Triton, Neptune's largest moon, was found just 17 days after the planet
On this day · 10 October 1846
British brewer-turned-astronomer William Lassell spotted Triton on October 10, 1846, barely two weeks after Neptune itself appeared.
Neptune was confirmed on September 23, 1846, and the hunt for moons began almost immediately. The astronomer John Herschel urged colleagues to look closer. In England, William Lassell — a Liverpool brewer who funded his own observatory — pointed his self-built reflecting telescope at the new planet and, on October 10, 1846, caught a faint point of light circling it.
That speck was Triton, today known as the largest of Neptune’s 13 moons. The discovery came just 17 days after Neptune’s own, a remarkable turnaround for a body roughly 2.7 billion miles from the Sun.
Lassell built his own mirror, then used it to nearly double the size of the known Neptunian system overnight.
Triton turned out to be deeply strange. It orbits backwards relative to Neptune’s spin — a retrograde path that hints it was a wanderer from the Kuiper Belt, captured by the planet’s gravity rather than born alongside it.
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