Editing Orcus (dwarf planet) (section)

== Orbit and rotation ==
Orcus is in a 2:3 orbital resonance with [[Neptune]], having an orbital period of 245 years,<ref name="Buie" /><ref name="jpldata"/> and is classified as a [[plutino]].<ref name="MPC-Orcus"/> Its orbit is moderately [[orbital inclination|inclined]] at 20.6° to the [[ecliptic]].<ref name="jpldata"/> Orcus's orbit is similar to Pluto's (both have [[Perihelion|perihelia]] above the [[ecliptic]]), but is oriented differently. Although at one point its orbit approaches that of Neptune, the resonance between the two bodies means that Orcus itself is always a great distance away from Neptune (there is always an [[angular separation]] of over 60° between them). Over a 14,000-year period, Orcus stays more than 18&nbsp;AU from Neptune.<ref name="MPC2004-D15" /> Because their mutual resonance with Neptune constrains Orcus and Pluto to remain in opposite phases of their otherwise very similar motions, Orcus is sometimes described as the "anti-Pluto".<ref name="MBP" /> Orcus last reached its [[aphelion]] (farthest distance from the Sun) in 2019 and will come to perihelion (closest distance to the Sun) around 10 January 2143.<ref name="Horizons2143"/> Simulations by the [[Deep Ecliptic Survey]] show that over the next 10 million years Orcus may acquire a perihelion distance (''q<sub>min</sub>'') as small as 27.8&nbsp;AU.<ref name="Buie" />

The rotation period of Orcus is uncertain, as different photometric surveys have produced different results. Some show low amplitude variations with periods ranging from 7 to 21 hours, whereas others show no variability.<ref name="Delsanti2010" /> The rotational axis of Orcus probably coincides with the orbital axis of its moon, Vanth. This means that Orcus is currently viewed pole-on, which could explain the near absence of any [[rotational modulation]] of its brightness.<ref name="Delsanti2010" /><ref name="Ortiz2011" /> Astronomer [[José Luis Ortiz Moreno|José Luis Ortiz]] and colleagues have derived a possible rotation period of about 10.5 hours, assuming that Orcus is not [[tidally locked]] with Vanth.<ref name="Ortiz2011">{{Cite journal | last1 = Ortiz | first1 = J. L. | last2 = Cikota | first2 = A. | last3 = Cikota | first3 = S. | last4 = Hestroffer | first4 = D. | last5 = Thirouin | first5 = A. | last6 = Morales | first6 = N. | last7 = Duffard | first7 = R. | last8 = Gil-Hutton | first8 = R. | last9 = Santos-Sanz | first9 = P. | last10 = De La Cueva | first10 = I. | title = A mid-term astrometric and photometric study of trans-Neptunian object (90482) Orcus | doi = 10.1051/0004-6361/201015309 | journal = Astronomy & Astrophysics | volume = 525 | pages = A31 | date = 2010 |bibcode = 2011A&A...525A..31O |arxiv = 1010.6187 | s2cid = 56051949 }}</ref> If, however, the primary is tidally locked with the satellite, the rotational period would coincide with the 9.7-day orbital period of Vanth.<ref name="Ortiz2011" />
{{Multiple image
| align = left
| direction = 
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| image1 = Orcus-motion.png
| width1 = 175
| caption1 = [[Mean-motion resonance]] of Orcus ([[rotating frame]] with a period equal to [[Neptune]]'s [[orbital period]])
| image2 = TheKuiperBelt Orbits Orcus2.svg
| width2 = 330
| caption2 = The orbits of Orcus (blue), Pluto (red) and Neptune (grey). Orcus and Pluto are shown in the April 2006 positions. The dates of their [[perihelia]] (q) and [[Apsis|aphelia]] (Q) are also marked.
}}
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