The same astrological thinking led them to correct the ancient Babylonian method of counting by 60, the sexagesimal system, per hour. Just as they divided 360 degrees of a circle or globe into 60 parts or minutes, they then divided each minute into 60 seconds.
The first section of the 24 hour day (known in Latin as partes minutae primae) gave them the length of a minute, which was one to 1440 days of an average solar day. The second division (partes minutae secundae) gave the duration – and name – of the second division, which was one to 86,400 parts of a day. This definition remained in effect until 1967. (There was a short inflection of something called the ephemeris time that was too complex for metrologists to use.)
But getting to know it has problems. The Earth is gradually slowing down in its daily rotation; The days get a little longer so the sidereal second is too. These small differences add up. Based on extrapolation from historical eclipses and other observations, Earth as an hour has lost more than three hours over the past 2,000 years.
Therefore, the standard unit of time, based on astronomical calculation, is not a constant, a fact that became increasingly improbable for metrologists during the early decades of the 20th century when they discovered how irregular the Earth’s rotation was. Science requires consistency, reliability, and repeatability. So did time – by the late 1960s, society had become increasingly dependent on radio signal frequencies, which required very precise timings.
Metrologists have turned to the more predictable motion of atomic particles. Atoms never wear out or slow down. Their properties do not change over time. It’s the perfect hours.
By the middle of the 20th century, scientists had convinced cesium-133 atoms to detect the secretive inner tick. Cesium, a silvery-golden metal that turns liquid at about room temperature, contains heavy, sluggish atoms, which means it’s relatively easy to keep track of.
Scientists put cesium atoms in a vacuum and exposed them to microwave energy, in the invisible range of the electromagnetic field. The task was to figure out the wavelength, or frequency, that would excite as many cesium atoms as possible to emit a beam of light, or photon. Photons were captured by a detector and counted.
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