Calendars and time
Terrestrial Time
Direct answer
Terrestrial Time is a uniform astronomical time scale for geocentric ephemerides, conventionally related by TT equals TAI plus 32.184 seconds.
Terrestrial Time is the uniform geocentric time argument used for astronomical ephemerides.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Greenwich Mean Time, International Atomic Time, Calendar, Julian Date
Terrestrial Time is the uniform geocentric time argument used for astronomical ephemerides.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Terrestrial Time
The IAU introduced TT as the successor to Ephemeris Time and Terrestrial Dynamical Time.
Why Terrestrial Time matters
TT avoids the irregularity of Earth rotation when calculating positions and dynamical models near Earth.
How to read Terrestrial Time
Terrestrial Time speaks to ephemeris arguments, J2000.0, and transformations among standardized astronomical time scales.
Terrestrial Time in practice: an example
J2000.0 is defined at Julian Date 2451545.0 TT, not at the same clock reading in UTC.
Where readings of Terrestrial Time go wrong
It is a mistake to compare a TT timestamp directly with civil UTC without applying the appropriate offsets.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Terrestrial Time - Official definition of TT, its relation to TAI, and the J2000.0 epoch.
- IAU SOFA: Time Scale and Calendar Tools - IAU-standard algorithms and guidance for civil calendars, Julian dates, and seven recognized astronomical time scales.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Name the ephemeris time scale
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Common questions
What is Terrestrial Time?
Terrestrial Time is a uniform astronomical time scale for geocentric ephemerides, conventionally related by TT equals TAI plus 32.184 seconds.
What do people get wrong about Terrestrial Time?
It is a mistake to compare a TT timestamp directly with civil UTC without applying the appropriate offsets.
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