Vocabulary. eclipse.

　　One way to think about time is to imagine a world without time.

　　There could be no movement because time and movement cannot be separated.

　　A world without time could exist only as long as there were no changes, for time and change are linked.

　　We know that time has passed when something changes.

　　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.