International Atomic Time(TAI) | Coordinated Universal Time(UTC) | Universal Time(UT) |
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Historically, time standards were often based on the Earth’s rotational period. From the late 18th to the 19th century it was assumed that the Earth’s daily rotational rate was constant. Astronomical observations of several kinds, including eclipse records, studied in the 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century (decreasing roughly by 0,002s per century).
For this reason, there are several different ways to measure time:
- based on Earth rotation around axis,
- based on atomic clocks and the SI second,
- based on Earth rotation around sun.
Based on Earth rotation around axis
Solar Time is based on the solar day, which is the period between one solar noon (passage of the real Sun across the meridian) and the next. Because the Earth’s orbit around the sun is elliptical, and because of the obliquity of the Earth’s axis relative to the plane of the orbit (the ecliptic), the apparent solar day varies a few dozen seconds above or below the mean value of 24 hours.
Mean Solar Time corrects Solar Time to compensate for the previously mentioned irregularities. At its core lies the assumption of a fictitious sun which moves uniformly across the sky. It gradually slows down to match the Earth’s rotation slowdown.
Greenwich Mean Time (GMT) is the Mean Solar Time at the Royal Observatory in Greenwich, London, with a twelve hour offset so that the astronomical day starts at midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a precise time unless a context is given.
Universal Time (UT) is a modern continuation of Greenwich Mean Time. It is Mean Solar Time at 0° longitude. Some implementations are
- UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources.
- UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in Earth’s rotation.
Based on atomic clocks
International Atomic Time (Temps Atomique International, TAI) is the primary international time standard from which other time standards, including UTC, are calculated. TAI is kept by the International Bureau of Weights and Measures (BIPM), and is based on the combined input of over 400 atomic clocks around the world, each corrected for environmental and relativistic effects. The scale unit of TAI is kept as close as possible to the SI second by using data from those national laboratories which maintain the best primary caesium standards.
Coordinated Universal Time (UTC) is an atomic time scale designed to approximate Universal Time. UTC differs from TAI by an integral number of seconds. The “atomistic” UTC is kept within 0.9 second of “solar” UT1 by the introduction of one-second steps to UTC, the “leap second“. To date these steps have always been positive. It is a compromise between the highly stable atomic time (TAI) and the irregular earth rotation.
Standard time or civil time in a region deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, now usually UTC. The offset (time zone) is chosen such that a new day starts approximately while the sun is crossing the nadir meridian. Alternatively the difference is not really fixed, but it changes twice a year a round amount, usually one hour, see Daylight saving time.
Based on Earth rotation around sun
Ephemeris Time (ET) was from 1952 to 1976 an official time scale standard of the International Astronomical Union; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year.
ET was replaced by other dynamic time standards which have been replaced themselves, namely by:
Terrestial Time (TT) is now defined as a coordinate time scale at Earth’s surface, based on SI seconds. It is a theoretical ideal, and real clocks can only approximate it.
Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of Earth’s mass. TCG is linearly related to TT.
Barycentric Coordinate Time (TCB) is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TT in rate and other mostly periodic terms.