So, from our perspective down on Earth, the sun and moon appear to be the same size during a total solar eclipse — which is why the sun gets completely blocked by the moon.

The sun’s diameter is 400 times wider than the moon’s, but the sun also happens to be 400 times farther away.

The penumbra is the region on Earth that is partially darkened by the moon’s shadow. Any region within the moon’s penumbra will experience a partial eclipse.

The umbra is the moon’s shadow that is cast on Earth. Any region that falls within the umbra’s path — known as the path of totality — will experience a total eclipse.

A solar eclipse happens when the moon passes between the Earth and the sun, covering the sun and casting a shadow on Earth.

For a few spectacular moments on Monday, Aug. 21, nature will hold its breath: the skies will darken, winds will calm, the temperature will drop and birds will go quiet. The reason? A total solar eclipse. Here’s what you need to know about this rare astronomical event.

What will you see?

The path of totality, the region which will get a total eclipse, stretches across the U.S. from the northwest to the southeast. About 12 million people live directly along this path, and another 200 million are within a day’s drive (including many Canadians).

Those in the penumbra are still likely to be treated to a partial eclipse (weather permitting). The penumbra is wide, stretching north to Canada, and south to Central America and parts of South America.

There are partial eclipses every year — that’s when the alignment of the moon is slightly off and does not fully obstruct the sun.

Total solar eclipses are more rare. They happen about once every year and a half, but since the moon’s shadow is relatively small, not many people get to see them. There hasn’t been a total solar eclipse across the entire U.S. since 1918.