Steve Seidel, age 13, of Hattiesburg, Miss., for his question:
WHEN DID WE START TELLING TIME?
Time is one of the deepest mysteries known to man. No one can say exactly what it is. For early peoples, there were no units of time measurement. But there were some cycles that repeated themselves evenly, and from this came our way of telling time.
The most obvious of the cycles was the alternating daylight and darkness, caused by the rising and setting of the sun. Each of these cycles of the sun came to be called a day.
Another regular change in the sky was the change in the visible shape of the moon. Each cycle of the moon's changing shape takes about 29 and a half days, or a month.
The cycle of the seasons gave man an even longer unit of time. The sun made a full cycle around the sky in one cycle of the seasons. This cycle takes about 365 and a quarter days, or one year.
For hundreds of years, people tried to fit days and months evenly into a year or a period of several years. But no system worked perfectly. Today, the calendar is based entirely on the year. Even though the year is divided into 12 so called months, the months have no relation to the actual cycle of the moon.
There is no regular change in the sky that lasts seven day:, as does the week. The seven day week came from the Jewish custom of observing a Sabbath or day of rest every seventh day.
The division of a day into 24 hours, an hour into 60 minutes and a minute into 60 seconds probably came indirectly from the ancient Babylonians. Babylonian astronomers and astrologers divided the imaginary circular path of the sun into 12 equal parts. Then they divided the periods of daylight and darkness into 12 parts each, resulting in a 24 hour day.
The Babylonians also divided the circle into 360 degrees or parts. Other ancient astronomers further divided each degree into 60 minutes and the minute into 60 seconds.
Directly above every spot on the earth, an imaginary curved line called the celestial meridian passes through the sky. As the earth rotates on its axis, the sun crosses every celestial meridian once each day.
When the sun crosses the celestial meridian above a particular place, the time there is noon. Twleve hours later, the time at that place is midnight.
The period from one midnight to the next is called a solar day. The length of a solar day varies because of the tilt of the earth's axis, the oval shape of its orbit and its changing speed along the orbit.
To make all solar days the same length, astronomers do not measure solar time with the apparent or real sun. Instead, they use an imaginary mean or average sun that moves at a steady speed around the sky.
Mean solar noon occurs when the mean sun crosses the celestial meridian above a particular place. The time between one mean solar noon and the next is always the same. Thus, all mean solar days are the same length.