Short answers, pulled from the story.
The second was redefined in 1967 when the world stopped measuring time by the movement of the sun and started measuring it by the vibration of a single atom. This decision replaced the Earth's rotation with the precise frequency of caesium-133 atoms to ensure reliability for mid-20th century technology.
One second is defined as exactly 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom. This number was chosen to ensure continuity with the previous definition based on the ephemeris year.
Christiaan Huygens invented the pendulum clock in 1656, creating a swing that took exactly one second. This invention transformed the second from a theoretical fraction of a day into a tangible unit that could be counted by machines.
The leap second was introduced to keep civil time synchronized with the day and night cycle as atomic time began to drift away from the Earth's rotation. This irregular adjustment is inserted at the end of June or December to ensure the difference between atomic time and universal time remains less than a second.
Draft resolutions from the 27th General Conference on Weights and Measures in November 2022 proposed a new definition based on optical transitions. This future redefinition aims to use optical lattice clocks that gain or lose less than one second in 15 billion years.