On March 25, 1655 Dutch astronomer Christian Huygens discovered the first moon of Saturn, later named Titan. Huygens did it with the help of his brother Constantijn, also an astronomer, with a telescope they built themselves.
Huygens called it Saturni Luna, Latin for Saturn’s Moon in his publication of the discovery, New Observations of Saturn’s Moon. When the next five moons of Saturn were discovered a few decades later, astronomers began referring to them by number, Saturn I through Saturn VI, though the list was not sequential and Titan was variously named Saturn I, IV and even VI. The name Titan was given 50 years after Huygen’s death by astronomer John Herschel, son of Anglo-German astronomer William Herschel in John Herschel’s 1857 publication Results of Astronomical Observations Made at the Cape of Good Hope. He named the moons after the twelve titans (Τῑτάν), the mythical race of deities that preceded the traditional canon of Greek deities.
The word titan was in common use in English by the 1500s, becoming an adjective by 1709, then applied to the element titanium in 1796 and finally the moon of Saturn.
Image of Titan from the Cassini program, courtesy NASA