# Geocentric Coordinate Time

﻿
Geocentric Coordinate Time

Geocentric Coordinate Time (TCG) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth: that is, a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Earth.

TCG was defined in 1991 by the International Astronomical Union, in [http://www.iers.org/MainDisp.csl?pid=98-133 Recommendation III of the XXIst General Assembly] . It was intended as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). Unlike former astronomical time scales, TCG is defined in the context of the general theory of relativity. The relationships between TCG and other relativistic time scales are defined with fully general relativistic metrics.

Because the reference frame for TCG is not rotating with the surface of the Earth and not in the gravitational potential of the Earth, TCG ticks faster than clocks on the surface of the Earth by about 7.0 × 10−10 (about 22 milliseconds per year). Consequently, the values of physical constants to be used with calculations using TCG differ from the traditional values of physical constants. (The traditional values were in a sense wrong, incorporating corrections for the difference in time scales.) Adapting the large body of existing software to change from TDB to TCG is a formidable task, and as of 2002 many calculations continue to use TDB in some form.

Time coordinates on the TCG scale are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used. For continuity with its predecessor Ephemeris Time, TCG was set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TCG instant 1977-01-01T00:00:32.184 exactly corresponds to TAI instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation.

TCG is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation. For practical purposes, TCG must be realised by actual clocks in the Earth system. Because of the linear relationship between Terrestrial Time (TT) and TCG, the same clocks that realise TT also serve for TCG. See the article on TT for details of the relationship and how TT is realised.

Barycentric Coordinate Time (TCB) is the equivalent of TCG for calculations relating to the solar system beyond Earth orbit. TCG is defined by a different reference frame from TCB, such that they are not linearly related. Over the long term, TCG ticks more slowly than TCB by about 1.6 × 10−8. In addition there are periodic variations, as Earth moves within the Solar system. When the Earth is at perihelion in January, TCG ticks even more slowly than it does on average, due to gravitational time dilation from being deeper in the Sun's gravity well and also velocity time dilation from moving faster relative to the Sun. At aphelion in July the opposite holds, with TCG ticking faster than in does on average.

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Geocentric Coordinate Time — Die Geocentric Coordinate Time (kurz TCG von frz. temps coordonné géocentrique) wurde 1991 von der Internationalen Astronomischen Union definiert als die Zeit, die eine Uhr messen würde, die sich mit der Erde mitbewegt, dabei aber weit genug von… …   Deutsch Wikipedia

• Coordinate time — In the theory of relativity, it is convenient to express results in terms of a spacetime coordinate system relative to an implied observer. In many (but not all) coordinate systems, an event is specified by one time coordinate and three spatial… …   Wikipedia

• Time from NPL — Map showing the location of the Anthorn VLF transmitter within Cumbria …   Wikipedia

• Time standard — A time standard is any officially recognized specification for measuring time: either the rate at which time passes; or points in time; or both. For example, the standard for civil time specifies both time intervals and time of day. A time scale… …   Wikipedia

• Time dilation — This article is about a concept in physics. For the concept in sociology, see time displacement. In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving… …   Wikipedia

• Time — This article is about the measurement. For the magazine, see Time (magazine). For other uses, see Time (disambiguation). The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as… …   Wikipedia

• Time zone — Timezone and TimeZone redirect here. For other uses, see Time zone (disambiguation). Local time redirects here. For the mathematical concept, see Local time (mathematics). This article is about time zones in general. For a list of time zones by… …   Wikipedia

• Time signal — These automatic signal clocks were synchronized by telegraphy in 1905 before the widespread use of radio A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Contents 1 Audible… …   Wikipedia

• Geocentric orbit — Earth orbit redirects here. For the motion of the Earth around the Sun, see Earth s orbit. Earth orbiter redirects here. For the shuttle simulator, see Earth Orbiter 1. The following words may have more than one definition or other non Earth… …   Wikipedia

• dynamical time —       specialized timescale used to describe the motion of objects in space.       As a practical matter, time can be defined as that coordinate which can most simply be related to the evolution of closed systems. Proper time is the time measured …   Universalium