Second
SI base unit · quantity: time
01 · Definition
For thousands of years the second was counted out by the rotation of the Earth. In 1967 it was untied from the planet and tied to the caesium atom — the Earth had turned out to be an insufficiently precise clock.
A second is 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. The number looks arbitrary, but it was chosen so that the new second matched the old, astronomical one to the precision of the day. The Earth has slowed since: a day is a couple of milliseconds longer than 86 400 seconds, and the difference is absorbed by inserting a leap second.
Below the second run the SI prefixes: ms, µs, ns, ps. Above it — the Babylonian inheritance: minute, hour, day. Kiloseconds never occur in real life, though formally they are legal.
02 · Conversion
Enter an interval — the passport will convert it
Time is the only quantity where the SI coexists with the sexagesimal system: minutes and hours are not part of the SI, yet they are accepted for use on a par with the second. The year in the table is the Julian one, 365.25 days: it is the year behind the light year.
The minute, the hour and the day are exact multiples of the second by definition: 60, 3600, 86 400. A calendar day, however, may contain 86 401 seconds: a leap second is inserted whenever the divergence from the Earth's rotation reaches 0.9 s.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| ns | nanosecond | 9·10¹⁰ | CPU cycles, network latency, GPS |
| µs | microsecond | 9·10⁷ | electronics measurements, response times |
| ms | millisecond | 90 000 | human reaction, video frames, ping |
| s | second | 90 | the base notation of time in the SI |
| min | minute | 1.5 | outside the SI, accepted: timetables, sport |
| h | hour | 0.025 | outside the SI, accepted: work, tariffs |
| d | day | 0.001042 | outside the SI, accepted: calendars, deadlines |
| wk | week | 1.488·10⁻⁴ | daily life and planning, outside the SI |
| a | year (Julian) | 2.852·10⁻⁶ | 365.25 days — the year of the light year |
1 min = 60 s · 1 h = 3600 s · 1 d = 86 400 s — exact multiples by definition
03 · Orders of magnitude
from the yoctosecond to the age of the UniverseSecond: a heartbeat
04 · Measuring instruments
What counts out the seconds
The primary standard: a cloud of cooled atoms is tossed upwards and probed twice with microwaves. Instability 10⁻¹⁶.
A strontium or ytterbium ion in a laser lattice. Accuracy 10⁻¹⁸ — the candidates for a new definition of the second.
A quartz tuning fork at 2¹⁵ hertz: halving fifteen times yields exactly one second. Watches, phones, meters.
Every satellite carries atomic clocks; the receiver takes the time from the sky for free. Nanoseconds — all of navigation rests on them.
05 · Writing rules
The symbol is lowercase: s, not "sec" and not "sec."
The second is not named after a person, so the symbol is lowercase and takes no full stop. "Sec" and "msec" belong to the typewriter era; in the SI they are s and ms. Minutes and hours are written min and h, and the time of day with a colon: 14:05, not 14 h 05 min.
06 · Neighbouring units
The second sits in the denominator of half the SI: the hertz is an inverse second, the becquerel is one decay per second, the metre per second is speed. Knock the second off, and all three drift.
1 / s
SI base
metre per second
The metre, too, grew out of the second: since 1983 it has been defined via the speed of light and the second — meaning the accuracy of length can never exceed the accuracy of time.
07 · Historical section
What the second was before the atom
A fraction of the mean solar day. The Earth slows unevenly, so this second slowly drifted.
A fraction of the tropical year 1900.0: time was tied to the Earth's orbit around the Sun rather than to the day.
A fraction of the sidereal day — the Earth's rotation relative to the stars. Shorter than the ordinary one by 0.27 %.
The sexagesimal series did not end at the second: after the "second small part" came the third, 1/60 of a second.
The second is the most precise unit in the SI, and the others depend on it
The kilogram is realised to 10⁻⁸, the metre to 10⁻¹¹, and the second to 10⁻¹⁶ in a caesium fountain and to 10⁻¹⁸ in optical clocks. That is why the definitions of the metre, the candela, the ampere and the volt are built on the second: it sets the precision ceiling for the whole system. Such clocks would err by less than a second over the lifetime of the Universe — which is exactly why another redefinition, no longer on caesium, is under discussion.
Catalogue · International System of Units
A passport for every quantity
Seven base units, twenty-two derived ones with names of their own, and an archive of systems withdrawn from use. Each is a separate sheet: definition, dimension, conversion, instruments, writing rules. In 33 languages.
Base units
open the passport of the secondDerived units with names of their own
in terracotta — those whose definition contains the secondHistorical systems
archive of notationsPassport language
33 languages. The symbol s and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.