Why do we have 24 hours in a day and 60 minutes in an hour?

The two numbers have different origins. The split of the day into 24 hours goes back to the ancient Egyptians, who divided daylight and night into 12 parts each. The division of hours and minutes into 60 comes from the base-60 counting system of the Sumerians and Babylonians, later applied to astronomy by the Greeks. Sixty has survived mainly because it divides evenly in so many ways. [1] [2]

The Royal Museums Greenwich says the Egyptians were the first to use 24 hours, with 12 hours from sunrise to sunset and 12 from sunset to sunrise. NIST adds that these hours were not equal: in summer, daytime hours were longer than nighttime hours. Because the length of an hour changed with the season, the system only produced equal hours near the equinoxes. [1] [2] [3]

Greek astronomers later proposed 24 equal hours. Science ABC credits Hipparchus with the idea of equinoctial hours. Even so, ordinary people kept using hours of uneven length until the late medieval period in Europe, when mechanical clocks, developed by monks to fix prayer and work periods according to NIST, made fixed hours standard. Science ABC dates the shift to the 14th century. [2] [3]

The Guardian's Notes and Queries credits the Babylonians, who used a sexagesimal system for mathematics and astronomy and inherited it from the Sumerians, in use perhaps as early as 3500 BC. Science ABC gives around 3000 BCE for the Sumerian origin. NIST notes that the Sumerians divided a circle into 360 degrees, and that Babylonian astronomers and then the Greeks applied this to the sky. Around 150 CE, Ptolemy's Almagest split each degree into 60 minutes and each minute into 60 seconds. These units later entered the mechanical clocks of medieval Europe. [4] [2] [3]

The sources agree that 60 is practical but do not fully agree on why it was chosen. The most common explanation is mathematical convenience: 60 divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30, so thirds and quarters of an hour are whole minutes. The Guardian's answer makes a similar point, noting that 60 has 12 divisors and combines the advantages of 10 and 12. Royal Museums Greenwich says only that 60 may have been chosen for this reason, and Science ABC says the precise reason is unknown. [1] [4] [3]

A second, more popular explanation is finger counting. One web article claims that counting the finger joints of one hand with the thumb reaches 12, giving a basis for base 12 and base 60. The search results cite no historical evidence for this, so it should be treated as a suggestion rather than an established fact. [3]

During the French Revolution, reformers tried to base timekeeping on ten, with 10 hours in a day and 100 minutes in an hour. It did not catch on. Today the second is defined atomically, as 9,192,631,770 oscillations of radiation from cesium atoms, but it still sits inside the old Babylonian divisions. [5] [2]