The diameter of our moon is about a quarter that of our planet's. This was thought to be an extraordinarily large size for a moon until the discovery of Pluto's moon. Pluto's moon is half that planet's diameter.
Pluto's moon was discovered in 1978 by an American astronomer James Christie. Christie had an old pile of Pluto's photographs that had been discarded because they were thought to be defective. They showed the planet to be elongated at one end. While going through the photographs the astronomer suddenly realised that the elongated portion was actually a moon that the camera had not been able to separate from the planet it was revolving around.
The newly-discovered moon was named Charon, who in Greek mythology is the boatman who ferries souls to Hades across the River Styx. The name was chosen to honour Christie's wife, Charlene.