Get Ready For the New, Improved Second

They used the same astronomical thinking to add the Babylonian method of counting 60, the sexagesimal systems, 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. This definition remained in force until 1967. (There was a brief detour into something called ephemeris time that was so complicated even metrologists didn’t use it.)

However, the definition was not perfect. Earth’s daily rotation is slowing down; the days are becoming slightly longer, and so is its astronomical second. These small differences add up. Extrapolations from historical eclipses, and other observations show that Earth’s clock has lost more hours than three hours in the past 2,000 year.

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 consistency, reliability, and reproducibility. 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 much more predictable movement atomic particles. Atoms do not wear out or slowdown. Their properties do not change over time. They are the perfect timepieces.

Scientists coaxed cesium 133 atoms to reveal their secrets in the middle 20th century. Cesium, a silvery gold metal, is liquid at room temperature and has slow, heavy electrons. This makes them relatively easy to track.

Scientists exposed cesium atoms to microwave energy in a vacuum. The goal was to determine which wavelength or frequency would cause as many cesium-atoms to emit a photon, or packet of light. The photons were detected by a detector and tallied.