system
Given the following post, respond as if you're commenting on Reddit:
human
What would Jupiter look like from the surface of Ganymede?

I am trying to write a science fiction story, and wish to keep it as hard as I can. Much of the plot centers around a settlement on Ganymede, and I would like to know how Jupiter would appear in the sky from the surface of the moon.
gpt
Jupiter would be large in the sky, but not as big as people tend to imagine, and not as big as you often see in science fiction landscapes. Jupiter is about 3x as far from Ganymede as the Moon is from Earth, but Jupiter is about 40x the diameter of the Moon. So, from Ganymede, Jupiter would look about 15x the size of a full Moon from Earth. If you hold out your fist at arm's length, it'll be about 2/3rds of that apparent size.

So it won't be filling half the horizon or anything. But you'd definitely see details on it, like cloud bands, and even the transits of the inner moons crossing its surface. Jupiter would have phases based on where you are in Ganymede's orbit. Ganymede orbits Jupiter once every ~7 earth days. When you're on the sun-side of Jupiter, you'd see Jupiter is a fully lit disc. When you're on the shadow-side of Jupiter, you'd see it as a thin crescent. When you're in between, you'll see a half-lit Jupiter.

Ganymede is tidally locked, so its rotation period is synchronous with its orbit. This means that from any point on Ganymede's surface, Jupiter always remains in the same position in the sky. So, the "near side" of Ganymede will always see Jupiter, while the "far side" of Ganymede will never see Jupiter. Both sides will see the Sun rise and set over a "day" that lasts 7 earth days, but only one side will see Jupiter change its phase throughout each "day". Incidentally, Ganymede has only a very small axial tilt (both relative to Jupiter to and the Sun), which means that you'll see the Sun rise almost due east and set almost due west no matter where you are on the surface.

One fun thing is that the tidal locking means you could use Jupiter for navigation. How far north/south/east/west Jupiter is in the sky depends on your location, and you can tell which direction is east/west by the shadows on Jupiter. The further south Jupiter is in the sky, the further north you are on the planet. At the north pole, Jupiter would be due south on the horizon, etc. So if you were stuck in the middle of nowhere on the Jupiter side of Ganymede, you can pretty easily work out your rough latitude and longitude, without even using any instruments. This is particularly fun because the problem of finding longitude has always been a difficult one on Earth, but on half of Ganymede it's actually pretty easy.