= Epoch
{disambiguate=astronomy}
{wiki}
= Astronomical epoch
{synonym}
An <astronomical epoch> is a reference instant for astronomical data. A catalogue's position <astronomical epoch> says when the listed source direction applies; <proper motion> advances that direction to another instant. The reference date for an <equatorial coordinate system>'s axes need not equal the position <astronomical epoch>. ESA's Gaia documentation explicitly associates both <right ascension> and <declination> with its reference-epoch field. https://gea.esac.esa.int/archive/documentation/GDR3/Gaia_archive/chap_datamodel/sec_dm_main_source_catalogue/ssec_dm_gaia_source.html[ESA Gaia DR3: Reference epoch and its associated coordinate and proper-motion fields]{ref}.
Back to article page
