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 can not see it. We can not hear it. We know it only by the way we mark its passing. For ail our success in measuring the tiniest parts of time, time remains one of the great mysteries of the universe. One way of thinking about time is to imagine a world without time. There could be no movement, because time and movement can not be separated. A world without time could exist only as long as there were no changes, for time and change are linked. When something changes, you know time has passed. In the real world, 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. People have always 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 rose in the eastern sky, producing light. It moved overhead and sank in the western sky, 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 moon's changing faces to tell time. The moon was 'full' when its face was bright and round. They 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. Early people hunted animals and gathered wild plants. They moved in groups, or tribes, from place to place in search of food. Then people learned to plant seeds and grow crops. They learned to raise animals. They found they no longer needed to move from one place to another to survive. As hunters, people did not need a way to measure time. As farmers, however, they had to plant crops in time to harvest them before 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 seems possible that it was based on moons, or lunar months. When people started farming, the wise men of the tribes became very important. They studied the sky. They gathered enough information to be able to say when the seasons would change. They announced when it was time to plant crops. The divisions of time we use today were developed in ancient Babylonia 4,000 years ago. Babylonian astronomers believed the sun moved around the Earth every 365 days. They divided the trip into 12 equal parts or months. Each month was 30 days. Then they divided each day into 24 equal pans, or hours. They divided each hour into 60 minutes, and each minute into 60 seconds. Humans have used many devices to measure time. The sundial was one of the earliest and simplest. A sundial measures the movement of the sun across the sky each day. It has a stick or other object that rises above a flat surface. The stick, blocking sunlight, creates a s