What creates a total solar eclipse? - Andy Cohen
On August 21, 2017 the shadow of the Moon will pass from the west coast to the east coast of the U.S. Our blue sky will turn black as night and fill with stars, and there will be a hole in the sky where the Sun used to be, surrounded by the fiery ring of the Sun's corona, a total eclipse of the Sun. This will truly be a historic event.
Accounts of solar eclipses date way back on the written record. The early Mesopotamians wrote that the Sun was put to shame during the solar eclipse of the 14th century B.C.E. and it may have started the Sun worship of the Egyptian pharaoh Akhenaten. Ancient Chinese astrologers paid with their lives if they failed to predict the solar eclipse and portend the fate of their emperors whose symbol was the Sun.
The earliest date of a specific event in human history, a battle between the armies of Lydia and Media, occurred on May 28, 535 B.C.E. when a solar eclipse caused the soldiers to lay down their arms and declare a truce. So how does it happen? 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 exactly the 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. Just like on the night side of the Earth, the sky during a total eclipse is black and filled with stars.
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.
The eclipse of 2017 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 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, as incredible as this event will be, total eclipses are one of the most dangerous as well. Only specially tinted filters, specifically designed to observe the Sun, should be used. The eclipse might put the Sun to shame, but even a shamed Sun can seriously damage your eyes.