Sky science

Reference Epoch

Direct answer

Reference Epoch: A Reference Epoch is the specified instant to which a catalogue position or time-varying astronomical quantity applies.

It dates an astrometric position or model.

Astronomy & SkywatchingThe Great Almanac DictionaryUpdated 2026-09-26

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At a glance

Found in: Astronomy & Skywatching
Related: Small Circle, Celestial Horizon, J2000 Epoch, Equinox of Date

A refracting telescope on its mount, pointed at a field of stars — Observable sky · visual guide **Reference Epoch**
It dates an astrometric position or model.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.

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The story behind Reference Epoch

Precision catalogues increasingly standardized epochs for comparing observations.

A one-minute story

The Wandering Stars

How the moving planets were watched, named and found, and what a transit is.

Why Reference Epoch matters

Reference Epoch prevents slowly changing coordinates from being mistaken for timeless values.

How to read Reference Epoch

Carry the epoch with positions, proper motions, and orbital elements.

Reference Epoch in practice: an example

A stellar position at epoch J2000 can be propagated using its proper motion.

Where readings of Reference Epoch go wrong

Epoch and equinox are related metadata but are not interchangeable.

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

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Small CircleIt is a spherical circle smaller than a great circle. Celestial HorizonIt is the mathematical horizon of the local sky. J2000 EpochIt is a widely used reference date for celestial coordinates. Equinox of DateIt uses the moving equatorial frame of a stated date. AstronomyAstronomy is the science of celestial objects, space, and the physical universe. SkywatchingSkywatching is direct observation of the changing day and night sky.

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