What Is a Solar Eclipse? Why the Moon Passes Between the Sun and Earth

Sun eclipse 1970
A total eclipse of the sun, observed by the Harvard College Observatory in 1970. Fox Photos/Getty Images

The sun, moon and Earth will line up in a rare celestial event Monday that, from coast to coast, will turn day to night. The total solar eclipse will be the first in the United States in nearly a century, and millions of Americans will be scrutinizing the event, recording it on social media and photographing it, making it perhaps the most watched eclipse in history.

"Going through life without ever experiencing totality is like going through life without ever falling in love," Rick Fienberg, spokesman for the American Astronomical Society, said of the event.

Eclipses occur when one heavenly body moves into the shadow of another heavenly body. Lunar eclipses happen when the Earth blocks the sun's light reflecting off the moon. A solar eclipse occurs when the moon moves between the Earth and the sun.

The diameter of the sun is 400 times that of the moon, but it lies 400 times further away, meaning that at certain points in its orbit people on Earth in exactly the right position will see the two exactly lined up.

NASA puts it this way: "The moon orbits, or goes around, Earth. Earth orbits, or goes around, the sun.… Sometimes when the moon orbits Earth, it lines up directly between the sun and Earth. When this happens, the moon blocks the light of the sun. This causes an eclipse of the sun, or a solar eclipse. During a solar eclipse, the moon casts shadows onto Earth. A solar eclipse happens during the daytime. The daylight grows dim. Sometimes the moon blocks almost all of the sunlight. Then the daytime can look as dark as night during a solar eclipse! Solar eclipses happen once every 18 months. Solar eclipses only last for a few minutes."

That means that on Monday, from Salem, Oregon, to Charleston, South Carolina, Americans will only be able to see the corona or halo of the sun as it is obscured for up to two minutes. For the rest of the country, there will be a partial eclipse, with the moon party obscuring the sun's disc.

Scientists describe the event as awe inspiring. Bill Cooke, chief of NASA's Meteoroid Environment Office, says: "It is the most weird, creepy, awe-inspiring astronomical event you will experience."