Uranus has 27 moons, all of which are named after characters from the works of Shakespeare and Alexander Pope. The five main satellites, in order of proximity to Uranus, are Miranda, Ariel, Umbriel, Titania, and Oberon. Before the Voyager 2 studied Uranus in 1986, these five moons were the only ones known to exist.
Oberon (King of the fairies in A Midsummer Night's Dream by Shakespeare) is similar to Umbriel but 35% larger. Heavily cratered and geologically inactive, Oberon is the second largest moon of Uranus. In many of the large craters that cover the surface there is a dark material that coats the crater floors. It is still unknown what causes this. Also of interest on Oberon is a mountain that is over 4 miles tall.