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.
