A unit of time 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 (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency,, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be when expressed in the unit Hz, which is equal to s−1."[1]
Historically, many units of time were defined by the movements of astronomical objects.
These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined for scientific purposes as multiples of seconds.
Units of time based on orders of magnitude of the second include the nanosecond and the millisecond.
See main article: History of calendars. 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 calendar.
Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres .
Note: The parsec is not a unit of time, but a unit of length of about 30.9 trillion kilometres, despite movie references otherwise.
| The amount of time light takes to travel one Planck length. | ||
| One nonillionth of a second. | ||
| One octillionth of a second. | ||
| One septillionth of a second. | ||
| jiffy (physics) | The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. | |
| One sextillionth of a second. Time measurement scale of the NIST and JILA strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds.[3] | ||
| One quintillionth of a second. | ||
| One quadrillionth of a second. Pulse time on fastest lasers. | ||
| Time unit used for sedimentation rates (usually of proteins). | ||
| One trillionth of a second. | ||
| One billionth of a second. Time for molecules to fluoresce. | ||
| 10 nanoseconds, also a casual term for a short period of time. | ||
| One millionth of a second. Symbol is μs | ||
| One thousandth of a second. Shortest time unit used on stopwatches. | ||
| jiffy (electronics) | ~ | Used to measure the time between alternating power cycles. Also a casual term for a short period of time. |
| One hundredth of a second. | ||
| One tenth of a second. | ||
| 1 s | SI base unit for time. | |
| Ten seconds (one sixth of a minute) | ||
| 1.44 minutes, or 86.4 seconds. Also marketed as a ".beat" by the Swatch corporation. | ||
| (on average) | Medieval unit of time used by astronomers to compute astronomical movements, length varies with the season.[4] Also colloquially refers to a brief period of time. | |
| centiday | (1 % of a day) | 14.4 minutes, or 864 seconds. One-hundredth of a day is 1 cd (centiday), also called "kè" in tradidional Chinese timekeeping. The unit was also proposed by Lagrange and endorsed by Rey-Pailhade[5] in the 19th century, named "centijours" (from French centi- 'hundred' and jour 'day'). |
| About 17 minutes. | ||
| deciday | (10 % of a day) | 2.4 hours, or 144 minutes. One-tenth of a day is 1 dd (deciday), also called "gēng" in tradidional Chinese timekeeping. |
| Longest unit used on stopwatches and countdowns. The SI day is exactly 86 400 seconds. | ||
| Historically sometimes also called "sennight". | ||
| decaday | 10 days. A period of time analogous to the concept of "week", used by different societies around the world: the ancient Egyptian calendar, the ancient Chinese calendar, and also the French Republican calendar (in which it was called a décade). | |
| About 11.6 days. | ||
| 14 days | ||
| Various definitions of lunar month exist; sometimes also called a "lunation". | ||
| Occasionally calculated as 30 days. | ||
| quarantine | 40 d (approximately 5.71 weeks) | To retain in obligatory isolation or separation, as a sanitary measure to prevent the spread of contagious disease. Historically it meant to be isolated for 40 days. From Middle English quarentine, from Italian quarantina ("forty days"), the period Venetians customarily kept ships from plague-ridden countries waiting off port, from quaranta ("forty"), from Latin quadrāgintā. |
| hectoday | 100 days, roughly equivalent to 1/4 of a year (91.25 days). In Chinese tradition "bǎi rì" (百日) is the hundredth day after one's birth, also called Baby's 100 Days Celebration. | |
| A division of the academic year.[6] Literally "six months", also used in this sense. | ||
| 52 weeks and 1 day. | ||
| [7] | Average. | |
| Average. | ||
| and | ||
| A quadrennium (plural: quadrennia or quadrenniums) is also a period of four years, most commonly used in reference to the four-year period between each Olympic Games.[8] It is also used in reference to the four-year interval between leap years, for example when wishing friends and family a "happy quadrennium" on February 29. | ||
| In early Roman times, the interval between censuses. | ||
| Interval for taxation assessments (Roman Empire). | ||
| About 31.7 years. | ||
| Also called "kiloannum". | ||
| 2 148 and 2/3 of a year | A superstitious unit of time used in astrology, each of them representing a star sign. | |
| About 31,709 years. | ||
| Also called "megayear". 1000 millennia (plural of millennium), or 1 million years (in geology, abbreviated as Ma). | ||
| About years. | ||
| The amount of time it takes the Solar System to orbit the center of the Milky Way Galaxy (approx years). | ||
| logarithmic (varies) | 10 times the length of the previous cosmological decade, with CD 1 beginning either 10 seconds or 10 years after the Big Bang, depending on the definition. | |
| Also refers to an indefinite period of time, otherwise is years. | ||
| Used in Hindu mythology. About years. | ||
| About years. Approximately 2.3 times the current age of the universe. | ||
| About years. | ||
| About years. | ||
| About years. | ||
| About years. | ||
All of the formal units of time are scaled multiples of each other. The most common units are the second, defined in terms of an atomic process; the day, an integral multiple of seconds; and the year, usually 365 days. The other units used are multiples or divisions of these 3.