Calendars and time
Greenwich Sidereal Time
Direct answer
Greenwich Sidereal Time is the hour angle of the equinox at the Greenwich meridian, combining Earth rotation with equinox motion.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Earth Rotation Angle, Local Sidereal Time, Calendar, Julian Date
Greenwich Sidereal Time expresses the equinox hour angle at Greenwich.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Greenwich Sidereal Time
Observatories used Greenwich sidereal clocks and star transits as central tools of navigation and time determination.
Why Greenwich Sidereal Time matters
Greenwich Sidereal Time indicates which right ascension is crossing the Greenwich meridian at a given instant.
Greenwich Sidereal Time in practice: an example
When Greenwich Sidereal Time is 6 hours, right ascension 6 hours lies on the Greenwich meridian in the classical model.
Sources and editorial method
Research references used for this entry include:
- 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.
- 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
Find the Greenwich meridian sky
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More in Calendars & Timekeeping
- Julian Century
- Lunisolar Calendar
- Reduced Julian Date
- Synodic Month
- Year Zero
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Common questions
What is Greenwich Sidereal Time?
Greenwich Sidereal Time is the hour angle of the equinox at the Greenwich meridian, combining Earth rotation with equinox motion.
Why does Greenwich Sidereal Time matter?
Greenwich Sidereal Time indicates which right ascension is crossing the Greenwich meridian at a given instant.
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