Epoch vs . equinoxDate-references for coordinate systemsEpochs and periods of validityChanging the regular equinox and epochSpecifying an epoch or equinoxBesselian yearsJulian yrs and J2000Epoch of the daySee alsoReferencesNotesCitationsSourcesFurther readingExternal backlinksAstronomical quantities may be specified in any of a number of methods, a