Calendars and time
Delta T
Direct answer
Delta T is the difference TT minus UT1, linking uniform terrestrial time with time based on Earth rotation.
Delta T is the offset between Terrestrial Time and UT1.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Leap Second, DUT1, Calendar, Julian Date
Delta T is the offset between Terrestrial Time and UT1.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Delta T
Researchers estimate past Delta T from telescopic observations, ancient eclipse records, occultations, and modern atomic data.
Why Delta T matters
Historical and future eclipse calculations depend on Delta T because Earth rotational irregularity changes where an event is seen.
Delta T in practice: an example
Changing an assumed Delta T shifts the terrestrial longitude of a reconstructed ancient eclipse track.
Where readings of Delta T go wrong
A common error is to treat modeled Delta T far from the observed era as an exact measured value.
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.
- IERS Conventions (2010), Technical Note 36 - International standards for Earth rotation angle, reference systems, time arguments, and Earth-orientation data.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
State the Delta T model
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Common questions
What is Delta T?
Delta T is the difference TT minus UT1, linking uniform terrestrial time with time based on Earth rotation.
What do people get wrong about Delta T?
A common error is to treat modeled Delta T far from the observed era as an exact measured value.
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