The Sky of the Satellites
Thanks to the Voyager space probes we have learned a great deal about the four large Galilean satellites * of Jupiter: the volcanoes of Io, the cracked glacier that covers Europa, the frozen craters of Ganymede and Callisto.
Yet if we could imagine ourselves on the surface of any of these worlds and somehow protected from the harsh conditions, it would probably not be the surfaces that would hold our attention most, not the volcanoes, not the craters, not the cracked glaciers. It would be the skies.
Consider Callisto, the farthest of the Galileans, 1,171,000 miles from Jupiter's center.Thanks to Jupiter's tidal effect, Callisto, like the other Galileans, faces one side always to Jupiter, as our moon faces one side always to us.This means that Callisto rotates on its axis, relative to the universe generally, in the same time it revolves about Jupiter—once in 16.69 days.
Standing on Callisto, we would see the sun make one complete circle in that time, which means it would move considerably more slowly in Callisto's sky than in our own.On Callisto, the time from sunrise to sunset would be some 200 hours rather than the average of 12 hours that the sun stays in Earth's sky.
The three Galilean satellites that are closer to Jupiter than Callisto is revolve about Jupiter more quickly, hence rotate more quickly, and therefore see the sun move more rapidly across the sky.On Ganymede, the sun moves from rising to setting in 84 hours; on Europa, in 42.7 hours; on____________________