Why does Io cast a hard shadow on Jupiter, but the Moon casts a soft shadow on Earth? Io (source):



The Moon (source):
It's due to the larger relative apparent size of the Sun. When the source of light is a point source the shadow is harder, and when it is extended it is softer.

Jupiter is approximately 5 times more distant from the Sun than the Earth, so the Sun is approximately 5 times smaller in the sky.


*Source: University of North Carolina CS