On July 19, 1846, a significant event occurred in the field of astronomy. On this day, the first known moon of Neptune, Triton, was discovered by English astronomer William Lassell, just 17 days after the discovery of Neptune itself.

Triton is quite an unusual moon, and its discovery was a thrilling moment in the history of astronomy. It is the only large moon in our solar system that orbits its planet in the opposite direction to the planet's rotation, which is known as a retrograde orbit. This suggests that Triton may have once been a dwarf planet that was captured by Neptune's gravity.

Triton is also one of the coldest known objects in our solar system, with surface temperatures reaching as low as -235°C (-391°F). Despite this extreme cold, Triton is geologically active. The Voyager 2 spacecraft, during its flyby in 1989, observed geyser-like eruptions spewing nitrogen gas and dust particles several kilometers into space. This makes Triton one of the few geologically active moons in our solar system.

The discovery of Triton opened up a new chapter in our understanding of the outer solar system and the diversity of moons that exist around the giant planets. It also sparked curiosity about the potential for life in extreme environments, as Triton's subsurface ocean and geological activity make it a fascinating candidate for astrobiological studies.

So, on this day in 1846, William Lassell's keen observations led to the discovery of a truly unique and captivating world, adding another piece to the cosmic puzzle that astronomers continue to explore to this day.