Editing Module:Uranus (section)

=== Axial tilt ===
[[File:Uranus orientation 1985-2030.gif|thumb|Simulated Earth view of Uranus from 1986 to 2030, from southern summer solstice in 1986 to equinox in 2007 and northern summer solstice in 2028.]]
The Uranian axis of rotation is approximately parallel to the plane of the Solar System, with an [[axial tilt]] of 82.23°.<!-- To match the description on the page [[Axial tilt]], 82.23° and not 97.8° is used throughout this page. See the following explanation. --> Depending on which pole is considered north, the tilt can be described either as 82.23° or as 97.8°. The former follows the [[International Astronomical Union]] [[axial tilt|definition]] that the north pole is the pole which lies on Earth's North's side of the [[invariable plane]] of the [[Solar System]]. Uranus has [[Retrograde and prograde motion|retrograde]] rotation when defined this way. Alternatively, the convention in which a body's north and south poles are defined according to the [[right-hand rule]] in relation to the direction of rotation, Uranus's axial tilt may be given instead as 97.8°, which reverses which pole is considered north and which is considered south and giving the planet prograde rotation.<ref>{{cite web |url=http://roger.ecn.purdue.edu/~masl/documents/masl/coords.html |title=Coordinate Frames Used in MASL |date=2003 |access-date=13 June 2007 |archive-url=https://web.archive.org/web/20041204061125/http://roger.ecn.purdue.edu/~masl/documents/masl/coords.html <!--Added by H3llBot--> |archive-date=4 December 2004}}</ref> This gives it seasonal changes completely unlike those of the other planets. Pluto and asteroid [[2 Pallas]] also have extreme axial tilts. Near the [[solstice]], one pole faces the Sun continuously and the other faces away, with only a narrow strip around the equator experiencing a rapid day–night cycle, with the Sun low over the horizon. On the other side of Uranus's orbit, the orientation of the poles towards the Sun is reversed. Each pole gets around 42&nbsp;years of continuous sunlight, followed by 42&nbsp;years of darkness.<ref>{{Cite web |last=Sromovsky |first=Lawrence |date=2006 |title=Hubble captures rare, fleeting shadow on Uranus |url=http://www.news.wisc.edu/releases/12826.html |url-status=dead |archive-url=https://web.archive.org/web/20110720221646/http://www.news.wisc.edu/releases/12826.html |archive-date=20 July 2011 |access-date=9 June 2007 |website=University of Wisconsin Madison}}</ref> Near the time of the [[equinox]]es, the Sun faces the equator of Uranus, giving a period of day–night cycles similar to those seen on most of the other planets.

One result of this axis orientation is that, averaged over the Uranian year, the near-polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Nevertheless, Uranus is hotter at its equator than at its poles. The underlying mechanism that causes this is unknown. The reason for Uranus's unusual axial tilt is also not known with certainty, but the usual speculation is that during the formation of the Solar System, an Earth-sized [[protoplanet]] collided with Uranus, causing the skewed orientation.<ref>{{Cite book |title=Uranus |date=1991 |publisher=[[University of Arizona Press]] |isbn=978-0-8165-1208-9 |editor-last=Bergstralh |editor-first=Jay T. |location=Tucson |pages=485–486 |editor-last2=Miner |editor-first2=Ellis D. |editor-last3=Matthews |editor-first3=Mildred Shapley}}</ref> Research by Jacob Kegerreis of [[Durham University]] suggests that the tilt resulted from a rock larger than Earth crashing into the planet 3 to 4&nbsp;billion years ago.<ref>{{cite news |url=https://www.apnews.com/d1e2c440af57450ab82b62d035adac61 |title=Science Says: A big space crash likely made Uranus lopsided |work=[[Associated Press]] |last=Borenstein |first=Seth |date=21 December 2018 |access-date=17 January 2019 |archive-date=19 January 2019 |archive-url=https://web.archive.org/web/20190119121444/https://www.apnews.com/d1e2c440af57450ab82b62d035adac61 |url-status=live }}</ref> Uranus's south pole was pointed almost directly at the Sun at the time of ''Voyager&nbsp;2''{{'s}} flyby in 1986.<ref>{{cite journal |url=http://www.hnsky.org/iau-iag.htm |title=Report of the IAU/IAG working group on cartographic coordinates and rotational elements of the planets and satellites: 2000 |volume=82 |issue=1 |pages=83 |journal=Celestial Mechanics and Dynamical Astronomy |date=2000 |access-date=13 June 2007 |bibcode=2002CeMDA..82...83S |last1=Seidelmann |first1=P. K. |last2=Abalakin |first2=V. K. |last3=Bursa |first3=M. |last4=Davies |first4=M. E. |last5=De Bergh |first5=C. |last6=Lieske |first6=J. H. |last7=Oberst |first7=J. |last8=Simon |first8=J. L. |last9=Standish |first9=E. M. |last10=Stooke |first10=P. |last11=Thomas |first11=P. C. |doi=10.1023/A:1013939327465 |s2cid=189823009 |archive-date=12 May 2020 |archive-url=https://web.archive.org/web/20200512151452/http://www.hnsky.org/iau-iag.htm |url-status=dead |url-access=subscription }}</ref><ref>{{cite web |url=http://pds.jpl.nasa.gov/documents/sr/stdref_021015/Chapter02.pdf |title=Cartographic Standards |work=NASA |access-date=13 June 2007 |url-status=dead |archive-url=https://web.archive.org/web/20040407151631/http://pds.jpl.nasa.gov/documents/sr/stdref_021015/Chapter02.pdf |archive-date=7 April 2004 }}</ref>

{| class="wikitable" style="text-align: center; margin-left: 20px;"
|+List of solstices and equinoxes<ref>{{cite conference |last=Hammel |first=Heidi B. |date=5 September 2006 |title=Uranus nears Equinox |url=http://www.apl.ucl.ac.uk/iopw/uworkshop_060905.pdf |archive-url=https://web.archive.org/web/20090225084057/http://www.apl.ucl.ac.uk/iopw/uworkshop_060905.pdf |archive-date=25 February 2009 |book-title=A report from the 2006 Pasadena Workshop}}</ref>
|-
! Northern hemisphere
! Year
! Southern hemisphere
|-
| Winter solstice
| 1902, 1986, 2069
| Summer solstice
|-
| Vernal equinox
| 1923, 2007, 2092
| Autumnal equinox
|-
| Summer solstice
| 1944, 2028
| Winter solstice
|-
| Autumnal equinox
| 1965, 2050
| Vernal equinox
|}