Telescopes were common enough and powerful enough in the 19th century for Urbain Jean Joseph Le Verrier, a French astronomer-mathematician to observe orbital irregularities suggesting Neptune’s existence. It was discovered about where Le Verrier predicted it would be, and credit for a century and a half has gone to him and the first observer, Germany’s Johann Gottfried Galle. But it is entirely possible that the first observation was actually made centuries before by Galileo Galilei, one of the most renowned astronomers of all time, who used only primitive magnification tools.

On this day, December 28, in 1612, Galileo Galilei was observing the four moons of Jupiter when he found a mysterious star near it. Galileo dutifully recorded the observation in his notebook – but it was thought until recently he never realized it was a planet.

Galileo recorded the “star” actually changed position in the sky, something that only planets could do. Several weeks later, his notebook records a black star where Neptune would then be. Nothing explicit is stated about Galileo’s conclusion about its star or planet status, but nevertheless he remains the first person to have seen Neptune.