In Greek mythology, Callisto was a nymph, or the daughter of King Lycaon; the myth varies in such details. She was believed to be one of the followers of Artemis who attracted Zeus. Many versions of Callisto's story survive. According to some writers, Zeus transformed himself into the figure of Artemis to pursue Callisto, and she slept with him believing Zeus to be Artemis. She became pregnant and when this was eventually discovered, she was expelled from Artemis's group, after which a furious Hera, the wife of Zeus, transformed her into a bear, although in some versions, Artemis is the one to give her an ursine form. Later, just as she was about to be killed by her son when he was hunting, she was set among the stars as Ursa Major by Zeus. She was the bear-mother of the Arcadians, through her son Arcas by Zeus.

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury, though only about a third as massive. Callisto is, with a diameter of 4,821 km, roughly a third larger than Earth's Moon and orbits Jupiter on average at a distance of 1,883,000 km, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.