James W. Christy

Who is James W. Christy?

James Walter Christy is an American astronomer.

On June 22, 1978 while working at the United States Naval Observatory, he discovered that Pluto had a moon, which he named Charon shortly afterwards. The name remained unofficial until its adoption by the IAU in 1986.

The discovery was made by carefully examining an enlargement of a photographic plate of Pluto and noticing it had a very slight bulge on one side. This plate and others had been marked "poor" because the elongated image of Pluto was thought to be a defect resulting from improper alignment. However, Christy noticed that only Pluto was elongated—the background stars were not.

Christy's earlier work at the Naval Observatory had included photographing double stars, so it occurred to him that this bulge might be a companion of Pluto. After examining images from observatory archives dating back to 1965, he concluded that the bulge was indeed a moon.

The photographic evidence was considered convincing but not conclusive. However, based on Charon's calculated orbit, a series of mutual eclipses of Pluto and Charon was predicted and observed, confirming the discovery.