Neptune's moon Triton was found just 17 days after Neptune

Triton, Neptune’s largest moon, was discovered on October 10, 1846 by English astronomer William Lassell, just 17 days after Johann Galle first sighted Neptune from Berlin.

Lassell was one of the era’s great amateurs, a brewer who used the fortune he made from beer to pay for his own telescopes. He was not looking for a moon. A week earlier he thought he had seen a ring around the new planet, and he turned his telescope back to confirm it. Instead he found a faint companion that moved with Neptune. He soon showed the ring was an illusion produced by distortion in his new instrument. The moon’s story after that night runs like this:

• 1846–1949: For more than a century the moon is usually called simply “the satellite of Neptune” — with no second moon known, there is nothing to tell it apart from. Its name, after the son of Poseidon, the Greek counterpart of Neptune, becomes needed only once Nereid is found in 1949.
• 1989: Voyager 2, the only spacecraft to visit, measures a surface temperature of −235°C and finds geysers throwing icy material more than 8 km high. It also photographs Neptune’s real rings — faint ones, one of which is now named Lassell.
• Since then: Triton’s thin nitrogen atmosphere has been detected from Earth several times and is growing warmer, for reasons scientists have not pinned down.

Triton, about 2,700 km across, is also the only large moon in the solar system that orbits backwards relative to its planet’s rotation. That retrograde path is the main reason scientists think it began as a Kuiper Belt object, a cousin of Pluto, later captured by Neptune’s gravity. Today it is the largest of Neptune’s 16 known moons.