When reading an article on using Jovian moons to calculate longitude, I came across this passage:
Periodically, Jupiter eclipses each of the four large moons as they pass into the planet's mammoth shadow. In a telescope, a moon's brightness takes several minutes to fade to black as it enters the shadow. Reappearances are just as gradual.
How can the moons take this long to fade in and out? Is it simply the shadow creeping over the surface of the moons? What does this look like?