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On August 21, 2017
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the shadow of the Moon will pass
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from the west coast to the east coast of the U.S.
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Our blue sky will turn black as night
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and fill with stars,
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and there will be a hole in the sky
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where the Sun used to be,
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surrounded by the fiery ring of the Sun's corona,
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a total eclipse of the Sun.
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This will truly be a historic event.
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Accounts of solar eclipses
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date way back on the written record.
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The early Mesopotamians wrote
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that the Sun was put to shame
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during the solar eclipse of the 14th century B.C.E.
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and it may have started
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the Sun worship of the Egyptian pharaoh Akhenaten.
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Ancient Chinese astrologers paid with their lives
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if they failed to predict the solar eclipse
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and portend the fate of their emperors
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whose symbol was the Sun.
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The earliest date of a specific event in human history,
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a battle between the armies of Lydia and Media,
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occurred on May 28, 535 B.C.E.
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when a solar eclipse caused the soldiers
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to lay down their arms and declare a truce.
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So how does it happen?
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During a total solar eclipse,
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the Moon moves between the Earth and the Sun.
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When this happens, the disc of the Moon
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appears to perfectly cover the disc of the Sun
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even though the Sun is much larger than the Moon.
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But how is this possible?
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The Sun is 400 times bigger than the Moon,
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but by sheer coincidence,
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the Moon is 390 times closer to Earth.
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Size and distance cancel each other out
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so that the Moon and Sun appear
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to be almost the exactly same size.
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Every time the Moon orbits the Earth,
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once every 27.3 days,
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it has to pass between the Earth and the Sun,
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a stage called the new moon phase.
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And every time it passes,
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the New Moon has a chance to block out the Sun.
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Most of the time,
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the Moon passes a little above
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or a little below the Sun,
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but if they align perfectly,
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the shadow of the Moon
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will make a narrow path across Earth
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and those in the shadows will see a total solar eclipse.
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Just like on night side of the Earth,
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the sky during a total eclipse is black
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and filled with stars.
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But while the moon perfectly covers
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the surface of the Sun,
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it doesn't block out the Sun's outer atmosphere,
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its corona, which appears as a fiery ring
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around the dark disc of the moon.
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Solar eclipses occur several times a year,
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but most often they are partial eclipses
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where the Moon doesn't quite line up with the Sun.
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And, when the Moon and Sun are perfectly aligned,
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the Moon is usually too far from Earth in its orbit
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to completely cover the Sun,
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creating an annular eclipse.
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During an annular or partial eclipse,
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the sky remains bright.
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Even on those rare occasions of a total eclipse,
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the Moon's shadow is most likely to fall
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on the 70% of Earth that is covered by water,
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and few people, if any, will see it.
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The eclipse of 2017 will be remarkable on a larger scale
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because the Moon is slowly moving away from Earth.
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If a furry ancestor of ours had bothered to look up
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during a solar eclipse a hundred million years ago,
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it wouldn't have seen the fiery corona of the Sun.
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It would have just been dark.
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Eventually, the Moon will have moved too far from Earth
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to completely cover the disc of the Sun.
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It is only during our little wink of Earth's history
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that the Moon is at just the right distance
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to cause a total solar eclipse
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yet not block the Sun's corona.
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So on August 21, 2017,
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when the Moon exactly lines up with the Sun
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and the Moon is close enough to the Earth,
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its shadow will cross the U.S.
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and, if you happen to be in its narrow path,
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you will witness one of the most
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awe-inspiring sights in the universe.
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But, as incredible as this event will be,
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total eclipses are one of the most dangerous as well.
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Only specially tinted filters,
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specifically designed to observe the Sun,
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should be used.
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The eclipse might put the Sun to shame,
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but even a shamed Sun
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can seriously damage your eyes.