From NY Times (Science Section, 2022/04/25
But time still has its roots and even its nomenclature in astronomical time keeping. Originally, it was based on the path of Earth in its daily spin, day to night and back again. Eventually, ancient Egyptian astronomers who used the duodecimal counting system, based on 12, divided the day and night into 12 hours each, giving us 24 hours in the day.
Those hours varied in length, depending on where Earth was in its orbit around the sun. A little more than 2,000 years ago, Greek astronomers, who needed fixed hours to calculate things like the movements of the moon, developed the revolutionary idea that a single day ought to be divided into 24 hours of the same length.
That same astronomical thinking led them to patch the ancient Babylonian method of counting by 60, the sexagesimal system, onto the hour. Just as they divided the 360 degrees of a circle or the sphere of Earth into 60 parts, or minutes, they then divided each minute into 60 seconds.
The first division of the day’s 24 hours (known in Latin as partes minutae primae) gave them the length of the minute, which was one-1,440th of an average solar day. The second division (partes minutae secundae) provided them the duration — and name — of the second, which was one-86,400th of a day. That definition stood, in effect, until 1967. (There was a brief detour into something called ephemeris time that was so complicated even metrologists didn’t use it.)
But the definition had problems. Earth is gradually slowing in its daily rotation; days are growing slightly longer and so the astronomical second is, too. Those small differences add up. Based on extrapolations from historical eclipses and other observations, Earth as a clock has lost more than three hours over the past 2,000 years.
Therefore, the standard unit of time, based on astronomical reckoning, isn’t constant, a reality that became increasingly intolerable for metrologists during the first decades of the 20th century as they discovered just how irregular Earth’s spin was. Science demands constancy, reliability and replicability. So does time — and by the late 1960s, society was becoming increasingly reliant on the frequencies of radio signals, which demanded extremely precise timings.