Triton with Neptune in the background NASA Classic Round Sticker
Triton (pronounced /ˈtraɪtən/, or as Greek Τρίτων) is the largest moon of the planet Neptune, discovered on October 10, 1846 by William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2700 km in diameter, it is the seventh-largest moon in the Solar System. Triton comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and twelve other known moons.