Pluto's giant moon Charon was discovered in a photographic smudge

On June 22, 1978, U.S. Naval Observatory astronomer James Christy was measuring photographs of Pluto to refine its orbit when he noticed something odd: the planet’s image had a small bulge on one side. The background stars stayed sharp, so the smear was not a flaw in the plate.

Christy had photographed double stars before, and the explanation clicked. The bulge was a companion moon, orbiting close enough to blur into Pluto on the 61-inch telescope at the observatory’s Flagstaff station. Checking archive plates back to 1965, he confirmed the body shifted in step with Pluto’s rotation.

With colleague Robert Harrington, Christy announced the find on July 7. He named it Charon, both for the mythological ferryman of the dead and, quietly, for his wife Charlene.

Charon is so large relative to Pluto that the two orbit a point in empty space between them.

The discovery let astronomers finally weigh Pluto, which proved far smaller than once feared.