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TT-UT1 2000+ ΔT vs. time from 1657 to 2022 [1] [2] In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time (86,400 seconds).
Atomic clocks are installed at sites of time signal radio transmitters. [113] They are used at some long-wave and medium-wave broadcasting stations to deliver a very precise carrier frequency. [114] Atomic clocks are used in many scientific disciplines, such as for long-baseline interferometry in radio astronomy. [115]
International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00. At that time, ΔT was already about 32.18 seconds. The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows: 1977 January 1.000 3725 TT = 1977 January 1.000 0000 TAI, i.e. TT − TAI = 32.184 seconds
The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock, [3] but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it. TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC).
International Atomic Time (abbreviated TAI, from its French name temps atomique international [1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. [2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. [3]
In the International System of Units (SI), the unit of time is the second (symbol: s). It has been defined since 1967 as "the duration of 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", and is an SI base unit. [12]
Screenshot of the UTC clock from time.gov during the leap second on 31 December 2016.. A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (), which varies due to irregularities and long-term ...
TDT is a uniform atomic time scale, whose unit is the SI second. TDT is tied in its rate to the SI second, as is International Atomic Time (TAI), but because TAI was somewhat arbitrarily defined at its inception in 1958 to be initially equal to a refined version of UT, TDT was offset from TAI, by a constant 32.184 seconds.