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Ever wonder why a day has 24 hours and a minute has 60 seconds?

Friday, July 6, 2012

The most widely used numerical system in the world is the decimal system, using 10 as a base. However, to measure time, we use the duodecimal (base 12) and sexagesimal (base 60) systems. This is because our method of dividing the day derives from the innovations of ancient Mediterranean civilizations. By 1500 BCE, Egyptians (who used base 12) developed a sundial which looked like a T-shaped bar placed in the ground with would divide the time between sunrise and sunset into 12 parts. Because of the seasonal change in the length of time between sunrise and sunset, summer hours were longer than winter hours! Historians theorize that the importance of 12 is based on the number of finger joints on each hand (not counting the thumb) or the number of lunar cycles in a year. The division of the night into 12 parts was achieved by Egyptian astronomers who observed the appearance of 12 key stars in the night sky. Out of these divisions was born the concept of a 24-hour day. However, seasonal hour length was used for many centuries, and fixed hours became common only after the appearance of mechanical clocks in 14th century Europe!

The Greek astronomer Eratosthenes (27 6-194 BCE) divided a circle into 60 parts to create a geographical system of latitude. The reason for the importance of the number 60 is not known, but historians note that it is conveniently divisible by 10, 12, 15, and 30. Hipparchus added a 360 degree system of longitude a century later, and in 150 CE Claudius Ptolemy subdivided each degree into 60 parts. The first division (each of the 360 degrees) was called the partes minutae primae, or first minute, and the second division (each of the 60 parts of a degree) was the partes minutae secundae, or “second minute”. Clock displays were in the shape of a circle, so the former became the modern minute, and the latter the modern second! However, like hours with fixed length, minutes and seconds took centuries to come into widespread use. The first clocks displaying minutes appeared in the late 16th century.


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