Game

Moon Transit Catcher

Jupiter’s four big moons go round at their true relative speeds, sped up a lot. When the moon we name crosses in front of Jupiter, hit Snap. You have 60 seconds.

The real thing

Io goes round Jupiter in 1.769 days, Europa in 3.551, Ganymede in 7.155 and Callisto in 16.689. NASA NSSDC: Jovian satellite fact sheet
Io, Europa and Ganymede are in step: each time Ganymede goes round Jupiter once, Europa goes round twice and Io four times. NASA Science: Ganymede facts
Their radii: Io 1,821.5 km, Europa 1,560.8 km, Ganymede 2,631.2 km, Callisto 2,410.3 km. NASA NSSDC: Jovian satellite fact sheet
The four largest moons, Io, Europa, Ganymede and Callisto, were first seen by Galileo Galilei in 1610. They are called the Galilean satellites. NASA Science: Jupiter moons

Our game is a simplified model. The speeds are in the true ratio from NASA’s fact sheet and the moons are sized against each other, but the distances from Jupiter are squeezed to fit the screen and the orbits are drawn as tilted circles. In the game, a moon passing in front of Jupiter is a transit; passing behind is an occultation.

Jupiter’s striped clouds with two small moons, Io and Europa, in front — A real double transit: Io and Europa in front of Jupiter, seen by Voyager 1 on 27 February 1979.Photo: NASA / JPL / Bjorn Jonsson, Public domain, Commons