This summer, for the first time since 1918, a total solar eclipse will cut a path across the mainland United States. On August 21, everyone in North America will be able to watch the moon pass in front of the sun, blotting out some or all of its light (depending on where you live). People near Lincoln City, Oregon will see the total eclipse around 9:05am PDT. Then the path of totality slants eastward, finishing up in South Carolina at 2:43pm EDT.

But hold on—if the moon rises in the east and sets in the west (or pretty close to it, anyway), why does the shadow of an eclipse move from west to east? The answer, says Angela Speck, an astronomer at the University of Missouri, is a matter of perspective.

Watching the sky from the ground, we can see the moon (and the sun and stars) cross from east to west, as if they were moving in a clockwise direction around us.

"It's almost like we are geocentric," says Speck, referring to the outdated idea that the sun and moon revolve around our own pale blue dot.