Calendars and time
Earth Rotation Angle
Direct answer
Earth Rotation Angle is the modern angle that measures Earth rotation from the Celestial Intermediate Origin to the Terrestrial Intermediate Origin.
Earth Rotation Angle is the IAU-standard angular measure of Earth daily rotation.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- DUT1, Greenwich Sidereal Time, Calendar, Julian Date
Earth Rotation Angle is the IAU-standard angular measure of Earth daily rotation.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Earth Rotation Angle
IAU 2000 resolutions introduced ERA as a clearer replacement for using sidereal time as the primary Earth-rotation measure.
Why Earth Rotation Angle matters
ERA separates rotation cleanly from precession and nutation and is a linear function of UT1.
Earth Rotation Angle in practice: an example
A geodetic transformation applies ERA after accounting for celestial pole motion and before polar-motion corrections.
Where readings of Earth Rotation Angle go wrong
It is a mistake to compute ERA from civil clock time alone; the required argument is UT1.
Sources and editorial method
Research references used for this entry include:
- IERS Conventions (2010), Technical Note 36 - International standards for Earth rotation angle, reference systems, time arguments, and Earth-orientation data.
- 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
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Common questions
What is Earth Rotation Angle?
Earth Rotation Angle is the modern angle that measures Earth rotation from the Celestial Intermediate Origin to the Terrestrial Intermediate Origin.
What do people get wrong about Earth Rotation Angle?
It is a mistake to compute ERA from civil clock time alone; the required argument is UT1.
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