Geocentric Coordinate Time (TCG) is a coordinatetime 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. Coordinate time is the interval of time independent of relativistic time dilation. ... A time scale specifies divisions of time. ... Precession refers to a change in the direction of the axis of a rotating object. ... Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a largely axially symmetric object, such as a gyroscope or a planet. ... Bulk composition of the moons mantle and crust estimated, weight percent Oxygen 42. ... For other uses, please see Satellite (disambiguation) A satellite is an object that orbits another object (known as its primary). ... Earth is the third planet in the Solar system. ... Proper time is time as measured by the clock for an observer who is traveling through spacetime. ... A gravity well is the scientific/science fictional term for the distortion in space-time caused by a massive body such as a planet. ... Gravitational time dilation is a consequence of Albert Einsteins theories of relativity and related theories under which a clock at a different gravitational potential is found to tick at a different rate than ones own clock. ...
TCG was defined in 1991 by the International Astronomical Union, in 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 and other relativistic time scales are defined with fully general relativisticmetrics. 1991 (MCMXCI) was a common year starting on Tuesday of the Gregorian calendar. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. ... It has been suggested that Einsteins theory of gravitation be merged into this article or section. ... Albert Einsteins theory of relativity is a set of two theories in physics: special relativity and general relativity. ... See: International System of Units, colloquially called the Metric System, and also metrication. ...
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. In physics, gravitational potential is the measure of potential energy an object possesses due to its position in a gravitational field. ... For the Cusco album, see 2002 (album). ...
Sidereal time is the measure of the earth's rotation with respect to distant celestial objects.
By convention, the reference points for Greenwich Sidereal Time are the Greenwich Meridian and the vernal equinox (the intersection of the planes of the earth's equator and the earth's orbit, the ecliptic).
It might seem strange that UT1, a solar time, is determined by measuring the earth's rotation with respect to distant celestial objects, and GMST, a sidereal time, is derived from it.
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