Dividing the day into two distinct sets of twelve hours dates back to ancient Mesopotamia and Egypt, well before mechanical pendulums existed. Egyptian astronomers divided the daytime into 10 hours of bright daylight, bookended by two twilight hours at dawn and dusk, producing 12 daylight units. They divided the night into 12 hours based on the observation of 36 specific star groups known as decans.
Because the length of daylight changes through summer and winter, ancient hours were seasonal. A daylight hour in midsummer lasted significantly longer than a daylight hour in midwinter. The invention of water clocks and subsequent European mechanical escapements in the 14th century eliminated dynamic hours, establishing equal 60-minute intervals throughout the year.
The 12-hour clock system remained popular across English-speaking countries because 12 is an exceptionally versatile composite number. It divides evenly into halves, thirds, and quarters. Traditional clockmakers found that designing an analog dial with 12 numerals made mechanical gear ratios far simpler and dials much easier to read than a cramped 24-numeral face.
