The Moons of Jupiter

Galileo Galilei discovered four moons of Jupiter in 1610. The two innermost moons are Io and Europa orbiting 421,700 km (3 diameters) and 671,034 km (4.8 diameters) from Jupiter's centre, where the diameter of Jupiter is 140,000 km. That must be how Jonathan Swift arrived at 3 and 5 diameters for the moons of Mars. Even so, to apply Kepler's Third Law he would have needed still more data.