Caliban orbits Uranus in the opposite direction from the regular moons and the rotation of the planet itself (called a retrograde orbit). Its orbit is also somewhat inclined and eccentric, and very far from the planet -- more than 10 times farther than Oberon, the farthest regular moon. These characteristics suggest that Caliban was an independent body that was captured by Uranus' gravity. It is thought to be about 72 km in diameter, and to be the second largest irregular satellite of Uranus (about half the size of Sycorax). The size estimate is based on the brightness of the moon and an assumed albedo of 0.04, typical of captured asteroids in the outer solar system.
Caliban was discovered on 6 September 1997 at Palomar Observatory by B. Gladman, P. Nicholson, J. A. Burns, and J. Kavelaars. This group of astronomers also discovered another Uranian moon at the same time, which was named Sycorax after the mother of Caliban in William Shakespeare's play The Tempest.
How Caliban Got its Name:
Originally called S/1997 U1, Caliban was named for a character in Shakespeare's play, "The Tempest." Caliban was forced into labor on an island owned by Prospero. He was human, but "not honour'd with a human shape," and is referred to as a mooncalf or freckled monster.
Moons of Uranus are named for characters in William Shakespeare's plays and from Alexander Pope's "Rape of the Lock."