The length of those hours varies depending on where the Earth is in its orbit around the Sun. A little over 2,000 years ago, Greek astronomers, who needed fixed times to calculate phenomena such as the motion of the moon, developed the revolutionary idea that a day should be divided into 24 hours of the same length.
The same astrological thinking led them to associate time with the ancient Babylonian method of counting up to 60, the sexagesimal system. Just like they divide 360 degrees of a circle or globe of the Earth into 60 parts or minutes, and then divide each minute into 60 seconds.
The first division of the 24 hours of a day (known in Latin as “parts minutae primae”) gave the length of a minute, which was 1/1440 of the mean solar day. The second division (“partes minutae secundae”) gave them the duration – and name – of the second division, which was 1/86.400 of the day. This definition held, in fact, until 1967 (there was a slight drift into something called ephemeris time that was too complex for metrologists to use).
However, the definition has problems. The Earth’s daily rotation is slowly slowing down; The days became a little longer, and as a result, the astronomical second also became. These small differences add up. Based on extrapolation from historical eclipses and other observations, Earth has lost more than three hours over the past 2,000 years.
Therefore, the standard unit of time, based on astronomical calculations, is not a constant, a fact that became increasingly improbable for metrologists during the early decades of the twentieth century, when they discovered how irregular the Earth’s rotation was. Science requires consistency, reliability, and reproducibility. The same over time … By the late 1960s, society had become increasingly dependent on the frequencies of radio signals, which required very precise timing.
Metrologists have turned to the more predictable motion of atomic particles. Atoms never run out or slow down. Its properties do not change over time. They are the perfect watches.
By the middle of the twentieth century, scientists were able to convince atoms of cesium-133 to reveal the secret of their inner resonance. Cesium, a gold-silver metal that is more or less liquid at room temperature, contains heavy, sluggish atoms, which means it is relatively easy to observe.