James W. Christy

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James Christy (left) and Robert Harrington in 1978.

James Walter Christy (born 1938) 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.[1]

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.[2]

The photographic evidence was considered convincing but not conclusive (it remained possible that the bulge was a giant mountain on Pluto). However, based on Charon's calculated orbit, a series of mutual eclipses of Pluto and Charon was predicted and observed, confirming the discovery.[3]

The 1965 plates included a note "Pluto image elongated", but observatory astronomers, including Christy, assumed that the plates were defective until 1978.[4]

In more modern telescopes, such as the Hubble or ground-based telescopes using adaptive optics, separate images of Pluto and Charon can very easily be resolved.[citation needed]

In late 2008, the asteroid 129564 Christy was named in his honor.


  1. ^ Marsden, Brian G., Satellites of Saturn and Pluto, IAUC 4157 (1986 January 3)
  2. ^ Planets Beyond: Discovering the Outer Solar System, by Mark Littmann, 1990, pp. 173-177, including the essay "A Moment of Perception" by James W. Christy.
  3. ^ Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, by Alan Stern and Jacqueline Mitton, 1999, p. 58
  4. ^ Littmann, p. 176

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