Why Does the Moon's Eclipse Shadow Cross the US from West to East?

Among the many excellent questions posed by visitors at the summer eclipse programs now being presented at the planetarium at De Anza College is one about the motion of the Moon's shadow during the upcoming eclipse. During the program we show the progress of the darkest part of the Moon's shadow moving eastward over the US (from Oregon to South Carolina) confusing some visitors in light of the familiar apparent westward drift of sky objects that we observe every night.

The reason for the confusion is probably that TWO independent motions are involved, 1) the Moon's monthly orbital motion around the Earth, and 2) the Earth's daily rotation about its own axis. The Earth's axial rotation eastward is the motion that results in the familiar nightly westward drift of the Sun, Moon, planets and stars. During a single night we don't become aware of the Moon's orbital motion due to the faster west to east motion of the Earth apparently carrying the Moon (and other celestial objects) westward. Both motions contribute to shadow movement, but the effect of the Moon's orbital motion dominates, which results in a net motion west to east of the shadow over the ground.