Historic Solar Eclipse!

Next Monday, August 21, people across the U.S. will get the chance to experience a total solar eclipse. A solar eclipse happens when the moon passes directly between the Earth and the sun. During a total eclipse, the moon completely blocks the view of the sun from certain places on Earth.

Monday’s eclipse marks the first time in nearly a century that the path of a total eclipse will cut across the nation from coast to coast. That hasn’t happened since 1918. And it won’t happen again until 2045. (But there will be a total eclipse visible in parts of the U.S. in 2024.) Millions of Americans and visitors from around the world are making eclipse-watching plans. Many will travel hundreds or thousands of miles to watch the spectacular sky show.

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. That makes both objects appear about the same size in the sky—and it’s why the moon can cover up the sun when it’s in just the right place.

When the moon blocks the sun, it casts a dark shadow on Earth. On August 21, that 113 kilometer (70 mile)-wide shadow will pass diagonally across the U.S. from Oregon to South Carolina. For viewers in this part of the moon’s 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, hot 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 parts of the U.S. outside the darkest part of the shadow, viewers will see a partial eclipse. The moon will cover a portion of the sun’s disk, making the sun look like a crescent. It will last for an hour or two.

Read more about the eclipse and analyze data about its path in our May 8, 2017 issue.