=Raymond |first6=C. A. |last7=Russell |first7=C. T. |date=6 July 2016 |title=The permanently shadowed regions of dwarf planet Ceres |journal=Geophysical Research Letters |volume=43 |issue=13 |pages=6783–6789 |bibcode=2016GeoRL..43.6783S |doi=10.1002/2016GL069368 |doi-access=free}}</ref><ref name=Carry2009>{{Cite journal|title=Physical properties of (2) Pallas|author=Carry, B.|date=2009|doi=10.1016/j.icarus.2009.08.007 |arxiv=0912.3626|display-authors=etal|bibcode = 2010Icar..205..460C|volume=205|issue=2|journal=Icarus|pages=460–472|s2cid=119194526}}</ref><ref name="Thomas1997b">{{cite journal | title=Vesta: Spin Pole, Size, and Shape from HST Images | date=1997 | author=Thomas, P. C. | bibcode=1997Icar..128...88T | display-authors=etal | journal=Icarus | volume=128 | issue=1 | pages=88–94 | doi=10.1006/icar.1997.5736| doi-access=free }}</ref> Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have a negligible axial tilt as a result of their proximity to their stars.<ref>{{cite journal |title=Obliquity Tides on Hot Jupiters | last1=Winn | first1=Joshua N. | last2=Holman | first2=Matthew J. |journal=The Astrophysical Journal |date=2005 | doi=10.1086/432834 | volume=628 |issue=2 |page=L159 |bibcode=2005ApJ...628L.159W|arxiv = astro-ph/0506468 |s2cid=7051928 }}</ref> Similarly, the axial tilts of the planetary-mass moons are near zero,<ref>{{cite book |title=Explanatory Supplement to the Astronomical Almanac |editor-first=P. Kenneth |editor-last=Seidelmann |publisher=University Science Books |date=1992 |page=384 }}</ref> with Earth's Moon at 6.687° as the biggest exception;<ref name="Lang2011">{{cite book |last=Lang |first=Kenneth R. |url=https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |title=The Cambridge Guide to the Solar System |publisher=Cambridge University Press |year=2011 |isbn=978-1139494175 |edition=2nd |archive-url=https://web.archive.org/web/20160101071141/https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA184 |archive-date=1 January 2016}}</ref> additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years.<ref name=galileantilt>{{cite journal |first=Bruce G. |last=Bills |title=Free and forced obliquities of the Galilean satellites of Jupiter |date=2005 |volume=175 |issue=1 |pages=233–247 |doi=10.1016/j.icarus.2004.10.028 |bibcode=2005Icar..175..233B |journal=Icarus |url=https://zenodo.org/record/1259023 |access-date=6 April 2023 |archive-date=27 July 2020 |archive-url=https://web.archive.org/web/20200727063125/https://zenodo.org/record/1259023 |url-status=live }}</ref>