Time is always running!
From VOA Learning English, this is SCIENCE IN THE NEWS, in Special English. I'm June Simms.
Our program today is about a mystery as old as time. Bob Doughty and Sarah Long tell about the mystery of time.
​​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.
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.
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 sun rises in the eastern sky, producing light. It moves across the sky and sinks in the west, causing 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.
People saw the sun rise higher in the sky during the summer than in winter. They counted the days that passed from the sun's highest position until it returned to that position. They counted 365 days. We now know that is the time Earth takes to move once around the sun. We call this period of time a year.
Early humans also noted changes in the moon. As it moved across the night sky, they must have wondered. Why did it look different every night? Why did it disappear? Where did it go?
Even before they learned the answers to these questions, they developed a way to use the changing faces of the moon to tell time.
The moon was "full" when its face was bright and round. The early humans counted the number of times the sun appeared between full moons. They learned that this number always remained the same -- about 29 suns. Twenty-nine suns equaled one moon. We now know this period of time as one month.