Who decides how long a second is? - John Kitching

As early as 1955, researchers started to develop atomic clocks, which relied on the laws of physics to establish a new foundation for timekeeping.

An atom consists of negatively charged electrons orbiting a positively charged nucleus at a consistent frequency. You can lightly disturb an electron's orientation if you expose an atom to an electromagnetic field such as light or radio waves.

If you briefly tweak an electron at the right frequency, the vibration will resemble a ticking pendulum and can tick for centuries.

Before timekeeping could go fully atomic, countries had to decide which atom would work best. 

At the Thirteenth General Conference of the International Committee for Weights and Measures in 1967, researchers decided on Cesium-133.

The element conformed to the following specifications:

• It had a long-lived and high-frequency electron oscillation for precise, long-term timekeeping.
• It maintained a reliably measurable quantum spin 
• It could easily vaporise.

Determining how many ticks of a caesium atom were in a second, the conference used the most precise astronomical measurement of a second available at the time. 

They started with the number of days in a year and divided it down. Compared to the atom’s ticking rate, one second was defined as exactly 9,192,631,770 ticks of a caesium-133 atom.