On this day in 1846, Neptune’s moon, Triton, is discovered by William Lassell while he was observing the recently discovered planet Neptune (discovered just one month earlier).
Lassell was trying to confirm his observation of the previousweek, that Neptune had a ring (later turned out to be a telescope distortion). Instead he discovered that Neptune had a satellite, Triton. Lassell. Triton is the only large satellite in the solar system that circles a planet in a direction opposite to the rotation of the planet (retrograde direction).
This image of Triton was taken in August, 1989 by Voyager 2, which found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes – Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune – but these would have been quite invisible to Lassell!