If you can read a clock, you can know the time of day. But no one knows what time itself is. We cannot see it. We cannot touch it. We cannot hear it. We know it only by the way we mark its passing. For all our success in measuring the smallest parts of time, time remains one of the great mysteries of the universe.如果你會看鐘，你就能知道時間。但沒人了解時間本身。它看不見，摸不著，聽不到。我們只能通過標記它的流逝來認識它。我們在測量極短時間的成就有限，時間仍舊是宇宙最大的謎之一。

In the real world—the world with time—changes never stop. Some changes happen only once in a while, like an eclipse of the moon. Others happen repeatedly, like the rising and setting of the sun. Humans always have noted natural events that repeat themselves. When people began to count such events, they began to measure time.在現實世界中——有時間的世界中，變化從未停止。有些變化只是偶爾發生一次，比如月蝕。還有其它重復發生的變化，比如日出和日落。人類一直在記載自然界中重復發生的事件。當人們開始記載這些事件時，他們就開始測量時間了。

In early human history, the only changes that seemed to repeat themselves evenly were the movements of objects in the sky. The most easily seen result of these movements was the difference between light and darkness. The appearance and disappearance of the sun was even and unfailing. The periods of light and darkness it created were the first accepted periods of time. We have named each period of light and darkness—one day.在早期的人類歷史中，天體運行是唯一有規律的重復發生的變化。而天體運行引起的最直觀的結果就是白天和黑夜的出現。太陽樂此不疲地出現和消失，而此產生的黑白交替的時段是首次被人們認可的時間。我們把每段白天黑夜的組合稱為“一天”。

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