As a result of the improvements Galileo Galilei had made to his telescope, now with a magnifying capability of 20x, he was able to observe celestial bodies more distinctly than it had been ever possible. On January 7, 1610, Galileo wrote a letter in which he mentioned Jupiter´s moons (actually known as the Galilean Moons) for the first time. At the time, he saw only three of the four and believed them to be fixed stars near Jupiter. In later observations he discovered the fourth moon and observed that they were not fixed stars, but rather bodies orbiting Jupiter.
In 1605, Galileo had been employed as a mathematics tutor for Cosimo de’ Medici, and seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter’s moons to gain it. On February 13, 1610, Galileo wrote to the Grand Duke‘s secretary:
“God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke”.
The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.