Proper Motion

Stars in catalogues usually have, in addition to the $[\,\alpha,\delta\,]$ coordinates, a proper motion $[\mu_\alpha,\mu_\delta]$.This is an intrinsic motion of the star across the background. Very few stars have a proper motion which exceeds $1\hspace{-0.05em}^{'\hspace{-0.1em}'}$/year, and most are far below this level. A star observed as part of normal astronomy research will, as a rule, have a proper motion which is unknown.

Mean $[\,\alpha,\delta\,]$ and rate of change are not sufficient to pin down a star; the epoch at which the $[\,\alpha,\delta\,]$ was or will be correct is also needed. Note the distinction between the epoch which specifies the coordinate system and the epoch at which the star passed through the given $[\,\alpha,\delta\,]$. The full specification for a star is $[\,\alpha,\delta\,]$, proper motions, equinox and epoch (plus something to identify which set of models for the precession etc. is being used - see the next section). For convenience, coordinates given in star catalogues are almost always adjusted to make the equinox and epoch the same - for example B1950 in the case of the SAO catalogue.

SLALIB provides one routine to handle proper motion on its own, sla_PM. Proper motion is also allowed for in various other routines as appropriate, for example sla_MAP and sla_FK425. Note that in all SLALIB routines which involve proper motion the units are radians per year and the $\alpha$ component is in the form $\dot{\alpha}$ (i.e. big numbers near the poles). Some star catalogues have proper motion per century, and in some catalogues the $\alpha$ component is in the form $\dot{\alpha}\cos\delta$ (i.e. angle on the sky).