The evolution of time measurement, Part 4: the atomic second [Recalibration]

Before the invention of atomic clocks, the second was defined by dividing the period of an astronomical event into a shorter time interval. For example, the second was once defined by dividing the average period of one revolution of the Earth on its axis. The mean solar second was equal to 1/86,400 of the mean solar day. To create a more stable unit of time interval, the second was redefined in 1956 as 1/31,556,925.9747 of the tropical year 1900. The ephemeris second was indeed more stable than the mean solar second but was nearly impossible to use as a time reference and of little use to metrologists or engineers. In retrospect, it seems almost ridiculous that another astronomical definition of the second was accepted during a period when atomic clocks were already being built [1], [2]. A clean transition from the mean solar second to the atomic second would have made more sense. Doomed from the start, the ephemeris second would be easy to forget about except for one thing - it became the comparison reference for the atomic second.