c. 3000 BCE – 150 CE
Why are there 24 hours in a day and 60 minutes in an hour?
The 24-hour day comes from ancient Egypt, where the night was divided into 12 parts by rising stars and daylight was later divided into 12 as well. The 60 minutes and 60 seconds come from Mesopotamia, where astronomers counted in base 60. Greek astronomers combined the two, and equal-length hours spread with mechanical clocks.
Key facts
| Base-60 counting | Sumer and Babylon, from the 3rd millennium BCE |
|---|---|
| 12 night hours | Egyptian decan star clocks, about 2100 BCE onward |
| Decans | 36 star groups, each rising about 10 days apart |
| Egyptian civil year | 365 days: 12 months of 30 days plus 5 extra days |
| Equal hours proposed | Hipparchus, about 2nd century BCE |
| Degrees in a circle | 360, from Babylonian astronomy |
| Why 60 works | Divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30 |
The way we split the day is a 4,000-year-old combination of two systems: an Egyptian habit of counting 12s and a Mesopotamian habit of counting 60s.
Egypt gives us 24
Egyptian astronomers tracked 36 groups of stars called decans. Each rose just before dawn for about ten days before the next took over. On any given night, about 12 decans rose one after another between dusk and dawn, so the night was divided into 12 parts. These star tables, painted on coffin lids from about 2100 BCE, are the earliest known hour system.
Daylight was measured with shadow clocks and sundials. Early versions split the day into 10 parts plus a twilight period at each end, and over time daylight settled into 12 parts as well. Twelve for the night plus twelve for the day gives 24.
These were seasonal hours: one-twelfth of the actual daylight or darkness. A summer daytime hour in Egypt was much longer than a winter one. People used seasonal hours for everyday life for the next 3,000 years.
Babylon gives us 60
The Sumerians, and the Babylonians after them, wrote numbers in base 60 (sexagesimal). Nobody knows exactly why. A popular explanation is finger counting: point with your thumb at the 12 finger joints on one hand, and use the 5 fingers of the other hand to count dozens, giving 60. Whatever the origin, 60 is practical because it divides evenly in so many ways, which made fractions easy for merchants and astronomers.
Babylonian astronomers used base 60 to divide the circle into 360 degrees and each degree into 60 smaller parts. The Latin names came much later: pars minuta prima, the "first small part," became the minute, and pars minuta secunda, the "second small part," became the second.
Greece puts them together
Greek astronomers in Alexandria inherited both systems. For calculations they used equal hours, 24 per day of exactly the same length, and they used Babylonian base 60 to subdivide them. Hipparchus (2nd century BCE) is credited with proposing equal "equinoctial" hours, and Ptolemy (2nd century CE) used sexagesimal fractions throughout his astronomy. Equal hours stayed an astronomer's tool until mechanical clocks, which can only tick at one rate, made them the everyday standard in medieval Europe.
What about the week and the year?
The seven-day week comes from a different line: the Babylonian and Jewish traditions, later named after the seven visible "planets" (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn). The Egyptian 365-day civil calendar fed into the Julian calendar of 46 BCE, which added a leap day every four years, and the Gregorian calendar of 1582, which fixed the remaining drift.
Attempts to replace all this with decimal time have failed. Revolutionary France tried a 10-hour day with 100-minute hours in 1793 and abandoned it in 1795.
Questions people ask
Who invented the 24-hour day?
The ancient Egyptians, who divided night into 12 parts using star groups called decans and later divided daylight into 12 parts too.
Why is an hour 60 minutes and not 100?
Because the subdivision comes from Babylonian base-60 mathematics, passed on through Greek astronomy. Sixty divides evenly in many more ways than 100.
Did France really use decimal time?
Yes. From 1793 to 1795, the French Republic officially used a 10-hour day with 100 minutes per hour and 100 seconds per minute. People ignored it, and it was dropped.
Sources
Last updated October 18, 2026. Found an error? Tell us.