The planet Neptune was discovered on this day in 1846, completing the set of eight planets in our solar system that kids learn in school today (we’re sidestepping the whole Pluto discussion today). Uranus, discovered 65 years earlier, had a strange orbital pattern that suggested some other object was influencing it. Mathematicians predicted a new planet could be seen at night if someone with a telescope bothered to look. Johann Gottfried Galle was the first to confirm it was a planet with his own eyes — but he wasn’t actually the first to observe Neptune.
It was Off-script alumnus Galileo Galilei who first saw Neptune in the sky, but there’s no direct evidence he thought it was a planet. In 1613, he’d been peering through a telescope at Jupiter’s four moons when he saw it, but the early astronomer figured it was a star.
Even after Galle’s eyeing of Neptune, people had trouble getting a good look at it given its distance; it’s the furthest planet from the sun, and the distorting effects of Earth’s atmosphere are strong enough to make items that far away hard to measure. Different details were figured out over time, like its size, density, and mass, but it took until the 1980s before we knew how long it even took to complete a full rotation. Finally, the Voyager 2 satellite was able to snap a detailed photo of Neptune on a flyby in 1989.
