The next major step was made by the Babylonians, over 2300 years ago. For their astronomical calculations, they used the sexgesimal, also called base-60, system. It is not known, why they chose 60, but it makes divisional operations easy. 60 is divisible by 2, 3, 4, 5, 6, 10, etc and, similar to base-10, can be divided into fractional places, the first place being called a minute, the second a second. The second was originally based on the Earth’s rotation, defined as 1/86400 of the mean solar day. Yet the irregularities detected in the rotation of the earth led to the replacement of the rotation of the earth as its basis. In 1967 the duration of a second was redefined as ‘the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.’