Editing Triton (moon) (section)

== Orbit and rotation ==
[[File:Triton orbit & Neptune.png|thumb|The orbit of Triton (red) is [[Retrograde and direct motion|opposite in direction]] and [[orbital inclination|tilted −23°]] compared to a typical [[Moons of Neptune|moon]]'s orbit (green) in the plane of Neptune's equator.]]
Triton is unique among all large moons in the Solar System for its [[retrograde orbit]] around its planet (i.e. it orbits in a direction opposite to the planet's rotation). Most of the outer [[irregular moon]]s of [[Jupiter]] and [[Saturn]] also have retrograde orbits, as do some of the irregular moons of [[Uranus]] and Neptune. However, these moons are all much more distant from their primaries and are small in comparison, with the largest of them ([[Phoebe (moon)|Phoebe]])<ref name="nLargest" group="lower-alpha"/> having only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton's orbit is associated with two tilts, the [[obliquity]] of Neptune's rotation to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's rotation, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's rotation with a period of about 678 Earth years (4.1 Neptunian years),<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/> making its Neptune-orbit-relative inclination vary between 127° and 173°. That inclination is currently 130°; Triton's orbit is now near its maximum departure from coplanarity with Neptune's.

Triton's rotation is [[tidally locked]] to be synchronous with its orbit around Neptune: it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane.<ref name="Davies1991-ControlNetwork"/> At present, Triton's rotational axis is about 40° from Neptune's [[orbital plane]], hence as Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other moves into the sunlight. Such changes were observed in 2010.<ref name="SpaceCom-TritonSeasons"/>

Triton's revolution around Neptune has become a nearly perfect circle with an [[Orbital eccentricity|eccentricity]] of almost zero. [[Viscoelastic]] damping from tides alone is not thought to be capable of [[tidal circularization|circularizing]] Triton's orbit in the time since the origin of the system, and [[Drag (physics)|gas drag]] from a [[Retrograde and prograde motion|prograde]] debris disc is likely to have played a substantial role.<ref name="JPL-SSD-Neptune"/><ref name="Jacobson2009-AJ"/> [[Tidal deceleration|Tidal interactions]] also cause Triton's orbit, which is already closer to Neptune than the [[Moon]] is to Earth, to gradually decay further; predictions are that 3.6&nbsp;billion years from now, Triton will pass within Neptune's [[Roche limit]].<ref name="Chyba"/> This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new [[planetary ring|ring]] system similar to that found around [[Saturn]].<ref name="Chyba"/>