Twenty-four hours in a day feels as natural as the sunrise. But it isn't built into nature. The Earth spins once a day, and people could have carved that spin into 10 pieces, or 20, or 100. So why 24?
The short answer is ancient Egypt. Egyptian astronomers divided the night into 12 parts by watching certain stars rise, and the daytime was later divided into 12 parts as well. Twelve plus twelve made 24. The idea spread to Greece and Rome, and Greek astronomers eventually turned it into 24 equal hours.
Counting Stars by Night
The Egyptians were dividing the night into units they called wnwt before the 24th century BC. By about 2150 BC, star diagrams painted inside coffin lids, sometimes called star clocks, show there were exactly 12 of them.
The Egyptians tracked 36 stars or star groups, now called decans, one for each ten-day week of their calendar. Each night they noted eleven of these rising, which split the darkness into 12 parts, the middle ones lasting about 40 minutes each. The number 12 may also echo the roughly 12 lunar cycles in a year.
The Nile played a role, too. Its annual flood, crucial for farming, came with the rise of the star Sirius before dawn and the appearance of 12 constellations across the night sky, which the Egyptians considered significant.
Hours That Stretched and Shrank
There was a catch: these early hours weren't equal. Each was one-twelfth of the night or one-twelfth of the daylight, so summer daytime hours were longer than winter ones. Historians call them seasonal or unequal hours.
By around 1350 BC, priests at Karnak were keeping time with water clocks. A vessel was filled to the brim at sunset, and the hour was read by comparing the water level against one of 12 gauges, one for each month.
The Greeks Even Things Out
The Greek astronomer Hipparchus of Nicaea was probably the first to divide the whole day and night into 24 hours. In Athens, during the first century BCE, a tower now called the Tower of the Winds tracked a 24-hour day using sundials and mechanical hour indicators.
Equal hours, each one twenty-fourth of a day, took a long time to win out in everyday life. The switch to twice twelve equal hours began as early as the 16th century. Because noon-to-noon days vary slightly over the year, the hour was later smoothed out by basing it on the average, or mean, solar day.
Minutes and seconds came from somewhere else. Medieval astronomers split the hour into 60 minutes of 60 seconds each, a habit that traces back to Babylonian astronomy, which counted in sixties.
The Time France Tried 10 Hours
Not everyone loved the system. During the French Revolution, France introduced decimal time, with 10 hours in a day. Midnight was called ten o'clock, and noon was five o'clock.
It never caught on. Mandatory use was suspended on April 7, 1795, though some clocks kept showing decimal time for years, and the scientist Pierre-Simon Laplace had a decimal watch made for himself.
So the next time you check the clock, remember you're using a system that began with Egyptian stargazers watching the night sky. It has been tweaked, standardized, and even challenged, but 24 has held on for more than four thousand years.
