If you can read a clock, you can know the time ofday. But no one knows what time itself is. We cannotsee it. We cannot touch it. We cannot hear it. Weknow it only by the way we mark its passing. For allour success in measuring the smallest parts of time,time remains one of the great mysteries of theuniverse.如果你会看钟，你就能知道时间。但没人了解时间本身。它看不见，摸不着，听不到。我们只能通过标记它的流逝来认识它。我们在测量极短时间的成就有限，时间仍旧是宇宙最大的谜之一。

In the real world—the world with time—changes never stop. Some changes happen only once ina while, like an eclipse of the moon. Others happen repeatedly, like the rising and setting ofthe sun. Humans always have noted natural events that repeat themselves. When people beganto count such events, they began to measure time.在现实世界中——有时间的世界中，变化从未停止。有些变化只是偶尔发生一次，比如月蚀。还有其它重复发生的变化，比如日出和日落。人类一直在记载自然界中重复发生的事件。当人们开始记载这些事件时，他们就开始测量时间了。

In early human history, the only changes that seemed to repeat themselves evenly were themovements of objects in the sky. The most easily seen result of these movements was thedifference between light and darkness. The appearance and disappearance of the sun waseven and unfailing. The periods of light and darkness it created were the first accepted periodsof time. We have named each period of light and darkness—one day.在早期的人类历史中，天体运行是唯一有规律的重复发生的变化。而天体运行引起的最直观的结果就是白天和黑夜的出现。太阳乐此不疲地出现和消失，而此产生的黑白交替的时段是首次被人们认可的时间。我们把每段白天黑夜的组合称为“一天”。