Why are there 24 hours in a day?

Ancient Egyptians used sundials to split daylight into 10 hours, plus an hour each for dawn and dusk, making 12. At night they tracked 12 star groups, called decans, rising in turn, giving another 12 hours. Early hours varied in length with the seasons, since summer days were longer than winter days. Greek astronomers such as Hipparchus later proposed dividing the full day into 24 equal hours, and the 60-minute hour came from the Babylonian base-60 number system. Equal hours became the everyday standard only after mechanical clocks spread across Europe in the 14th century.