The movement of the sun creates periods of ______.

W: Dr. Smith, How did early humans start to note, or record the passing of time?
M: Well, this is a long story. No one knows what time itself is. But we can notice its passing in a number of ways. 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 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.
W: I am sure the moon was also very important to our ancestors.
M: Yes, of course. Early humans also 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. 29 suns equaled one moon. We now know this period of time as one month.
    As early humans learned to plant seeds and grow crops, 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.
    The divisions of time we use today were developed in ancient Babylonia 4000 years ago. Babylonian astronomers believed the sun moved around the Earth every 365 days. They divided the trip into twelve equal parts, or months. Each month was thirty days. Then, they divided each day into 24 equal parts, or hours. They divided each hour into sixty 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 shadow. As the sun moves, so does the shadow of the stick across the flat surface. Marks on the surface show the passing of hours, and perhaps, minutes.
W: But the sundial works well only when the sun is shining. So, what are the other ways to measure the passing of time?
M: Or course. One device is the hourglass. It uses a thin stream of falling sand to measure time. The hourglass is shaped like the number eight—wide at the top and bottom, but very thin in the middle. In a true "hour" glass, it takes exactly one hour for all the sand to drop from the top to he bottom through a very small opening in the middle. When the hourglass is turned with the upside down, it begins to mark the passing of another hour.
    By the 18th century, people had developed mechanical clocks and watches. And today, many of our clocks and watches are electronic.
    So, we have devices to mark the passing of time. But what time is it now? Clocks in different parts of the world do not show the same time at the same time. This is because time on Earth is set by the sun’s position in the sky above.
    As international communications and travel increased, it became clear that it would be necessary to establish a common time for all parts of the world.
    In 1884, an international conference divided the world into 24 time areas, or zones. Each zone represents one hour. The astronomical observatory in Greenwich, England, was chosen as the starting point for the time zones. Twelve zones are west of Greenwich. Twelve are east. The time at Greenwich—as measured by the sun—is called Universal Time. For many years it was called Greenwich Mean Time.
W: That brings us to the modern time. Thanks very much, Dr. Smith.