An Etymological Dictionary of Astronomy and Astrophysics
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A → coordinate time having its spatial origin at the solar system barycenter. It is intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the solar system. → Barycentric Dynamical Time (IDB).

→ barycenter; → coordinate; → time.

In relativity, the proper time in the specified reference frame. Because of time dilation, this may differ from the time experienced by any participant in the events being considered. It is the time basis (or coordinate) to be used in the theory of motions referred to this system.

→ coordinate; → time.

The proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth, i.e. a clock that performs exactly the same movements as the Earth but is outside the Earth's gravity well. TCG was defined in 1991 by the International Astronomical Union as one of the replacements for Barycentric Dynamical Time (TDB).

→ geocentric; → coordinate; → time.