Unit of time
A unit of time or midst unit is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern definition, from the National Institute of Standards and Technology is:
"The duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."
Historically units of time were defined by the movements of astronomical objects.
- Sun-based: the year was the time for the earth to revolve around the sun. Year-based units include the olympiad, the lustrum, the indiction, the decade, the century, and the millennium.
- Moon-based: the month was based on the moon's orbital period around the earth.
- Earth-based: the time it took for the earth to rotate on its own axis, as observed on a sundial. Units originally derived from this base include the week at seven days, and the fortnight at 14 days. Subdivisions of the day include the hour, which was further subdivided into minutes and finally seconds. The second became the international standard unit for science.
- Celestial sphere-based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year.
Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.
Historical
The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Buddhist, Hindu, Islamic, Icelandic, Mayan, and French Republican calendars.The modern calendar has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian.
Scientific time units
- The jiffy is the amount of time light takes to travel one fermi in a vacuum.
- Planck time is the time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today.
- The TU is a unit of time defined as 1024 µs for use in engineering.
- The Svedberg is a time unit used for sedimentation rates. It is defined as 10−13 seconds.
- The galactic year, based on the rotation of the galaxy, and usually measured in million years.
- The geological time scale relates stratigraphy to time. The deep time of Earth’s past is divided into units according to events which took place in each period. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. It is not a true mathematical unit, as all ages, epochs, periods, eras or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually.
List
| Unit | Length, Duration and Size | Notes |
| Planck time unit | The amount of time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today. | |
| yoctosecond | ||
| jiffy | The amount of time light takes to travel one fermi in a vacuum. | |
| zeptosecond | Time measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 850 zeptoseconds. | |
| attosecond | ||
| femtosecond | Pulse time on fastest lasers. | |
| Svedberg | Time unit used for sedimentation rates. | |
| picosecond | ||
| nanosecond | Time for molecules to fluoresce. | |
| shake | 10 nanoseconds, also a casual term for a short period of time. | |
| microsecond | Symbol is µs | |
| millisecond | Shortest time unit used on stopwatches. | |
| jiffy | or | Used to measure the time between alternating power cycles. Also a casual term for a short period of time. |
| second | SI Base unit. | |
| decasecond | ||
| minute | ||
| moment | Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season. | |
| hectosecond | 1 minute and 40 seconds | |
| ke | Usually calculated as 15 minutes, similar to "quarter" as in "a quarter past six". | |
| kilosecond | 16 minutes and 40 seconds | |
| hour | ||
| day | Longest unit used on stopwatches and countdowns. | |
| week | Also called "sennight". | |
| megasecond | 277.777778333333 hours or about 1 week and 4.6 days. | |
| fortnight | 14 days | |
| lunar month | Various definitions of lunar month exist. | |
| month | Occasionally calculated as 30 days. | |
| quarter and season | ||
| semester | A division of the academic year. Literally "six months", also used in this sense. | |
| year | ||
| common year | 52 weeks and 1 day. | |
| tropical year | Average. | |
| Gregorian year | Average. | |
| sidereal year | - | |
| leap year | and | |
| - | ||
| triennium | - | |
| quadrennium | - | |
| olympiad | - | |
| lustrum | - | |
| decade | - | |
| indiction | - | |
| gigasecond | 16,666,666.6667 minutes or About 31.7 years. | |
| jubilee | - | |
| century | - | |
| millennium | Also called "kiloannum". | |
| terasecond | about 31,700 years. | |
| Also called "Megayear." About 1,000 millennia, or 1 million years. | ||
| petasecond | About 31,700,000 years or 380,399,583.12 months | |
| galactic year | The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy one time. | |
| cosmological decade | varies | 10 times the length of the previous cosmological decade, with CÐ 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition. |
| aeon | Also spelled "eon". Also refers to an indefinite period of time. | |
| exasecond | About 31,700,000,000 years or 380,399,583,123.74 months | |
| zettasecond | About 31.7 trillion years or 3,803,995,983,123,744.56 months | |
| yottasecond | About 31.7 quadrillion years or 380,399,583,123,744,510 months |