Mean Places

The main effect of the precession/nutation is a steady increase of about $50\hspace{-0.05em}^{'\hspace{-0.1em}'}$/year in the ecliptic longitudes of the stars. It is therefore essential, when reporting the position of an astronomical target, to qualify the coordinates with a date, or epoch. Specifying the epoch ties down the equator and equinox which define the $[\,\alpha,\delta\,]$ coordinate system that is being used. For simplicity, only the smooth and steady ``general precession'' part of the complete precession/nutation effect is included, thereby defining what is called the mean equator and equinox for the epoch concerned. We say a star has a mean place of (for example) $12^{h}\,07^{m}\,58^{s}.09$ $-19^{\circ}\,44^{'}\,37^{''}.1$ ``with respect to the mean equator and equinox of epoch J2000''. The short way of saying this is ``$[\,\alpha,\delta\,]$ equinox J2000'' (not ``$[\,\alpha,\delta\,]$ epoch J2000'', which means something different to do with proper motion).