The astronomers of the ancient world noticed that three lengths of time were regular and predictable: the cycle of nighttime and daytime (a day), the cycle of the lunar phases (a month), and the cycle of the amount of daylight per day over a large number of days (a year). These ancients did not realize that a day is the time it takes Earth to rotate once about its axis; that a month is the time it takes the Moon to make one orbit around Earth; and that a year is the time it takes Earth to make one orbit around the Sun. Astronomers eventually figured out these relative motions of Earth, the Moon, and the Sun, and further refined their timekeeping. For example, they realized that the difference between the Moon’s orbital period (27.3 days) and the cycle of the Moon’s phases (29.5 days) was caused by the additional motion of Earth around the Sun.

Eventually, days, months, and years were all subdivided into units based on their utility, and based on long-standing customs and traditions. The differences between the common time units and the astronomical motions that spawned them are made compatible through the use of devices such as leap-years and leap-seconds.