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.

Early humans 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 use animals to help them work, and for food.