second
Timeexact by definition
The second is the SI base unit of time. Since 1967 it has been defined as the duration of exactly 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom, at rest at 0 K. That definition is exact by construction and underpins the definitions of the metre, the candela and the ampere.
| 1 s | 1 s |
The second began as 1/86,400 of a mean solar day, dividing the day by 24, then twice by 60. From 1960 to 1967 it was defined instead as a fraction of the tropical year 1900, the so-called ephemeris second, before the caesium definition removed any dependence on the Earth's motion.
About the second
The second is the most accurately realised quantity in science, and everything else leans on it. Caesium fountain clocks keep it to a few parts in 10¹⁶, which is a drift of less than a second over the age of the universe, and optical clocks in development are better still. Because the metre is defined by fixing the speed of light, and the ampere and candela are defined using the second as well, an improvement in timekeeping quietly improves most of the SI at once. GPS is the everyday consequence: a receiver computes position from signal travel times, and an error of one nanosecond in a satellite clock becomes an error of about 30 cm on the ground.
Its subdivision is a Babylonian inheritance that no reform has dislodged. The day was split into 24 hours, the hour into 60 first small parts (minutes), and the minute into 60 second small parts, which is literally where "second" comes from. Revolutionary France tried decimal time, with 10 hours of 100 minutes of 100 seconds, and it lasted barely two years. The 1967 caesium definition finally cut the second loose from astronomy altogether, which matters because the Earth is not a good clock: its rotation is slowing irregularly, so a mean solar day is no longer exactly 86,400 SI seconds, and leap seconds exist to paper over the difference.