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A letter and year fixing a coordinate frame.
Also known as: J2000 notation, Coordinate epoch
The notation naming the reference date for a set of celestial coordinates, which drift as the Earth's axis precesses.
Names the reference date for a set of celestial coordinates.
Contexts: History
Is necessary because the Earth's axis precesses, so a right ascension or declination quoted without an epoch is ambiguous by an amount that grows every year it is left unstated.
Contexts: History
A capital letter followed by a four-figure year, written before or after the coordinates it qualifies.