Figure 2-11 Jupiter and Its Largest Moons In 1610, Galileo discovered four “stars” that move back and forth across Jupiter. He concluded that they are four moons that orbit Jupiter just as our Moon orbits Earth. (a) Observations made by Jesuits in 1620 of Jupiter and its four visible moons. (b) Photograph of the four Galilean satellites alongside an overexposed image of Jupiter. Each satellite would be bright enough to be seen with the unaided eye were it not overwhelmed by the glare of Jupiter.