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How did we come to divide the hour into 60 minutes and the minute into 60 seconds? These smaller divisions of time have been in practical use for only about 400 years, but they were vital to the advent of modern science.

For millennia, ancient civilizations looked to the sky to measure the big units of time. There's the year, which is the time it takes Earth to complete one orbit around the sun; the month, which is approximately how long it takes the moon to orbit our planet; the week, which is approximately the time between the four phases of the moon; and the day, which is the duration of one rotation of the Earth's on its axis.

Dividing the day was not so straightforward, though hours and minutes have their origins in traditions tracing back thousands of years.

Number systems

The use of 60 began with the Sumerians who used different number systems. While you and I write numbers using base 10, or "decimal" this civilization used base 12 ("duodecimal") and base 60 ("sexigesimal"). It is not known exactly why they chose these systems, but there are a few theories:
  • Many ancient cultures used the three segments of each finger to count to 12 on one hand, writes Georges Ifrah in his book, "The Universal History of Numbers" (Wiley, 2000; translated by David Bello). It is hypothesized 60 arose from using five fingers of one hand with the twelve segments of the other.
  • Fewer fractions have repeating decimals (1/3 = 0.333...) when written in sexagesimal. This is particularly important because the Sumerians had no notion of repeating-digit fractions. In "An Introduction to the History of Algebra" (American Mathematical Society, 2009), author Jacques Sesiano describes a tablet that reads, "I don't know the inverse of 7/6."
  • Twelve was an important number to the Sumerians, and later to the Egyptians. For example, it was the number of lunar cycles in a year and the number of constellations of the Zodiac. Day and night were each divided into 12 periods, and the 24-hour day was born.
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