Astronomer James W. Christy was examining routine images of Pluto when he noticed something strange: in several of the shots, the planet appeared elongated. His first thought was the images were somehow defective, but images of background stars on the same plate did not show the same elongation. As Christy dug in further, he noticed the “bump” on the planet appeared to travel over semicircular route at a regular speed. This could only mean one thing, he figured: Pluto was not alone.

On this day, June 22, in 1978, James Christy observed what would eventually be determined to be Pluto’s companion, a moon.

As the discoverer of the planetary body, Christy had the honor of choosing its name. He proposed “Charon,” in keeping with the Greek god-of-the-underworld theme, after the ferryboat pilot who took the souls of the dead across the River Styx into Pluto’s domain. Christy’s wife’s name was Charlene — nicknamed Char — which made the name an honor to her as well. Just don’t try to pronounce it: “Charon” in Greek was more like “Karon.”