Io, Europa, and Ganymede have orbital periods in the ratio 1:2:4. At some time all three satellites are lined up on the same side of Jupiter as seen from Earth. After Europa goes once around Jupiter:

Question

Quizplus · Accepted Answer

Since Ganymede's orbital period is twice that of Europa, Europa and Ganymede will always be on opposite sides of Jupiter. When Europa completes one orbit, it will be on the same side of Jupiter as Io, but Ganymede will be on the opposite side. Therefore, all three satellites will be lined up, but Ganymede will be on the opposite side of Jupiter as seen from Earth.

Io, Europa, and Ganymede Have Orbital Periods in the Ratio