On 22 June 1978, an astronomer at the U.S. Naval Observatory in Washington,
was making routine measurements of photographic plates taken with the
(61-inch) Kaj Strand Astrometric Reflector at the USNO Flagstaff Station in
Arizona. The purpose of these images was to refine the orbit of the far-flung
planet Pluto to help compute a better ephemeris for this distant object.

Astronomer James W. Christy had noticed that a number of the images of Pluto
appeared elongated, but images of background stars on the same plate did not.
Other plates showed the planet as a tiny, round dot. Christy examined a number
of Pluto images from the USNO archives, and he noticed the elongations again.
Furthermore, the elongations appeared to change position with respect to the
stars over time. After eliminating the possibility that the elongations were
produced by plate defects and background stars, the only plausible explanation
was that they were caused by a previously unknown moon orbiting Pluto at a
distance of about 19,600 kilometers (12,100 miles) with a period of just over
six days.

On 7 July 1978, the discovery was formally announced to the astronomical
community and the world by the IAU Central Bureau for Astronomical Telegrams
IAU Circular 3241. The discovery received the provisional designation "1978 P
1"; Christy proposed the name "Charon", after the mythological ferryman who
carried souls across the river Acheron, one of the five mythical rivers that
surrounded Pluto’s underworld.

Over the course of the next several years, another USNO astronomer, the late
Robert S. Harrington, calculated that Pluto and its newly-found moon would
undergo a series of mutual eclipses and occultations, beginning in early 1985.
On 17 February 1985 the first successful observation of one of these transits
was made at with the 0.9-meter (36-inch) reflector at the University of Texas
McDonald Observatory, within 40 minutes of Harrington’s predicted time. The IAU
Circular announcing these confirming observations was issued on 22 February
1985. With this confirmation, the new moon was officially named Charon.

Pluto was discovered at Lowell Observatory in 1930 by the late Clyde W.
Tombaugh, an amateur astronomer from Kansas who was hired by the Observatory
specifically to photograph the sky with a special camera and search for the
planet predicted by the Observatory’s founder, Percival Lowell.

Lowell had deduced the existence of a "Planet X" by studying small anomalies in
the orbits of Uranus and Neptune. As it turned out, Pluto’s discovery was
entirely serendipitous; Pluto’s tiny mass was far too small to account for the
anomalies, which were resolved when Voyager 2 determined more precise masses
Uranus and Neptune.

The discovery of Charon has led to a much better understanding of just how tiny
Pluto is. Its diameter is about 2274 km (1413 miles), and its mass is 0.25% of
the mass of the Earth. Charon has a diameter of about 1172 kilometers (728
miles) and a mass of about 22% that of Pluto. The two worlds circle their
center of mass with a period of 6.387 days and are locked in a
"super-synchronous" rotation: observers on Pluto’s surface would always see
Charon in the same part of the sky relative to their local horizon.

Normally Pluto is considered the most distant world in the solar system, but
during the period from January 1979 until February 1999 it was actually closer
to the Sun than Neptune. It has the most eccentric and inclinced orbit of any
the major planets. This orbit won’t bring Pluto back to its discovery position
until the year 2178!

Pluto is in a 2:3 resonance with Neptune; that is, it orbits the Sun twice in
the time it takes Neptune to orbit three times. It is the largest of a family
objects called "Plutinos", objects about the size of Charon and smaller, which
also have a 2:3 resonance with Neptune. Plutinos are one of several classes of
minor planets that make up the Kuiper Belt, a region just beyond the orbit of
Pluto thought to be a source of periodic comets. A current plot of these
shows their relationship to Pluto and other transneptunian objects in the outer
solar system.

The U. S. Naval Observatory continues to monitor the positions of Pluto and
Charon, as well as those of other bodies in the solar system as a part of its
continuing Mission. Ephemerides for Pluto and Charon may be found in our annual
Astronomical Almanacs and Computer Almanacs.


[Image 1:
http://www.usno.navy.mil/pao/press/chardis.jpg (477KB)]
Discovery images of Pluto/Charon, taken with the 1.55-meter (61-inch) Kaj
Astrometric Reflector at the USNO Flagstaff Station:

Left: showing elongated image of Pluto, with Charon to the upper right.
Right: showing "undistorted" image of Pluto with Charon along line of sight
the planet.

Official USNO Photograph.

[Image 2:
http://www.usno.navy.mil/pao/press/chrishar.jpg (332KB)]
Charon discoverer James Christy (seated), with the late Robert S. Harrington.

Official USNO Photograph.