The Tiny Bump That Revealed Charon

On June 22, 1978, astronomer James Christy was examining photographic plates of Pluto at the U.S. Naval Observatory in Washington, D.C. In those days, astronomers did not have modern digital cameras attached to their telescopes. Instead, they recorded images of the night sky onto special glass photographic plates. Christy was trying to calculate Pluto's orbit more precisely, but when he placed one particular plate under a magnifying viewer, he noticed something strange. Pluto did not look like a neat, round circle of light. Instead, it appeared slightly elongated, as if it had an odd bulge or bump on one side.

At first, Christy wondered if the image was defective. A telescope can shift during a long exposure, smearing the light of a star or planet. A tiny flaw on the glass plate itself could also create a false mark. To test these possibilities, Christy looked closely at the background stars on the same plate. If the telescope had shifted or shaken, every single star would be smeared in the exact same direction. Yet all the background stars appeared perfectly crisp and round. The distortion belonged exclusively to Pluto.