aside from the sun, the most prominent objects in jupiter's sky are the four galilean moons. io, the nearest to the planet, would be slightly larger than the full moon in earth's sky, though less bright, and would be the largest moon in the solar system as seen from jupiter. the higher brightness of europa would not overcome its greater distance from jupiter, so it would not outshine io. ganymede, the largest moon and third from jupiter, is almost as bright as io and europa, but appears only half the size of io. callisto, further out, would appear only a quarter the size of io.