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The earth completes a full rotation once per day, and hence
地球自转一周需要一天
the moon passes overhead pretty much once daily.
而月球只有一次经过头顶
So then why are there two high and low tides every day?
那为什么会出现两次潮汐呢
Simple: the moon's gravitational attraction is stronger the closer you are to it.
简单 月球的引力在近的一端更强
So the close side of the earth gets pulled away from the middle,
使得靠近月球一端的海水上涨
which in turn gets pulled away from the far side.
同理 背对月球一端的海水也会上涨
As the earth rotates, these so-called "tidal forces"
这种被称为潮汐力的相互作用
cause water on the surface to slosh around as "high tides" twice per day -
使海水能够每天高潮两次
when the moon is directly over your head and directly below your feet.
一次月亮在你头顶 一次月亮在你脚下
So why is gravity weaker at a greater distance?
为何引力随距离而减小
Well, imagine you're spraying a stream of water at a sheep (flamingo).
假设你向一只羊喷水
If the sheep's more distant, less of the water hits it than when it's close, so there's less force pushing it away.
羊离你越远 越少的水溅到羊 从而羊收到的力越小
Gravity behaves the same way, except it's attractive.
引力是同样的道理
And there are tidal forces from the sun as well:
太阳同样能产生潮汐力
they're weaker than the lunar ones,
但比月球引起的潮汐力小
but when the sun and moon line up (on the same or opposite sides of the earth) the tides pack an extra punch.
但太阳地球月亮共线时 潮汐会达到最大
It's not just water that sloshes,
不止江河湖海会产生潮汐
though - tidal forces cause the earth's crust to bulge as well -
潮汐力还能使地面隆起
we just don't notice because we get lifted along with it.
我们感觉不到是因为我们随地表一起运动
And tidal forces from the earth cause the moon to bulge, too!
地球的潮汐力也能使月球表面隆起
All this bulging and sloshing dissipates some of the earth's rotational energy,
不管是潮汐还是隆起 都在减小地球的旋转动能
causing it to gradually slow its rotation.
使地球逐年减速
Eventually, the earth will spin slowly enough
最终 地球的自转速度
that the same side of the earth will always face the moon
使地球始终只有一面对着月球
- just as the same side of the moon already always faces the earth!
就像现在月球只有一面对着地球一样
This means that in the distant future, a day and a lunar month will each have the same length:
也就是说 以后一个地球日会和一个月球月一样长
about 50 of our current earth days.
差不多是现在的50天那么长
What's more, if the moon were to get too close to us,
同时月球也在不断靠近地球
tidal forces from the earth would be stronger than the gravity holding the moon together,
而总有一天地球的潮汐力将会大过hold住月球的引力
so rocks on the moon's surface would literally start flying off into space,
理论上月球表面的岩石会开始脱离
and the moon would disintegrate.
然后月球就会解体
This would also happen if the earth were to get too close to the sun
同理 地球如果太靠近太阳也会解体
- and if you decided to fly into a black hole,
如果你决定飞向黑洞
it's what would actually do you in:
最有可能发生的情况是
your spaceship would get ripped apart from tip to tail by tidal forces.
你的飞船在到达黑洞前就被潮汐力撕成粉碎了
Now that's bad tidings.
这可不是一个好消息
