dynamical time

dynamical time

The family of timescales that was introduced in 1984 to replace ephemeris time, ET, as the independent variable of dynamical theories and ephemerides. Like ET these timescales are independent of the Earth's rotation.

Terrestrial dynamical time (TDT) is used as the timescale of ephemerides for observations from the Earth's surface. It is used for apparent geocentric ephemerides. For practical purposes

TDT = TAI + 32.184 seconds

where TAI is the International Atomic Time. This takes advantage of the direct (broadcast) availability of coordinated universal time, UTC, which is an integral number of seconds offset from TAI. Continuity with pre-1984 practices has been achieved by setting the difference between TDT and TAI to the 1984 estimate of the difference between ET and TAI. The increment to be applied to universal time, UT1, to give TDT is called delta TT ). The unit of TDT is the day, 86 400 seconds, 24 hours, at mean sea level.

Barycentric dynamical time (TDB) is used for ephemerides and equations of motion that are referred to the barycenter of the Solar System. TDB differs from TDT only by periodic variations.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
Davidovic, "Dynamical time versus system time in quantum mechanics," Chinese Physics B, vol.
For the difference between uniform Dynamical Time (now called Terrestrial Time, TT) and Universal Time, the value [DELTA]T = +67 seconds has been used.
Focussing initially on how the conflict between the directional chronological time of history and the reversible dynamical time of classical physics has been resolved by the invalidation of the dynamical sense of time by non-linear dynamics, Velasco then shows how non-linear dynamics has been complemented by symbol mediated language.
It is the difference, at a given instant, between Universal Time and the uniform time scale known as Dynamical Time. While the latter is defined by atomic clocks, Universal Time is based on the rotation of the Earth around its axis.
Dynamical time is the time embraced by most of the laws of physics.
(3) The indifferent time interval or dynamical time of classical mechanics is responsible for the philosophical position called Laplacian determinism--that the distinction between past, present and future is illusory because both the past and the future are already implicit in the present.
(22) Accordingly, it instructs human affairs with dynamical time. That is to say, only with present conditions.
How inimical to complexity the government of human affairs by present conditions alone or what is effectively dynamical time secured through temporal myopia is may be appreciated as follows: Temporal myopia militates against sustainable logging because agriculture yields quicker, more reliable, less risky returns to investment.
To put it another way, the Bayesian statistical objection is only valid if we operate in an atemporal or dynamical time rather than in temporal or chronological time.
In 1984 ET was replaced by Terrestrial Dynamical Time (TDT); the name was shortened in 1991 to Terrestrial Time (TT).
Also created in 1984 was Barycentric Dynamical Time (TDB), which is referred to the solar system's center of mass; it accounts for tiny effects on the flow of time in Einstein's theory of general relativity.
To mention only a few: relativistic effects, Barycentric Dynamical Time, measurement of Atomic Time, clock synchronization by very-long-baseline interferometry, planetary terms in nutation (used only for arc-millisecond accuracy!), the FK5 system, laser ranging, and rotation parameters for satellites of the planets.

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