The total solar eclipse in August 2017 will travel across the United States from Oregon to South Carolina. But if the moon rises in the east and sets in the west, why does the eclipse shadow travel from west to east? Dear Science is on the case. (Daron Taylor,Jorge Ribas/The Washington Post)

Dear Science,

I'm really excited about the total solar eclipse that's going to happen in the U.S. on Aug. 21. But some of the details have me confused. People say that the eclipse will start on the West Coast, in Oregon, and travel southeast toward South Carolina. But if the moon rises in the east and sets in the west, shouldn't the eclipse go in that direction too? Is the eclipse moving backward?

Here's what science has to say:

No, the moon will not reverse its course around the Earth for a few hours on Aug. 21. But we don't blame you for feeling confused. This definitely falls into the category of Space Questions That Are Really Tough to Wrap Your Mind Around.

“It is actually something even astronomers struggle to see,” said C. Alex Young, a heliophysicist at NASA's Goddard Space Flight Center. When the issue came up at a recent conference, “it wound up with a bunch of astronomers sitting around trying to figure out how to explain it.”

When the moon passes in front of the sun on Aug. 21, it will cast a long shadow across the Earth's surface. As the moon moves in its orbit, that shadow will race from the west coast of the U.S. to the east — the opposite path we're used to watching the moon take.

The phenomenon is so difficult to understand because it shifts our perspective. Even though we learned in elementary school that everything in space, including the Earth, is in motion, we are very bad at remembering that we are not standing still at the center of things. When we look up at night and see the moon rising in the east, we assume that's because the moon is circling from east to west around our planet.


The Hinode spacecraft captured this image of the January 6, 2011 solar eclipse. (NASA)

In reality, the moon appears to rise in the east because that's the way the Earth is spinning. If you looked down on Earth from above the North Pole, our planet would be rotating counterclockwise — or spinning toward the east. That's why you catch sight of the moon first peeping above the eastern horizon, and your last view of it is as it sets in the west.

Meanwhile, the moon is actually revolving in the same direction that Earth spins — and twice as fast. The moon is traveling eastward in its orbit at about 2,100 miles per hour, while the Earth spins at a sluggish 1,040 mph (at the equator). So the path of the moon's shadow will track eastward across the Earth's surface at a speed of about 2,100 minus 1,040 mph — roughly 1,060 mph.

Head still spinning? That's understandable — this issue is easier to understand if you can visualize it. Take a look at this video of the moon taken by a camera aboard NASA's Deep Space Climate Observatory (DSCOVR) satellite last year.

NASA released images July 11 of the moon crossing the face of the Earth for the second time this year. The images were captured by NASA's Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) satellite between July 4-5 (Reuters)

See how the moon travels quickly from west to east across the globe?

The moon doesn't do anything differently during the eclipse. Instead, the eclipse is exposing us to a reality that exists every day of the year. For many of us, for the first time in our lives, the eclipse will let us witness the movement of the moon in its own orbit, and not as a result of the Earth's rotation. That's part of what makes astronomical events like this so cool: they are a reminder of our place in the universe.

Have a question for Dear Science? Ask it here.

More eclipse coverage:

The first solar eclipse to cross America in 99 years is coming. To some, it's an act of God.

Don't let clouds ruin your solar eclipse — here's where it's most likely to be clear

Scientists are gearing up to take the longest-ever video of a solar eclipse