Dictionary term
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.
Greenwich Sidereal Time expresses the equinox hour angle at Greenwich.
A cycle or timeline schematic for comparing the named timekeeping concept.Quick reference
- Use when
- Use Greenwich Sidereal Time in classical telescope pointing and convert it to local sidereal time with longitude.
- Do not use it for
- Distinguish mean from apparent sidereal time and use current IAU models for precision work.
- Reference category
- Calendars & Timekeeping
- Related terms
- Earth Rotation Angle, Local Sidereal Time, Calendar, Julian Date
The Chapter · Mystery School
Learn to read the sky for yourself
A mystery school of astrology with a worldwide chapter: letters on the chart and the soul, readings under one sky, and a rule of honesty about what the stars can and cannot say.
- The founding letter, with what the school studies and how it meets
- First word each season, when the doors open
- A seat at the gatherings, wherever in the world you are
Reading about Greenwich Sidereal Time? The chapter is where readers like you meet.
Founding circle · forming now
The list costs nothing. Word of the school and its gatherings, nothing else. Unsubscribe at any time.
The story behind Greenwich Sidereal Time
Observatories used Greenwich sidereal clocks and star transits as central tools of navigation and time determination.
Greenwich Sidereal Time in spiritual tradition
Spiritually, Greenwich Sidereal Time can deepen orientation while remaining a coordinate tool.
Greenwich Sidereal 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.
Greenwich Sidereal Time in numerology
Greenwich Sidereal 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 Greenwich Sidereal Time matters
It indicates which right ascension is crossing the Greenwich meridian at a given instant.
Greenwich Sidereal Time at work: an example
When Greenwich Sidereal Time is 6 hours, right ascension 6 hours lies on the Greenwich meridian in the classical model.
Where interpretation stops
Greenwich Sidereal 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 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
- Also appears in
- Calendars & Timekeeping
- Aliases
- None listed
- Last updated
- 2026-07-16
Find the Greenwich meridian sky
Greenwich Sidereal Time in a book is one thing; Greenwich Sidereal 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.
Speak with the OracleRelated terms
More in Calendars & Timekeeping
- Julian Century
- Lunisolar Calendar
- Reduced Julian Date
- Synodic Month
- Year Zero
- Before Common Era
- Common Year
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.
How is Greenwich Sidereal Time used in an almanac?
Use Greenwich Sidereal Time in classical telescope pointing and convert it to local sidereal time with longitude.
What should I avoid when interpreting Greenwich Sidereal Time?
Distinguish mean from apparent sidereal time and use current IAU models for precision work.
