We are so ready(!!!!!)
During a total solar eclipse, the Moon moves between the Earth and the Sun. When this happens, the disc of the Moon appears to perfectly cover the disc of the Sun even though the Sun is much larger than the Moon. But how is this possible?
The Sun is 400 times bigger than the Moon, but by sheer coincidence, the Moon is 390 times closer to Earth. Size and distance cancel each other out so that the Moon and Sun appear to be almost the exactly same size. Every time the Moon orbits the Earth, once every 27.3 days, it has to pass between the Earth and the Sun, a stage called the new moon phase. And every time it passes, the New Moon has a chance to block out the Sun. Most of the time, the Moon passes a little above or a little below the Sun, but if they align perfectly, the shadow of the Moon will make a narrow path across Earth and those in the shadows will see a total solar eclipse.
But while the moon perfectly covers the surface of the Sun, it doesn’t block out the Sun’s outer atmosphere, its corona, which appears as a fiery ring around the dark disc of the moon. Solar eclipses occur several times a year, but most often they are partial eclipses where the Moon doesn’t quite line up with the Sun. And, when the Moon and Sun are perfectly aligned, the Moon is usually too far from Earth in its orbit to completely cover the Sun, creating an annular eclipse. During an annular or partial eclipse, the sky remains bright. Even on those rare occasions of a total eclipse, the Moon’s shadow is most likely to fall on the 70% of Earth that is covered by water, and few people, if any, will see it.
TODAY’S eclipse will be remarkable on a larger scale because the Moon is slowly moving away from Earth. If a furry ancestor of ours had bothered to look up during a solar eclipse a hundred million years ago, it wouldn’t have seen the fiery corona of the Sun. It would have just been dark. Eventually, the Moon will have moved too far from Earth to completely cover the disc of the Sun. It is only during our little wink of Earth’s history that the Moon is at just the right distance to cause a total solar eclipse yet not block the Sun’s corona. So today, on August 21, 2017,when the Moon exactly lines up with the Sun and the Moon is close enough to the Earth, its shadow will cross the U.S. and, if you happen to be in its narrow path, you will witness one of the most awe-inspiring sights in the universe. BUT - remember - you can’t look directly at the sun, so….
Don’t forget your (hopefully legit) protective eclipse glasses!
From the TED-Ed Lesson A rare, spectacular total eclipse of the sun - Andy Cohen
Animation by Bevan Lynch