Fact
A second is defined by a caesium atom, not the Earth
Exactly 9,192,631,770 oscillations. The planet turned out to be an unreliable clock.
Earth’s rotation is irregular and slowing, so the second was redefined in 1967 in terms of a specific transition in caesium-133. Because the two no longer match, leap seconds are occasionally inserted to keep clocks aligned with the planet – a practice due to be discontinued by 2035.