On August 21, 2017, the shadow of the Moon will pass from the west coast to the east coast of the U.S.
2017年8月21日，月亮的阴影会横扫美国，从西岸到东岸。
Our blue sky will turn black as night and fill with stars,
我们的蓝天会变得像晚上一样黑，还会有满天星斗，
and there will be a hole in the sky where the Sun used to be,
同时天空中也会出现一个洞，就是太阳之前的位置，
surrounded by the fiery ring of the Sun's corona, a total eclipse of the Sun.
会被日冕形成的火环所围绕，这就是日全食。
This will truly be a historic event.
这将会是一个历史性事件。
Accounts of solar eclipses date way back on the written record.
关于日食的纪录可以追溯回古时史书。
The early Mesopotamians wrote that the Sun was put to shame during the solar eclipse of the 14th century B.C.E.
早期的美索不达米亚人写道，在十四世纪的一场日食中，太阳蒙羞了，
and it may have started the Sun worship of the Egyptian pharaoh Akhenaten.
这可能是导致埃及法老阿肯那顿太阳膜拜的开端。
Ancient Chinese astrologers paid with their lives
古代的中国占星术士是要赔上性命的，
if they failed to predict the solar eclipse and portend the fate of their emperors whose symbol was the Sun.
如果他们漏报了日食或无法预示皇帝的命兆，因为太阳就是皇帝的象征。
The earliest date of a specific event in human history, a battle between the armies of Lydia and Media,
人类史上最早记载日期的特定事件为利底亚与米底亚之间的战争，
occurred on May 28, 535 B.C.E. when a solar eclipse caused the soldiers to lay down their arms and declare a truce.
发生于公元前535年5月28日，当时发生的日食造成士兵弃械并宣告停战。
So how does it happen?
那日食是怎么形成的？
During a total solar eclipse, the Moon moves between the Earth and the Sun.
当日全食发生时，月球运行到地球与太阳之间。
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.
而且月球的盘面看起来完全挡住太阳，即使太阳比月球大很多。
But how is this possible?
但这怎么可能？
The Sun is 400 times bigger than the Moon, but by sheer coincidence, the Moon is 390 times closer to Earth.
太阳比月球大了400倍，但是就有这么巧。月球到地球的距离比日地距离近390倍。
Size and distance cancel each other out so that the Moon and Sun appear to be almost the exactly same size.
尺寸及距离的效应互相抵销，因此月球与太阳看起来大小几乎相同。
Every time the Moon orbits the Earth, once every 27.3 days,
每次月球绕地球公转一圈27.3天，
it has to pass between the Earth and the Sun, a stage called the new moon phase.
它总会穿过地球与太阳之间一次，这时期称做新月。
And every time it passes, the New Moon has a chance to block out the Sun.
每次它穿过，新月就有机会挡住太阳。

Most of the time, the Moon passes a little above or a little below the Sun, but if they align perfectly,
多数情况下，月球穿过的位置可能比太阳高一点或低一点，但如果它们完全对齐，
the shadow of the Moon will make a narrow path across Earth and those in the shadows will see a total solar eclipse.
月球的阴影就会在地球上扫出一条狭长的食带，只有在食带中的人才能看到日全食。
Just like on night side of the Earth, the sky during a total eclipse is black and filled with stars.
就像地球在夜晚那侧一般，天空在日全食期间是黑暗的，并可见满天星斗。
But while the moon perfectly covers the surface of the Sun, it doesn't block out the Sun's outer atmosphere,
但是就算月球完全遮盖太阳的表面，它也挡不住太阳的大气层，
its corona, which appears as a fiery ring around the dark disc of the moon.
即日冕，看起来就像一圈火环，在月球暗影的外围。
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.
一年会发生好几次日食，但多数情况是日偏食，也就是月球与太阳并非完全对齐。
And, when the Moon and Sun are perfectly aligned,
再者，有时月球与太阳排成一直线时，
the Moon is usually too far from Earth in its orbit to completely cover the Sun, creating an annular eclipse.
月球在公转轨道上的位置离地球太远，无法完全遮盖太阳，就会发生日环食。
During an annular or partial eclipse, the sky remains bright.
在日环食或日偏食发生时，天空依然明亮。
Even on those rare occasions of a total eclipse,
即使是很罕见的日全食，
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.
月球的阴影最有可能落在占地球70%的海面上，所以只有极少数人有幸能见到此奇景。
The eclipse of 2017 will be remarkable on a larger scale because the Moon is slowly moving away from Earth.
2017年的日食很值得注意，因为长久来看，月球正逐渐远离地球。
If a furry ancestor of ours had bothered to look up during a solar eclipse a hundred million years ago,
如果我们有着毛皮的祖先愿意费神在几亿年前的一场日食中抬头仰望，
it wouldn't have seen the fiery corona of the Sun. It would have just been dark.
他不会看到太阳的日冕火环。只有一片漆黑。
Eventually, the Moon will have moved too far from Earth to completely cover the disc of the Sun.
总有一天，月球会因离地球太远，再也无法完全遮蔽太阳。
It is only during our little wink of Earth's history
只有在地球历史上这段转瞬般短暂的时间内，
that the Moon is at just the right distance to cause a total solar eclipse yet not block the Sun's corona.
月球处于能造成日全食、却不会挡住日冕的恰好的距离。
So on August 21, 2017, when the Moon exactly lines up with the Sun and the Moon is close enough to the Earth,
所以在2017年8月21日，当月球与太阳恰恰形成一直线，而且月球与地球的距离够近，
its shadow will cross the U.S. and, if you happen to be in its narrow path,
它的暗影才会横扫美国，如果你刚好位在月影的狭长食带上，
you will witness one of the most awe-inspiring sights in the universe.
你将会见证一场宇宙间最令人敬畏的景象。
But, as incredible as this event will be, total eclipses are one of the most dangerous as well.
但是，就像这起事件非常奇妙一样，日全食也非常危险。
Only specially tinted filters, specifically designed to observe the Sun, should be used.
只有专为观察太阳而设计的特殊染色滤镜能拿来看日食。
The eclipse might put the Sun to shame, but even a shamed Sun can seriously damage your eyes.
日食可能使太阳蒙羞了，但是即使是蒙羞的太阳，也能严重伤害你的眼睛。