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ANNCR:

This is SCIENCE IN THE NEWS, in VOA Special English. I'm SteveEmber. This week our program is about a mystery as old as time. BobDoughty and Sarah Long tell about the mystery of time.

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VOICE ONE:

If you can read a clock, you can know the time of day. But no oneknows what time itself is. We cannot see it. We cannot touch it. Wecannot hear it. We know it only by the way we mark its passing.

For all our success in measuring the smallest parts of time, timeremains one of the great mysteries of the universe.

VOICE TWO:

One way to think about time is to imagine a world without time.There could be no movement, because time and movement cannot beseparated.

A world without time could exist only as long as there were nochanges. For time and change are linked. We know that time haspassed when something changes.

VOICE ONE:

In the real world -- the world with time -- changes never stop.Some changes happen only once in a while, like an eclipse of themoon. Others happen repeatedly, like the rising and setting of thesun. 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 repeatthemselves evenly were the movements of objects in the sky. The mosteasily seen result of these movements was the difference betweenlight and darkness.

The sun rises in the eastern sky, producing light. It movesacross the sky and sinks in the west, causing darkness. Theappearance and disappearance of the sun was even and unfailing. Theperiods of light and darkness it created were the first acceptedperiods of time. We have named each period of light and darkness --one day.

VOICE TWO:

People saw the sun rise higher in the sky during the summer thanin winter. They counted the days that passed from the sun's highestposition until it returned to that position. They countedthree-hundred sixty-five days. We now know that is the time Earthtakes to move once around the sun. We call this period of time ayear.

VOICE ONE:

Early humans also noted changes in the moon. As it moved acrossthe night sky, they must have wondered. Why did it look differentevery night? Why did it disappear? Where did it go?

Even before they learned the answers to these questions, theydeveloped 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 earlyhumans counted the number of times the sun appeared between fullmoons. They learned that this number always remained the same --about twenty-nine suns. Twenty-nine suns equaled one moon. We nowknow this period of time as one month.

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VOICE TWO:

Early humans hunted animals and gathered wild plants. They movedin groups or tribes from place to place in search of food. Then,people learned to plant seeds and grow crops. They learned to useanimals to help them work, and for food.

They found they no longer needed to move from one place toanother to survive.

As hunters, people did not need a way to measure time. Asfarmers, however, they had to plant crops in time to harvest thembefore winter. They had to know when the seasons would change. So,they developed calendars.

No one knows when the first calendar was developed. But it seemspossible that it was based on moons, or lunar months.

When people started farming, the wise men of the tribes becamevery important. They studied the sky. They gathered enoughinformation so they could know when the seasons would change. Theyannounced when it was time to plant crops.

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VOICE ONE:

The divisions of time we use today were developed in ancientBabylonia four-thousand years ago. Babylonian astronomers believedthe sun moved around the Earth every three-hundred-sixty-five days.They divided the trip into twelve equal parts, or months. Each monthwas thirty days. Then, they divided each day into twenty-four equalparts, or hours. They divided each hour into sixty minutes, and eachminute into sixty seconds.

VOICE TWO:

Humans have used many devices to measure time. The sundial wasone of the earliest and simplest.

A sundial measures the movement of the sun across the sky eachday. It has a stick or other object that rises above a flat surface.The stick, blocking sunlight, creates a shadow. As the sun moves, sodoes the shadow of the stick across the flat surface. Marks on thesurface show the passing of hours, and perhaps, minutes.

The sundial works well only when the sun is shining. So, otherways were invented to measure the passing of time.

VOICE ONE:

One device is the hourglass. It uses a thin stream of fallingsand to measure time. The hourglass is shaped like the number eight--- wide at the top and bottom, but very thin in the middle. In atrue "hour" glass, it takes exactly one hour for all the sand todrop from the top to the bottom through a very small opening in themiddle. When the hourglass is turned with the upside down, it beginsto mark the passing of another hour.

By the eighteenth century, people had developed mechanical clocksand watches. And today, many of our clocks and watches areelectronic.

VOICE TWO:

So, we have devices to mark thepassing of time. But what time is it now. Clocks in different partsof the world do not show the same time at the same time. This isbecause time on Earth is set by the sun's position in the sky above.

We all have a twelve o'clock noon each day. Noon is the time thesun is highest in the sky. But when it is twelve o'clock noon whereI am, it may be ten o'clock at night where you are.

VOICE ONE:

As international communications and travel increased, it becameclear that it would be necessary to establish a common time for allparts of the world.

In eighteen-eighty-four, an international conference divided theworld into twenty-four time areas, or zones. Each zone representsone hour. The astronomical observatory in Greenwich, England, waschosen as the starting point for the time zones. Twelve zones arewest of Greenwich. Twelve are east.

The time at Greenwich -- as measured by the sun -- is calledUniversal Time. For many years it was called Greenwich Mean Time.

VOICE TWO:

Some scientists say time is governed by the movement of matter inour universe. They say time flows forward because the universe isexpanding. Some say it will stop expanding some day and will beginto move in the opposite direction, to grow smaller. Some believetime will also begin to flow in the opposite direction -- from thefuture to the past. Can time move backward?

Most people have no trouble agreeing that time moves forward. Wesee people born and then grow old. We remember the past, but we donot know the future. We know a film is moving forward if it shows aglass falling off a table and breaking into many pieces. If the filmwere moving backward, the pieces would re-join to form a glass andjump back up onto the table. No one has ever seen this happen.Except in a film.

VOICE ONE:

Some scientists believe there is one reason why time only movesforward. It is a well-known scientific law -- the second law ofthermodynamics. That law says disorder increases with time. In fact,there are more conditions of disorder than of order.

For example, there are many ways a glass can break into pieces.That is disorder. But there is only one way the broken pieces can beorganized to make a glass. That is order. If time moved backward,the broken pieces could come together in a great many ways. Only oneof these many ways, however, would re-form the glass. It is almostimpossible to believe this would happen.

VOICE TWO:

Not all scientists believe time is governed by the second law ofthermodynamics. They do not agree that time must always moveforward. The debate will continue about the nature of time. And timewill remain a mystery.

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ANNCR:

Our program was written by Marilyn Christiano and read by SarahLong and Bob Doughty. I'm Steve Ember. You can download all of ourprogram script text and audio files from WWW.51VOA.COM.Listen again next week for Science in the News, in VOA Special English.