Ferdinand (moon)

Ferdinand was a moon of Uranus.

While helping William Shakespeare find inspiration for The Tempest, the Tenth Doctor mentioned two moons of the then-undiscovered planet Uranus, Ferdinand and Miranda. This gave Shakespeare the character names for a boy and girl featured in the play. (PROSE: The Tempest – A Work in Progress)

Behind the scenes
Ferdinand and Miranda were named after the characters from The Tempest in the real world, advancing the obvious implication that the Doctor created a bootstrap paradox by giving Shakespeare the information he did.