Dictionary term
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.
Terrestrial Time is the uniform geocentric time argument used for astronomical ephemerides.
A cycle or timeline schematic for comparing the named timekeeping concept.Quick reference
- Use when
- Use Terrestrial Time for ephemeris arguments, J2000.0, and transformations among standardized astronomical time scales.
- Do not use it for
- Do not compare a TT timestamp directly with civil UTC without applying the appropriate offsets.
- Reference category
- Calendars & Timekeeping
- Related terms
- Greenwich Mean Time, International Atomic Time, Calendar, Julian Date
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The story behind Terrestrial Time
The IAU introduced TT as the successor to Ephemeris Time and Terrestrial Dynamical Time.
Terrestrial Time in spiritual tradition
Spiritually, Terrestrial Time exemplifies the precision required to describe the created sky without mystifying measurement.
Terrestrial Time: fact and faith, kept distinct
This is a factual calendar, astronomy, Earth-science, cultural-history, or observational reference. It can support wonder and practical planning without becoming a claim of fate, spiritual causation, or private revelation.
Terrestrial Time in numerology
Terrestrial Time is not a numerology claim. Any numbers attached to it should retain their ordinary scientific, calendrical, navigational, or historical meaning unless a separate symbolic interpretation is clearly labeled.
Why Terrestrial Time matters
TT avoids the irregularity of Earth rotation when calculating positions and dynamical models near Earth.
Terrestrial Time at work: an example
J2000.0 is defined at Julian Date 2451545.0 TT, not at the same clock reading in UTC.
Where interpretation stops
Terrestrial Time is presented as sourced astronomy, calendrical, Earth-science, cultural, or instrument-history reference. Definitions should distinguish measured phenomena from civil convention and historical interpretation, preserve location and time dependencies, and avoid turning factual sky data into a deterministic personal claim.
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
- Also appears in
- Calendars & Timekeeping
- Aliases
- None listed
- Last updated
- 2026-07-16
Name the ephemeris time scale
Terrestrial Time in a book is one thing; Terrestrial Time in your own birth chart is your life. The Oracle casts your real chart and tells you what it means for your love, your work, and your year ahead, in conversation, free.
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More in Calendars & Timekeeping
- Year Zero
- Before Common Era
- Common Year
- Earth Rotation Angle
- Gregorian Calendar
- Mean Solar Day
- Regnal Year
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.
How is Terrestrial Time used in an almanac?
Use Terrestrial Time for ephemeris arguments, J2000.0, and transformations among standardized astronomical time scales.
What should I avoid when interpreting Terrestrial Time?
Do not compare a TT timestamp directly with civil UTC without applying the appropriate offsets.
