Telling Time

Telling time is based on an ancient tradition of dividing the day and night into 12 equal parts called hours. Each hour can be divided into 60 minutes and each minute is divided into 60 seconds. If you look at a clock with hands (pointers), the face of the clock is divided into 12 sections, each representing either 1 hour or 5 minutes, depending on which hand you are "reading", the short hand (hours) or the long hand (minutes).

Why do we count 60 minutes to an hour?

According to Live Science writer Rober Coolman, the use of a counting system based on 12 (duodecimal) and 60 (sexagesimal) dates back to the Sumerian civilization in the 24th century B.C. The center of this civilization lay between the Tigris and Euphrates rivers, near the Persian Gulf, which is now part of the modern country of Iraq. The importance of 12 and 60 can be seen in astronomy and geometry. A year can be divided into 12 months or "lunar cycles". Day and night can each be divided into 12 hours to make a 24 hour day, which is the time it takes the earth to make one rotation on it's axis. Many ancient civilizations used the 3 segments of 4 fingers to count to 12. Using the 5 fingers of the other hand would have made it easy to count to 60 (5 times 12).

Six hundred years later, the Babylonian Empire had taken over the same general area, previously occupied by the Sumerians. The Babylonians invented the measurement of angles in degrees. It was thought that a year was approximately 360 days, so a circle was defined as 360 degrees. When the sun was measured with respect to the other stars (along the ecliptic plane of the galaxy), the sun appeared to move one degree per day. This also fit with geometry, since 6 triangles with equal sides (equilateral triangles) can be centered around a common point (or vertex) to make a hexagon. A circle fits neatly around the vertices of the hexagon. A sixth of a circle (60 degrees) was used as a common angle measurement. The Babylonians studied the stars and understood enough about the motion of the moon and the sun to predict when eclipses would occur.

The sexagesimal (base 60) system was so useful in astronomy that the Greek scholars kept using it. When they realized that the earth was round, (Eratosthenes did a very nice experiment to measure the circumference of the earth) , they started using degrees to divide the earth by lines of latitude and longitude, which they used along with the stars to find their way at sea.

Minutes and seconds have been used for a very long time to describe location; however, the measurement of time using very accurate clocks to measure minutes and seconds did not happen until the 1600s. Tycho Brahe was able to make very accurate measurements of star locations by knowing the time to an accuracy of 8 seconds. Johannes Kepler based his laws of planetary motion on these very accurate measurements by Brahe. Kepler was not the first scientist to believe that the earth rotated around the sun, but he was the first to describe the orbits of the planets as ellipses.

The modern definition of a second has been redefined based on the energy transitions in an atom of Cesium, which is commonly called an atomic clock.