Charon*, is the only moon of the ninth planet in our solar system, Pluto. The moon was discovered in 1978 quite by accident, when routine photographs taken of Pluto showed that the planet had an elongated shape rather then the spherical planet scientests were expecting. They then noticed that the elongation appeared to change shape over time, moving its position on Pluto's surface. Having ruled out technical errors as a cause for the fault, it was concluded they must have discovered Pluto's only moon.
James W. Christy, the astronomer responsible for spotting the anomaly, chose the name Charon for the lonely moon, as this was the name of the the ferryman who transported souls across the River Styx to Pluto's underworld kingdom. (His wife is also called Charlene, which is pretty close too!) Despite it's discovery in 1978, Charon only became an official moon in 1985 after a series of observances were made of transits by Pluto across the surface of Charon, and Charon across the surface of Pluto. These measurements were important to working out what areas of the moon and planet's surface received light from the sun, and at what times. They also helped in working out the separate densities of the two bodies, and from this data it was concluded that the two have formed independently as Pluto has a very high density, and Charon rather lower density.
Charon and Pluto share the same length of axis, 6.4 days, and Charon's orbit around Pluto is also 6.4 days. Because of this, the planet and the moon always show the same face to one another. Also because of this strange relationship, Charon would appear stationary in Pluto's sky, but could only be viewed on one hemisphere. Unlike Pluto, Charon is thought to have a surface covered in frozen water, whereas Pluto's surface is thought to be that of frozen methane, another possible clue as to how they formed.
* There are several different ideas about how this is pronounced.
- SHAR-on - The unofficial term used by the founding astronomers, as it sounds the closest to Charlene, the wife of Jim Christy who found it.
- liveforever suggests KAIR-on for the English pronunciation, or KAH-ron for the Greek.
So feel free to take your pick from the above! :o)
For more information on Charon see:
http://www.jpl.nasa.gov/ice_fire/CharonDiscovery.htm
Pictures of Pluto and Charon can be seen at:
http://www.star.le.ac.uk/edu/solar/pluto.html#charon