The sun is about 400 times larger than the moon. How can a smaller object block our view of a much bigger one? The answer lies in their distances from our planet. The sun is about 400 times farther away from Earth than the moon is. That makes both objects appear about the same size in the sky. That means the moon can cover up the sun when both are in just the right places.
When the moon blocks the sun, it casts a dark shadow on Earth. On August 21, that shadow will be 113 kilometers (70 miles) wide. The shadow will pass diagonally across the U.S. from Oregon to South Carolina. For viewers in this shadow, the sun will completely disappear for a minute or two. The daytime sky will become as dark as evening twilight. Bright stars will become visible. The wispy outer layer of the sun, called the corona, which is usually invisible, will come into view. The air will grow cooler. The moon will create the appearance of an inky black hole over the sun.
In many other parts of the U.S., viewers will see a partial eclipse. The moon will cover just a portion of the sun, making the sun look like a crescent. It will last for an hour or two.