“Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell . . . . I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto..”
– Johannes Kepler
Kepler named the moons of Jupiter after Jupiter/Zeus’ lovers but Galileo wanted them to be called the Cosmica Sidera – the stars of Cosimo, after the Medici who was Galileo’s patron. He wrote a lavish letter to Cosimo referring to him as having immortal graces and saying that the Maker of Stars himself wished Galileo to name these stars after Cosimo. Cosimo asked that his brothers be included n the honour of having stars named after them.
Galileo discovered Callisto on this date in 1610. But Kepler’s names stuck.