| Seconds (s) | Centiseconds (cs) |
|---|---|
| 1 Second | 100 cs |
| 2 Seconds | 200 cs |
| 3 Seconds | 300 cs |
| 4 Seconds | 400 cs |
| 5 Seconds | 500 cs |
| 10 Seconds | 1000 cs |
| 20 Seconds | 2000 cs |
| 25 Seconds | 2500 cs |
| 50 Seconds | 5000 cs |
| 100 Seconds | 10000 cs |
| Reference | Seconds (s) | Centiseconds (cs) |
|---|---|---|
| A blink of an eye | 0.1 s | 10 cs |
| One heartbeat at rest | 0.83 s | 83 cs |
| Half of a football match | 2700 s | 270000 cs |
| A full day | 86400 s | 8640000 cs |
The second is the base unit of time in the International System of Units. Its symbol is s. It is defined by fixing the frequency of the caesium-133 hyperfine transition at exactly 9,192,631,770 cycles per second, so a second is the duration of that many oscillations.
The name records a division that no longer exists in practice. Medieval astronomers divided the hour into sixty minutae primae, first small parts, and each of those into sixty minutae secundae, second small parts. English kept the first as minute and the second as second, leaving the ordinal in the name of a unit almost nobody thinks of as ordinal.
For most of history the second was defined from the rotation of the Earth, as one 86,400th of a mean solar day. That definition failed once clocks became better than the planet: the Earth's rotation is irregular, slowing over centuries from tidal friction and wobbling on shorter timescales. The atomic definition adopted in 1967 severed the link, which is why leap seconds are occasionally inserted to keep civil time aligned with the sun.
Every other time unit is now built on it, and the second underpins most of the rest of the SI as well. The metre is defined through the speed of light in metres per second, the kilogram through the Planck constant which carries a second in its units, and the ampere through the elementary charge per second. Caesium fountain clocks realise the unit to a few parts in ten to the sixteenth, and optical clocks now do better still.
Because atomic clocks keep better time than the planet does, the two have to be reconciled. The Earth's rotation is slowing irregularly, so a day is now a millisecond or two longer than 86,400 atomic seconds, and the difference accumulates. Since 1972 the answer has been the leap second: an extra second inserted at the end of a June or December, announced months in advance, which has happened twenty-seven times. Computer systems handle the repeated timestamp badly enough that outages have been traced to it, and in 2022 the world's metrology bodies voted to abandon the practice by 2035, letting civil time drift from solar time instead.
One second equals 1000 milliseconds, one sixtieth of a minute, or about 0.000277778 hours.
The centisecond is a unit of time equal to one hundredth of a second. Its symbol is cs. The name is almost never spoken, but the quantity it describes is one of the most familiar in the world, because it is the resolution at which competitive sport reports its results.
Swimming, athletics, cycling, rowing and speed skating all publish times to the hundredth of a second. The choice is not arbitrary. Electronic timing can resolve far finer intervals, but the physical uncertainties of the events themselves make finer figures meaningless. In swimming, the tolerance permitted on the length of a pool lane is such that two lanes may differ by enough to matter at the thousandth of a second, which is why the sport declines to separate swimmers below the hundredth and awards shared medals instead.
Athletics reaches the same conclusion by a different route. Sprint times are recorded to the thousandth internally but published to the hundredth, rounded upward so that no athlete is credited with a time faster than they ran. The rule means a hand-timed result and an electronically timed one are not comparable, and world records require full automatic timing precisely because the difference between the two is several centiseconds.
Outside sport the unit appears mostly in disguise. Video at fifty frames a second advances one frame every two centiseconds, and the perceptual threshold at which a delay begins to feel like lag in an interactive system sits around ten centiseconds. Industrial control loops and programmable logic controllers often run on cycle times in this range, fast enough to keep up with machinery and slow enough to be computationally cheap.
The name itself is unusual because the SI prefix centi is rarely applied to the second. Milli and micro are far more common, and where a hundredth of a second is needed people usually say exactly that rather than reaching for a prefixed name.
Photography meets the quantity in shutter speeds around a hundredth, which is roughly the slowest a steady hand can hold a standard lens without visible blur.
One centisecond equals 0.01 seconds, 10 milliseconds, or 10,000 microseconds.