Divisions into hours, minutes, and seconds probably began with the Sumerians around 3000 B.C.E., as they divided the day into 12 periods, and the periods into 30 sections. About one thousand years later, the Babylonian civilization, which was then in the same area as the Sumerians, broke the day into 24 hours, with each hour composed of 60 minutes, and each minute having 60 seconds.

It is unknown why the Babylonians chose to divide by 60 (also called a base number). Theories range from connections to the number of days in a year, weights and measurements, and even that the base-60 system was easier to use. Whatever the explanation, their methods proved to be important to us centuries later. We still use 60 as the basis of our timekeeping system (hours, minutes, seconds) and in our definitions of circular measurements (degrees, minutes, seconds). (For more information about the Sumerian counting system, see “History of Mathematics.”)