Second (s)
The SI base unit of time, defined by the radiation frequency of the cesium-133 atom: 9,192,631,770 cycles per second.
The second is the reference unit for time on this site.
About the second
Our word second is a leftover of Latin. Medieval scholars divided a degree, and later the hour, into 60 parts called pars minuta prima, the first small part, and divided those again into pars minuta secunda, the second small part. NIST traces the roots further back: Babylonian and Greek astronomers split the circle into 360 degrees, and around 150 C.E. Ptolemy split each degree into 60 minutes and each minute into 60 seconds. Clocks that could show seconds appeared only in the later 16th century, and Christiaan Huygens's pendulum clock of 1656 was the first able to keep time accurately to the second.
For most of history the second was a fraction of a day, and in 1956 it was redefined as a fraction of the tropical year, a definition the SI took over in 1960. In 1967 the 13th CGPM switched to atoms: 9,192,631,770 periods of the radiation tied to a hyperfine transition of caesium-133. The 2019 SI revision restated this by fixing the caesium frequency at 9,192,631,770 hertz. NIST reports that commercial caesium clocks stay within about a three-millionth of a second per year, and experimental clocks based on other atoms are roughly ten thousand times more precise.
Sources: NIST: the second and the SI redefinition; BIPM: SI Brochure, 9th edition; Wikipedia: Second
Second conversion chart
1 second expressed in every other time unit:
| Unit | 1 second |
|---|---|
| Minutes | 0.01666666667 |
| Hours | 0.0002777777778 |
| Days | 0.00001157407407 |
| Weeks | 0.000001653439153 |
| Months | 3.802648621×10⁻⁷ |
| Years | 3.168873851×10⁻⁸ |
| Milliseconds | 1,000 |
| Microseconds | 1,000,000 |
| Nanoseconds | 1,000,000,000 |
| Decades | 3.168873851×10⁻⁹ |
| Centuries | 3.168873851×10⁻¹⁰ |
| Millennia | 3.168873851×10⁻¹¹ |
| Picoseconds | 1,000,000,000,000 |
| Common Years | 3.170979198×10⁻⁸ |
| Leap Years | 3.162315321×10⁻⁸ |
| Julian Years | 3.168808781×10⁻⁸ |
| Tropical Years | 3.16887654×10⁻⁸ |
| Sidereal Years | 3.168753579×10⁻⁸ |
| Synodic Months | 3.919350773×10⁻⁷ |
| Sidereal Days | 0.00001160576284 |
| Aeons | 3.168873851×10⁻¹⁷ |
| Shakes | 100,000,000 |
| Fortnights | 8.267195767×10⁻⁷ |
| Score | 1.584436925×10⁻⁹ |
| Lustra | 6.337747701×10⁻⁹ |
| Olympiads | 7.922184627×10⁻⁹ |
| Indictions | 2.112582567×10⁻⁹ |
| Jubilees | 6.337747701×10⁻¹⁰ |
| Ghati | 0.0006944444444 |
| Muhurtas | 0.0003472222222 |
| Prahar | 0.00009259259259 |
| Shichen | 0.0001388888889 |
| Moments | 0.01111111111 |
| Ship's Watches | 0.00006944444444 |
| Kin (Maya) | 0.00001157407407 |
| Winals | 5.787037037×10⁻⁷ |
| Tun (Maya) | 3.215020576×10⁻⁸ |
| Katuns | 1.607510288×10⁻⁹ |
| Baktuns | 8.03755144×10⁻¹¹ |
More Metric (SI) units
- millisecond (time)
- microsecond (time)
- nanosecond (time)
- minute (time)
- hour (time)
- day (time)
- meter (length)
- kilometer (length)
- centimeter (length)
- millimeter (length)
- micrometer (length)
- nanometer (length)
- picometer (length)
- decimeter (length)
- hectometer (length)
- decameter (length)
Browse all time units.