This is an astronomy homework question that my classmates and I are having an awful time with. If the penumbra of the Earth’s shadow is 16,000 km across, and if the Moon moves 3400 km/hr with respect to the shadow, why does it take 6 hours instead of only 5 hours to get completely through the penumbra? We first thought that the extra hour was due to the Earth's revolution around the Sun pushing the edge of the penumbra ahead so the moon would have to play "catch up" with that. But now that I am re-reading the question, it says that the moon moves "with respect to" the shadow so I am thinking that the Earth's rotation has already been factored in. The other theory is that the extra hour would be due to the moon travelling in an arc instead of a straight line through the penumbra, and that would account for the time delta. Are either of these close? Thanks in advance for your help.