most of us appreciate the beauty of a full moon sedately moving across our night skies.
我们中的大多数人都会因满月静静地从我们的夜空滑过而产生美的感觉。
the moon seems unchanging, as if for eons it has been there, controlling the tides, going through its phases every month and inspiring poets and lovers.
月球看起来似乎不曾改变，就像千万年来它一直就在那里那样，掌控着地球的潮水涨落，经历着自身的一月一次的阴晴圆缺和激发诗人和情人们的灵感与情思。
it might surprise you to know that in the past, things were much different, and they are still changing.
可能你会惊奇，当你得知，在过去，情况是非常不同的，并且“不同”仍在加剧。
a billion years ago, our relationship with the moon was quite different.
10亿年前，我们与月球的关系完全不同。
for one thing, the moon was much closer and therefore appeared much larger: and we saw the entire moon, not just one face as we do now.
首先，月球近得多，因此看起来更大，并且我们能看到月球的全貌，而不是像现在，仅仅看到一面。
it took the moon only twenty days to circle the earth, and earth's day was only eighteen hours long.
月球绕行地球一周只需二十天，地球的一天也只有十八小时长。
massive tides, over a kilometer in height, would ebb and flow every few hours.
每几个小时，超过一千米高的大规模潮汐就发生一次。
from the beginning though, things were changing, as the moon's orbit was getting larger.
一开始，事情就在发生变化，月球的轨道在越变越大。
every year, the moon moves about four centimeters (about 1.6 inches,) farther out into space, away from earth.
每一年，月球都会深入太空，远离地球约4厘米（约1.6英寸）。
right now, the moon's rotation rate, and the time it takes to orbit earth are the same.
现在，月球的自转速度与绕地球公转速度是一样的。
is this all just some cosmic coincidence?
难道这一切只是宇宙巧合吗？
would it surprise you to know that at one time, earth didn't even have a moon?
当你得知，曾几何时，地球甚至还不曾拥有月亮，你是否会感到惊奇？
where then, did our nearest neighbor in space come from, and what will the future bring? to answer these questions we must go very far back into time.
那么，我们这个最近的邻居来自何方，未来又将怎样？为了回答这些问题，我们必须回到遥远的过去。
黑暗面月球
four and a half billion years ago the earth was about sixty percent formed, though it did have a differentiated core, crust and mantle.
45亿年前，地球大约只有现在百分之六十，尽管它确有能够区分的地核，地壳和地幔。
in these primal skies, there was no moon.
在原始的天空里，没有月亮。
it was a very violent time in the solar system with other planets forming from clumps of gas, dust and rock and impacts among these bodies were the norm.
这是一段变化剧烈的时间，在太阳系里，气团，尘埃和岩石遵循一定的规则相互影响变化形成其他的行星。
orbits were not the sedate, orderly system they are today.
轨道还不是今天那样静止、有序的系统。
one planet, about the size of mars, probably had an orbit which crosses earth's, and eventually a collision occurred.
一个大小与火星相当，轨道可能与地球轨道交叉的行星，最终与地球发生了碰撞。
the impacting planet made a hard, but glancing blow off earth at just the right angle.
撞击行星以一个恰当的角度硬生生地几乎是一瞬间地砸向了地球。
it almost bounced off earth, but was consumed instead.
它几乎弹起，但最终为地球所吞噬。
but the blow sent shock waves across earth, spewing gas and debris into space, giving earth, for a very short time, a ring around it.
但是，撞击产生的冲击波穿透了地球，导致它向太空喷出了环绕自己的气体和碎片，但这种情况持续的时间很短。
the debris around earth began to condense into clumps and quite rapidly, a blink in cosmic time, formed a large glowing ball: our primal moon.
那些围绕地球的碎片开始迅速地凝结，依宇宙时间的标准，就在瞬间形成了一个大灼热球：即我们的原始月球。
the moon would have looked about ten times larger than it does today, and the earth was changed forever.
那个月球看起来比今天的要大约10倍，地球也就从此改变了。
this story, about the birth of the moon, is still a theory, one among several attempting to explain the source of our satellite.
这个关于月球诞生的故事，还只是几个试图解释我们这个卫星来源的理论中的一个。
currently, this is the most widely accepted of the theories.
