Working at the U.S. Naval Observatory in Flagstaff, Arizona, astronomer James Christy noticed an odd elongation in images of Pluto and identified it as a previously unknown moon. Named Charon after the ferryman of Greek mythology who carried souls across the river Styx, the discovery revealed that Pluto was smaller than previously calculated and allowed astronomers to finally determine its mass accurately.
Christy was reviewing old photographic plates of Pluto taken to refine its orbit when he noticed something strange: the tiny planet's image seemed to bulge in one direction on some plates and another direction on others, like a lopsided blob rather than a clean point of light.
At first he suspected a flaw in the telescope's optics or the plates themselves. But checking archival images going back years, he found the bulge shifted position in a predictable pattern, consistent with an orbiting body too close to Pluto to be resolved as separate by the telescope.
He proposed the name Charon—fittingly, the ferryman who rowed the dead across the river to the underworld ruled by Pluto in Greek mythology. The discovery was a turning point: by observing Charon's orbit, astronomers could finally calculate Pluto's mass precisely, revealing it to be far smaller than Earth's moon—a key piece of evidence in the debate decades later over Pluto's status as a planet.
Key people: James Christy