Editing Umbriel (section)

== Orbit ==
Umbriel orbits Uranus at the distance of about {{convert|266000|km|mi|abbr=on}}, being the third farthest from the planet among its [[moons of Uranus|five major moons]].{{efn|The five major moons are [[Miranda (moon)|Miranda]], [[Ariel (moon)|Ariel]], Umbriel, Titania and Oberon.}} Umbriel's orbit has a small [[orbital eccentricity|eccentricity]] and is [[orbital inclination|inclined]] very little relative to the [[equator]] of Uranus.<ref name="orbit" /> Its [[orbital period]] is around 4.1&nbsp;Earth days, coincident with its [[rotational period]], making it a [[synchronous orbit|synchronous]] or [[tidally locked]] satellite, with one face always pointing toward its parent planet.<ref name="Smith Soderblom et al. 1986" /> Umbriel's orbit lies completely inside the [[Magnetosphere of Uranus|Uranian magnetosphere]].<ref name="Grundy Young et al. 2006" /> This is important, because the trailing [[Sphere#Hemisphere|hemispheres]] of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetospheric [[Plasma (physics)|plasma]], which co-rotates with the planet.<ref name="Ness Acuña et al. 1986" /> This bombardment may lead to the darkening of the trailing hemispheres, which is observed for all Uranian moons except [[Oberon (moon)|Oberon]] (see below).<ref name="Grundy Young et al. 2006" /> Umbriel also serves as a sink of the magnetospheric charged particles, which creates a pronounced dip in energetic particle count near the moon's orbit as observed by ''Voyager 2'' in 1986.<ref name="Krimigis Armstrong et al. 1986" />

Because Uranus orbits the [[Sun]] almost on its side, and its moons orbit in the planet's equatorial plane, Umbriel and the other moons are subject to an extreme seasonal cycle. Both northern and southern [[Poles of astronomical bodies#Geographic poles|poles]] spend 42 years in complete darkness, and another 42 years in continuous sunlight, with the Sun rising close to the [[zenith]] over one of the poles at each [[solstice]].<ref name="Grundy Young et al. 2006" /> The ''Voyager 2'' flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was unilluminated. Once every 42 years, when Uranus has an [[equinox]] and its equatorial plane intersects the Earth, mutual [[occultation]]s of Uranus's moons become possible. In 2007–2008, several such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007, as well as of Ariel by Umbriel on August 19, 2007.<ref name="Miller Chanover 2009" /><ref name="Arlot Dumas et al. 2008" />

Currently, Umbriel is not involved in any [[orbital resonance]] with other Uranian satellites. Early in its history however, it may have been in a 1:3 resonance with [[Miranda (moon)|Miranda]]. This would have increased Miranda's orbital eccentricity, contributing to the internal heating and geological activity of that moon, while Umbriel's orbit would have been less affected.<ref name="Tittemore Wisdom 1990" /> Due to Uranus's lower [[oblate spheroid|oblate]]ness and smaller size relative to its satellites, its moons can escape more easily from a mean motion resonance than those of [[Jupiter]] or [[Saturn]]. After Miranda escaped from this resonance (through a mechanism that probably resulted in its anomalously high orbital inclination), its eccentricity would have been damped, turning off the heat source.<ref name="Tittemore Wisdom 1989" /><ref name="Malhotra Dermott 1990" />