The Moon's steady gravitational influence keeps the Earth spinning at the right speed and angle to provide the sort of stability necessary for the long and successful development of life. This won't go on forever. The Moon is slipping from our grasp at a rate of about 1.5 inches a year. In another two billion years it will have receded so far that it won't keep us steady and we will have to come up with some other solution, but in the meantime you should think of it as much more than just a pleasant feature in the night sky.
由于月球持久的引力影响，地球能以合适的速度、合适的角度自转，为生命的长久和成功的发展提供一个必需的稳定环境。这种情况不会永远下去。月球在以每年大约4厘米的速度脱离我们的控制。再过20亿年，它会退缩到很远的地方，无法再维持我们的稳定，我们不得不像个别的解决办法。但是，与此同时，你该认识到，它远不只是夜空中一道悦目的风景线。

For a long time, astronomers assumed that the Moon and Earth either formed together or that the Earth captured the Moon as it drifted by. We now believe, as you will recall from an earlier chapter, that about 4.5 billion years ago a Mars-sized object slammed into Earth, blowing out enough material to create the Moon from the debris. This was obviously a very good thing for us—but especially so as it happened such a long time ago. If it had happened in 1896 or last Wednesday clearly we wouldn't be nearly so pleased about it. Which brings us to our fourth and in many ways most crucial consideration:
很久以来，天文学家一直有两种看法：一、月球和地球是同时形成的；二、是地球在月球飘过时把它抓住的。我们在前面有一章已经谈到，现在我们认为，大约44亿年以前，一个火星大小的物体撞击了地球，撞飞了足够的材料来形成月球。这对我们来说显然是一件好事情——尤其是因为这件事恰好发生在遥远的过去。要是它发生在1896年或者上个星期三，我们肯定不会很高兴的。于是，我们就谈到第四个因素，在许多方面也是最重要的因素：
Timing. The universe is an amazingly fickle and eventful place, and our existence within it is a wonder. If a long and unimaginably complex sequence of events stretching back 4.6 billion years or so hadn't played out in a particular manner at particular times—if, to take just one obvious instance, the dinosaurs hadn't been wiped out by a meteor when they were—you might well be six inches long, with whiskers and a tail, and reading this in a burrow.
合适的时间。宇宙是个变幻无常、变化多端的地方，我们在宇宙里的存在是个奇迹。假如在过去的46亿年时间里发生的一长串极其复杂的事件没有在特定的时间以特定的方式告一段落——举一个最明显的例子，假如恐龙当时不是因为一块陨石的撞击而灭绝——你很可能只有几厘米长，长着触角和尾巴，趴在哪个洞穴里看这本书。