Calendars and time
Geocentric Coordinate Time
Direct answer
Geocentric Coordinate Time: Geocentric Coordinate Time, or TCG, is the relativistic coordinate time of the geocentric celestial reference system.
TCG is coordinate time for a reference frame centered on Earth.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Barycentric Coordinate Time, GPS Time, Calendar, Julian Date
TCG is coordinate time for a reference frame centered on Earth.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Geocentric Coordinate Time
The IAU introduced TCG with TCB to make astronomical time scales consistent with general relativity.
Why Geocentric Coordinate Time matters
Geocentric Coordinate Time supplies the time coordinate for relativistic models near Earth while remaining distinct from time realized on the rotating geoid.
Geocentric Coordinate Time in practice: an example
TCG and TT share an epoch relationship but advance at slightly different rates by definition.
Where readings of Geocentric Coordinate Time go wrong
It is a mistake to see TCG as the time shown by ordinary Earth clocks.
Sources and editorial method
Research references used for this entry include:
- IAU SOFA: Time Scale and Calendar Tools - IAU-standard algorithms and guidance for civil calendars, Julian dates, and seven recognized astronomical time scales.
- U.S. Naval Observatory Circular 179: The IAU Resolutions on Astronomical Reference Systems, Time Scales, and Earth Rotation Models - Technical reference for TAI, TT, TCG, TCB, TDB, UT1, sidereal time, epochs, and Earth rotation.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
- Intercalation
- Local Apparent Solar Time
- Old Style Date
- Unix Time
- Astronomical Day
- Callippic Cycle
- Draconic Month
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Common questions
What is Geocentric Coordinate Time?
Geocentric Coordinate Time, or TCG, is the relativistic coordinate time of the geocentric celestial reference system.
What do people get wrong about Geocentric Coordinate Time?
It is a mistake to see TCG as the time shown by ordinary Earth clocks.
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