Editing Sedna (dwarf planet) (section)

== Orbit and rotation ==
{{See also|List of Solar System objects most distant from the Sun}}
{{multiple image
| direction         = vertical
| image1            = Sedna solar system Jan1 2017.png
| caption1          = The orbit of Sedna set against the orbits of outer Solar System objects (top and side views, Pluto's orbit is purple, Neptune's is blue)
| alt1              = A large oval represents the orbit of Sedna around the offset Sun and smaller, more circular planetary orbits
| image2            = Sednoid apparent magnitudes.png
| caption2          = The 10,000 year [[apparent magnitude]]s of Sedna and two other sednoids
| alt2              = A grid chart showing smoothly varying brightness over time
}}

Sedna has the longest [[orbital period]] of any known object in the Solar System of its size or larger with an orbital period of around 11,400 years.<ref name="barycenter"/>{{refn|1=Given the [[orbital eccentricity]] of this object, different [[Epoch (astronomy)|epochs]] can generate quite different heliocentric unperturbed [[Two-body problem|two-body]] [[curve fitting|best-fit]] solutions to the orbital period. Using a 1990 epoch, Sedna has a 12,100-year orbit,<ref name="DES" /> but using a 2019 epoch Sedna has a 10,500-year orbit.<ref>{{cite web | url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Sedna | publisher=JPL | title=SBDB Epoch 2019 | archive-url=https://web.archive.org/web/20191113081024/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Sedna | archive-date=13 November 2019 }}</ref> For objects at such high eccentricity, the Solar System's [[barycenter]] (Sun+Jupiter) generates solutions that are more stable than heliocentric solutions.<ref name="Kaib2009" /> Using [[JPL Horizons On-Line Ephemeris System|JPL Horizons]], the barycentric orbital period is consistently about 11,388&nbsp;years, with a variation of 2 years over the next two centuries.<ref name="barycenter" />|name=footnoteG|group=lower-alpha}} Its [[orbit]] is extremely eccentric, with an [[aphelion]] of approximately 937 AU<ref name="barycenter"/> and a [[Perihelion and aphelion|perihelion]] of 76.19 AU. Near aphelion, Sedna is one of the coldest places in the [[Solar System]], located far past the [[Heliosphere|termination shock]], where temperatures never exceed −240°[[Celsius|C]] (−400°[[Fahrenheit|F]]) due to its extreme distance.<ref>{{Cite web |title=Mysterious Sedna {{!}} Science Mission Directorate |url=https://science.nasa.gov/science-news/science-at-nasa/2004/16mar_sedna/ |access-date=31 March 2023 |website=science.nasa.gov |archive-date=16 May 2017 |archive-url=https://web.archive.org/web/20170516234331/https://science.nasa.gov/science-news/science-at-nasa/2004/16mar_sedna |url-status=dead }}</ref><ref>{{Cite web | title=Most Distant Object in Solar System Discovered | date=15 March 2004 | url=https://www.jpl.nasa.gov/news/most-distant-object-in-solar-system-discovered | access-date=31 March 2023 | website=NASA Jet Propulsion Laboratory (JPL) | language=en-US | archive-date=22 October 2023 | archive-url=https://web.archive.org/web/20231022073655/https://www.jpl.nasa.gov/news/most-distant-object-in-solar-system-discovered | url-status=live }}</ref> At aphelion, the Sun as viewed from Sedna is a particularly bright star, among the other stars, in the otherwise black sky, being about 45% as bright as the full moon as seen from Earth.<ref>Apparent magnitude of Sun at 937 AU = -26.74 + 5log(937) = -11.88

Full moon magnitude = -12.74

Ratio = 10^(0.4*(11.88-12.74)) = 0.453 ≈ 45%.</ref> Its perihelion was the largest for any known Solar System object until the discovery of the [[sednoid]] {{mpl|2012 VP|113}}.<ref name="Trujillo2007"/><ref name="Trujillo2014"/> At its aphelion, Sedna orbits the Sun at a meagre 377&nbsp;m/s,<ref>Calculated: https://www.wolframalpha.com/input?i=G*solar+mass%2FAU%281%2F76.19-1%2F937%29%3D1%2F2*%28x+m%2Fs%29%5E2%28%28937%2F76.19%29%5E2-1%29</ref> 1.3% that of Earth's average orbital speed.<ref>377.4 m/s for Sedna divided by 29.78 km/s for Earth.</ref>

When Sedna was first discovered, it was 89.6&nbsp;AU<ref name="AstDys2003"/> away from the Sun, approaching perihelion, and was the most distant object in the Solar System observed. Sedna was later surpassed by [[Eris (dwarf planet)|Eris]], which was detected by the same survey near its aphelion at 97&nbsp;AU. Because Sedna is near perihelion {{as of|2024|lc=y}}, both Eris and {{dp|Gonggong}} are farther from the Sun, at 96&nbsp;AU and 89&nbsp;AU respectively, than Sedna at 84&nbsp;AU, despite both of their semi-major axes being shorter than Sedna's.<ref name="AstDys-Eris" /><ref name="AstDys-OR10" /><ref name="AstDys" /> The orbits of some long-period comets extend further than that of Sedna; they are too dim to be discovered except when approaching perihelion in the inner Solar System. As Sedna nears its perihelion in mid-2076,<ref name="Perihelion2076"/>{{refn|1=Different programs using different [[Epoch (astronomy)|epochs]] and/or [[data set]]s will produce slightly different dates for Sedna's [[apsis|perihelion]] as they generate instantaneous unperturbed 2-body solutions. Using a 2020 epoch, the [[JPL Small-Body Database]] has a perihelion date of 9{{nbsp}}March 2076.<ref name="jpldata"/> Using a 1990 epoch the Lowell [[Deep Ecliptic Survey|DES]] has perihelion on [http://www.boulder.swri.edu/~buie/kbo/astrom/90377.html 2479285.9863] ([http://ssd.jpl.nasa.gov/tc.cgi 14{{nbsp}}December 2075]). {{As of|2021}}, the [[JPL Horizons On-Line Ephemeris System|JPL Horizons]] (using much more accurate [[numerical integration]]) indicates a perihelion date of 18{{nbsp}}July 2076.<ref name="Perihelion2076" />|name=footnoteC|group=lower-alpha}} the Sun will appear merely as a very bright pinpoint in its sky, too far away to be visible as a disc to the naked eye.<ref name="HubbleSite2004image"/>

When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days).<ref name="HubbleSite2004"/> It was initially speculated that Sedna's rotation was slowed by the gravitational pull of a large binary companion, similar to [[Pluto]]'s moon [[Charon (moon)|Charon]].<ref name="mikebrown" /> However, a search for such a satellite by the [[Hubble Space Telescope]] in March 2004 found no such objects.<ref name="HubbleSite2004"/>{{refn|1=The HST search found no satellite candidates to a limit of about 500 times fainter than Sedna (Brown and Suer 2007).<ref name="largest" />|group=lower-alpha}} Subsequent measurements from the [[MMT Observatory|MMT]] telescope showed that Sedna in reality has a much shorter rotation period of about 10 hours, more typical for a body its size. It could rotate in about 18 hours instead, but this is thought to be unlikely.<ref name="Gaudi2005"/>