目前，这是接受度最高的理论。
since that cosmic collision, the earth and moon have both have an effect on each other, and will continue to have an affect far into the future. since the earth is so much more massive than the moon, the earth has had the larger effect.
宇宙撞击之后，地球和月球相互影响对方，并且这种影响将持续到久远的将来。由于地球比月球大很多，所以地球施加的影响更大。
earth's gravity has caused the moon to become tidally locked us as well as increasing the distance between the two worlds.
地球的引力造成了月球潮锁，也增加了两个世界之间的距离。
by tidally locked, we mean that the moon's rotation rate is the same as the time it takes the moon to go once around earth, which also results in us seeing only one side of the moon.
潮锁，是指月球的自转率与月球绕地球一周所花费的时间相同，这也是导致我们只能看到月球的一面的原因。
月球大小的比较
the dynamics of this earth/moon relationship come basically down to earth's tides so we need to take a closer look at them.
地/月动态关系变化基本归结到地球潮汐，所以我们必须仔细看看他们。
we know that the moon is the primary cause of earth's ocean tides by the moon “pulling” on the earth as it goes around it.
我们知道，月球是地球形成海洋潮汐的首要原因，因为当月球绕过地球时，它在“拉扯”地球。
but gravity lessens with distance and this means that the pull from the moon is stronger on the side of earth that is facing the moon.
但是，引力随距离增大而减小，这意味着月球施加在朝向自己那一面的地球部分的拉力要大些。
keep in mind that the moon is not pulling on the earth as a whole, but rather on the part of earth beneath the moon.
请记住，月球不是将地球作为一个整体拉动，而是在拉扯月下的那部分。
so, as the moon passes over a part of earth, the moon pulls on that area.
所以，月球经过地球哪里，就拉扯它哪里。
if it is over water, the moon actually pulls on the water, creating a void for more water to flow into that area and creating a high tide.
如果经过水体，月球对水产生一个拉力，创造出一个虚空，让更多的水流入这一领域，从而产生一个高潮。
directly opposite that spot, the moon's gravity is pulling on the earth itself, pulling it away from the water and allowing more water to flow into that area creating another high tide.
正背面，月球的引力作用在地球球体上，拉动它远离其表面的水，并让更多的水流入该地区，从而产生又一个高潮。
but because the distance from the moon is greater, the pull from gravity on the far side of the earth is about six percent less than on the side facing the moon.
但因为距离月球更远，地球背面受到的拉力比朝向月球那面受到的拉力小约6%。
the sun also has an effect on tides, but it only about forty six percent of the tidal effect from the moon.
太阳也对潮汐有影响，但其潮汐效应只有月球的约46%。
that might sound confusing – seeing as how the sun is so much bigger than the moon, but remember – gravity decreases with distance – and the sun is much farther away from earth than the moon.
听起来可能有些令人困惑——明明太阳是远远大于月球的，但请记住——引力随距离减小——比起地球与月球的距离，太阳遥远得多。
when the sun and moon team up together (both on the same side of the earth) we have even larger high tides.
当太阳和月亮在一起时（均在地球的同一侧） ，我们将有更大的潮。
the point though, is that earth is being tugged from several bodies, rotating under the shifting tides.
原因是，地球正在被几方面用力拉，并且是在大潮下自转。
also, remember that as the moon is moving around the earth, the earth itself is spinning.
此外，请记住，当月球环绕地球，地球本身就在自转。
the earth spins much faster than the moon moves around our planet.
地球自转速度远远超过了月球绕地球旋转速度。
the effect this has, is that that the tidal bulge caused by the moon is actually pulled ahead of the moon by the earth's faster rotation.
这样的结果是，由于地球更快的自转速度，月球所造成的地球潮汐隆起实际上在月球拉力点的前面。
the moon therefore is pulling back on the tidal bulge with the result that there is friction between the ocean floor and the water.
因此，月球在把潮汐隆起往回拉，导致洋底和海洋水体之间产生摩擦。
the friction actually slows earth down, therefore explaining why our days on earth are getting longer.
摩擦实际上减缓了地球自转，因此解释了为什么我们在地球上的天变得越来越长。
these tidal bulges on earth, have an affect on the moon, in essence pulling the moon and forcing it into a higher orbit.
