Four centuries ago, Galileo Galilei turned his tiny telescope to Jupiter and discovered its four largest moons. The satellites later received Roman names, as well as numerals reflecting the order of their distance from Jupiter: Io (I), Europa (II), Ganymede (III), and Callisto (IV). Were it not for their close proximity to brilliant Jupiter, all four would be viable naked-eye targets. Nevertheless, several well-documented historical accounts exist of sightings of at least one (and sometimes more) of these moons.
Stephen James O’Meara’s Secret Sky: The Daughters of Dawn’s Heart
October 2010: It may be tricky, but you too can spot Jupiter's largest moons with your naked eyes.