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James W. Christy

James W. Christy
James Christy (left) and Robert Harrington in 1978.
Born (1938-09-15) September 15, 1938
Milwaukee, Wisconsin, U.S.
Nationality American
Fields Astronomy, astrometry
Institutions United States Naval Observatory
Hughes Missile Systems
Alma mater University of Arizona

James Walter "Jim" Christy (born September 15, 1938) is an American astronomer.

Christy was born in 1938 in Milwaukee, Wisconsin. He attended the University of Arizona and earned a Bachelor of Science degree in astronomy from there in 1965.[1] 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.[2]

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. The 1965 plates included a note "Pluto image elongated", but observatory astronomers, including Christy, assumed that the plates were defective until 1978.[3]

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.[4]

The photographic evidence was considered convincing but not conclusive (it remained possible that the bulge was due to Pluto having an unexpectedly irregular shape). However, based on Charon's calculated orbit, a series of mutual eclipses of Pluto and Charon was predicted and observed, confirming the discovery.[5]

Charon viewed by New Horizons on 12 July 2015

In more modern telescopes, such as the Hubble or ground-based telescopes using adaptive optics, separate images of Pluto and Charon can be resolved, and the New Horizons probe took images showing some of Charon's surface features.

In late 2008, the asteroid 129564 Christy was named in his honor.† As of 2015, he resides in Flagstaff, Arizona.[6] He has been married to Charlene Mary since 1975 and has four children.[7] On July 14, 2015, Christy and the children of Clyde Tombaugh were guests at the Johns Hopkins Applied Physics Laboratory when the New Horizons spacecraft successfully performed the first flyby of the Pluto-Charon system.


  1. ^ [2]
  2. ^ Marsden, Brian G., Satellites of Saturn and Pluto, IAUC 4157 (1986 January 3)
  3. ^ Littmann, p. 176
  4. ^ 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.
  5. ^ Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, by Alan Stern and Jacqueline Mitton, 1999, p. 58
  6. ^
  7. ^

External links

  • Pluto's Companion from the website "Pluto: The Discovery of Planet X," by Brad Mager
  • 25th Anniversary of the Discovery of Pluto's moon CHARON from NASA JPL website
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