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  • 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 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.

  • Just like on 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.

On August 21, 2017

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B1 TED-Ed moon eclipse sun solar eclipse solar

【TED-Ed】A rare, spectacular total eclipse of the sun - Andy Cohen

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    稲葉白兎 posted on 2015/01/17
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