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The earth completes a full rotation once per day,
地球每天完成一次完整的自转，
and hence the moon passes overhead pretty much once daily.
因此月球几乎每天都经过我们头顶一次。
So then why are there two high and low tides every day?
那么为什么每天都有两次涨潮和落潮呢？
Simple: the moon's gravitational attraction is stronger the closer you are to it.
这很简单：越接近月球，月球的引力就越强。
So the close side of the earth gets pulled away from the middle,
因此，离月球近的那一边受引力影响从中间挣脱出来，
which in turn gets pulled away from the far side.
继而从离月球远的那一边挣脱出来。
As the earth rotates, these so-called "tidal forces"
当地球自转时，这些所谓的“潮汐力”
cause water on the surface to slosh around as "high tides" twice per day
使水面上的水每天以“涨潮”的形式来回晃动两次。
when the moon is directly over your head and directly below your feet.
这时候月亮要么正对着你的头顶，要么在脚的正下方时。
So why is gravity weaker at a greater distance?
那么，为什么引力在更远的距离会减弱呢？
Well, imagine you're spraying a stream of water at a sheep.
好吧，想象一下你正对着一只绵羊喷水。
If the sheep's more distant, less of the water hits it than when it's close,
如果绵羊离你比较远，喷到绵羊身上的水就很少，离你比较近的话反而更多，
so there's less force pushing it away.
所以把绵羊推开的力量更小。
Gravity behaves the same way, except it's attractive.
引力也是一样的，只是它很吸引人。
And there are tidal forces from the sun as well: they're weaker than the lunar ones,
还有来自太阳的潮汐力：太阳的潮汐力要比月球的潮汐力小，
but when the sun and moon line up, the tides pack an extra punch.
但当太阳和月亮在一条直线上时，潮汐就会产生额外的冲击力。
It's not just water that sloshes, either, tidal forces cause the earth's crust to bulge as well
不仅是水的晃动，潮汐的力量也会导致地壳的膨胀。
we just don't notice because we get lifted along with it.
我们只是没有注意到而已，因为我们就站在地壳上面，随着地壳隆起，我们也站在更高处。
And tidal forces from the earth cause the moon to bulge, too!
来自地球的潮汐力也引起了月球的膨胀！
All this bulging and sloshing dissipates some of the earth's rotational energy, causing it to gradually slow down.
所有这些膨胀和晃动都会消耗掉一些地球的自转能量，导致地球逐渐减速。
Eventually, the earth will spin slowly enough that the same side of the earth will always face the moon
最终，地球将以足够慢的速度自转，使地球的同一侧始终面向月球，
just as the same side of the moon already always faces the earth!
就像月球的同一侧一直面向地球一样！
This means that in the distant future,
这意味着在遥远的未来，
a day and a lunar month will each have the same length: about 50 of our current earth days.
一天和一个农历月的长度是一样的：大约是我们当前地球日的50天。
What's more, if the moon were to get too close to us,
更重要的是，如果月球离我们太近，
tidal forces from the earth would be stronger than the gravity holding the moon together,
来自地球的潮汐力会比将月球固定在一起的引力更强，
so rocks on the moon's surface would literally start flying off into space, and the moon would disintegrate.
因此月球表面的岩石就会开始飞向太空，而月球就会解体。
This would also happen if the earth were to get too close to the sun,
如果地球离太阳太近，地球就会解体。
and if you decided to fly into a black hole, it's what would actually do you in:
如果你决定飞入黑洞，它实际上会对你造成的影响是：
your spaceship would get ripped apart from tip to tail by tidal forces.
你的太空船会被潮汐的力量撕成碎片。
Now that's bad tidings.
那样的话就是坏消息了。
