Question: When we 'accidentally' discovered Ganymede, how did we know that it was a moon and not a planet?

Nawapat Kaweeyanun answered on 20 Nov 2019:

Through a telescope, planets like Jupiter move rapidly across the night sky, so astronomers can trace out Jupiter’s path. Since Ganymede orbits Jupiter, astronomers would see a small spot moving rapidly back and forth around Jupiter. Galileo saw four such moons during his Jupiter observation, and that’s the origin of the four Galilean moons (Ganymede, Callisto, Io, and Europa).