Short answers, pulled from the story.
The International System of Units adopted the caesium-133 atomic frequency standard in 1967. This definition fixed the value at exactly 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels.
Christiaan Huygens invented the first pendulum clock in 1656 with a swing lasting one second. The device featured a pendulum length just under one meter to achieve this timing and an escapement mechanism that allowed the clock to tick every single second accurately.
Strontium clocks achieved accuracy records by gaining or losing less than one second in 15 billion years. This duration exceeds the estimated age of the universe itself.
Metrologists recognized Earth's orbit around the Sun as more stable than its rotation. The IAU chose the tropical year over the sidereal year in 1955 for this purpose.
A stone falls about 4.9 meters from rest in exactly one second according to standard gravity. Light takes 1.3 seconds to travel from the Moon surface to Earth covering 384,400 kilometers.