Sky science
J2000 Epoch
Direct answer
J2000 Epoch: The J2000 Epoch is the standard Julian epoch centered on 2000 January 1 at 12:00 Terrestrial Time.
It is a widely used reference date for celestial coordinates.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Celestial Horizon, Reference Epoch, Equinox of Date, Precession of the Equinoxes
It is a widely used reference date for celestial coordinates.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind J2000 Epoch
J2000 replaced older standard epochs in modern astronomical practice.
Why J2000 Epoch matters
J2000 Epoch gives catalogues and software a common baseline for positions and orientations.
How to read J2000 Epoch
Do not assume J2000 coordinates equal the apparent sky of a later date.
J2000 Epoch in practice: an example
A planetarium may store J2000 coordinates and precess them to the observing date.
Where readings of J2000 Epoch go wrong
The noon-based epoch and its time scale should be stated precisely.
Sources and editorial method
Research references used for this entry include:
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- International Astronomical Union: Standards of Fundamental Astronomy - IAU-maintained standards and algorithms for astronomical reference systems and coordinate transformations.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Use the J2000 reference correctly
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Common questions
What is the J2000 Epoch?
The J2000 Epoch is the standard Julian epoch centered on 2000 January 1 at 12:00 Terrestrial Time.
What do people get wrong about the J2000 Epoch?
The noon-based epoch and its time scale should be stated precisely.
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