page_title: Triton (moon)
section_title: Orbit and rotation
hierarchical_section_title: Triton (moon) / Orbit and rotation
caption_attribution_description: English: The orbit of Triton (red) has a 157° tilt in comparison to the orbit of most moons (green) 中文（中国大陆）‎: 海卫一的轨道和运行方向（红）与其他海王星的卫星轨道（绿）相比有157°的倾斜
context_page_description: Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered. The discovery was made on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. At 2,710 kilometres in diameter, it is the seventh-largest moon in the Solar System, the only satellite of Neptune massive enough to be in hydrostatic equilibrium and the second-largest planetary moon in relation to its primary, after Earth's Moon. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet captured from the Kuiper belt.
  Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. The mean density is 2.061 g/cm³, reflecting a composition of approximately 15–35% water ice.
  During its 1989 flyby of Triton, Voyager 2 found surface temperatures of 38 K and also discovered active geysers; Voyager 2 remains the only spacecraft to visit Triton.
context_section_description: 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 moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons. However, these moons are all much more distant from their primaries, and are small in comparison; the largest of them (Phoebe) has 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 spin to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's spin, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's spin with a period of about 678 Earth years (4.1 Neptunian years), making its Neptune-orbit-relative inclination vary between 127° and 180°. 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. At the present time, Triton's rotational axis is about 40° from Neptune's orbital plane, and hence at some point during Neptune's year each pole points fairly close to the Sun, almost like the poles of Uranus. 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.
  Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. Viscoelastic damping from tides alone is not thought to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role. Tidal interactions also cause Triton's orbit, which is already closer to Neptune than the Moon's is to Earth, to gradually decay further; predictions are that 3.6 billion years from now, Triton will pass within Neptune's Roche limit. This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn.
caption: The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.
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