这些地球上隆起的潮，会拉扯月球，迫使它进入了更高的轨道。（译者注：是否像放风筝？）
this is why the moon is moving away from us.
这就是为什么月球正在远离我们。
we mentioned tides on earth caused by the moon – but earth actually causes tides on the moon's surface as well.
我们提到月球引起的地球上的潮汐——但实际地球也会引起月球表面的潮汐。
even though the moon has a solid surface of rock, small tidal bulges actually occur.
尽管月球是固体岩石表面，小的潮汐隆起实际上也会发生。
earth tugging on the moon's surface has slowed down the rotation of the moon over time.
随着时间的推移，地球对月球表面的牵引逐渐减缓了月球的自转。
over time the moon rotation slowed until it equaled the time it took the moon to go around the earth.
月球自转一直变慢，直到相当于环绕地球一周所花费的时间。
at that point, those lunar tidal bulges lined up with earth and the rotation ceased slowing down: the moon was tidally locked with earth and that's why we see only one side of the moon.
那一刻，那些月球潮汐隆起与地球成一线，其自转不再放缓：月球被地球潮锁，这就是为什么我们看到的只是月球的同一面。
just because the moon's rotation has ceased to slow down doesn't mean that the earth/moon system isn't changing in other ways.
单纯地月球的自转放慢停止并不意味着地球/月球系统不会以其他的方式改变。
we already know that the moon is moving farther away from earth but other changes are happening as well.
我们已经知道，月球正在远离地球，但其他变化也正在发生。
we mentioned that earth's rotation was being slowed by the friction between the oceans and the ocean floor.
我们提到，海洋与洋底之间的摩擦导致地球的自转减速。
this will continue to happen until earth's tidal bulges align with an imaginary line running through the center of the earth/moon system then earth's rotation will cease slowing down.
这种情况将继续下去，直到地球的潮汐隆起与一个贯穿中心地/月系统假想线重合，那时地球的自转将停止放缓。
this will take a few billion years but when it does happen: earth's day will be a month long (960 hours a day) and our month will be forth days long!
这将需要数亿年，但是当它发生：地球的一天将像一个月那样漫长（ 960小时一天） ，我们的一个月只有1/4天长！
by then the moon will be twenty-five percent farther away.
届时月球将远上25%。
if we were on the moon looking back at earth – we would see the same face of earth – just as now we see only one face of the moon.
如果我们在月球上回首地球——我们将看到地球的同一面——就像现在我们看到的只是月球的一面。
and if someone were still on earth: the moon will have moved far enough away that it appears much smaller – and there are no more solar eclipses!
如果仍然有人在地球上：月球将离到足够远，以致看起来要小得多——也不再有太阳蚀！
we have yet to answer that question about a cosmic coincidence.
我们还没有回答关于宇宙巧合的问题。
do we “just happen” to be here on earth when the moon is close enough to give us tides, and slowed the earth's rotation until we have a twenty-four hour day and thirty day month?
难道刚好月球近到足以引发潮汐，并减缓地球自转，直到一天一个二四小时一个月三十天时，我们才“偶然”出现在地球上的吗？
some scientists believe that it may be thanks to the moon that we are here at all.
一些科学家认为这可能正是由于月球，我们才在这里的。
although the moon is slowing down earth's rotation, it has stabilized it as well.
虽然月球在减缓地球的自转速率，但它也稳住了地球。
if we had no moon, we would suffer movement of the earth's poles, an axial tilt of up to ninety degrees, causing major climate swings.
如果没有月亮，地球的两极不稳，轴向倾斜会高达90度，从而造成重大的大气波动。
some scientists believe that under those circumstances: intelligent life simply could not have evolved.
在这种情况下，一些科学家认为：智能生命根本无法形成。
it is hard to imagine that our world as we know it, and all that is familiar to us, would likely not exist if not for the moon.
很难想象，如果不是月球，我们现在所知的这个世界，以及对我们来说非常熟悉的一切，很可能不存在。
we may truly owe our lives to a lunar legacy·
我们能有现在这样的生活，可能还要真正感谢月球的馈赠。