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 a circle or the sphere of Earth into 60 parts, and then 60 again — making 360 degrees — so they divided the hour.
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.
Metrologists turned to the far more predictable movement of atomic particles. Atoms never wear out or slow down. Their properties do not change over time. They are the perfect timepieces.
By the middle of the 20th century, scientists had coaxed atoms of cesium 133 into divulging their secret inner ticks. Cesium, a silvery-gold metal that is liquid at about room temperature, has heavy, slow atoms, which means they are relatively easy to track.
Scientists put cesium atoms in a vacuum and exposed them to the energy of microwaves, in the nonvisible range of the electromagnetic field. The task was to figure out which wavelength, or frequency, would excite as many cesium atoms as possible into emitting a packet of light, or photon. The photons were picked up by a detector and counted.