Section 1.1
Why a day has 24 hours
The 24-hour day traces back to ancient Egypt, where each interval of darkness, and later each interval of daylight, was divided into twelve equal parts, a system Scientific American links to the importance the Egyptians placed in the 12 decans.A Chronicle Of Timekeeping | Scientific AmericanThe cosmic significance the Egyptians placed in the 12 decans led them to develop a system in which each interval of darkness (and later, each interval of daylight) was divided into a dozen equal parts.
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Why twelve?
The reason for choosing 12 is uncertain; it may come from star patterns or from a number system built around 12.Time, Clocks and Sun Dials in Ancient Egypt | Middle East And North Africa — Facts and DetailsThe origin of the 12-hour division might come from star patterns in the sky or from the Sumerian number system which was based on the number 12.
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From seasonal to equal hours
The Greek astronomer Hipparchus proposed equal-length hours in the second century BC, but ordinary people kept using seasonal hours until mechanical clocks arrived in Europe.How 10 Fingers Became 12 Hours | EarthDateBetween 147 and 127 BC, the Greek astronomer Hipparchus proposed setting the length of the hour based on the equal length days and nights that occur on the equinox, but common people continued to use hours that varied with the season because they still depended on sundials.
Open source The system became so entrenched that France's 1790s attempt at a decimal day lasted only 16 months.