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<H2><A NAME="SECTION00057000000000000000">
Proper Motion</A>
</H2>
Stars in catalogues usually have, in addition to the
<IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> coordinates, a <I>proper motion</I> <IMG WIDTH="54" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img264.gif"
ALT="$[\mu_\alpha,\mu_\delta]$">.This is an intrinsic motion
of the star across the background. Very few stars have a
proper motion which exceeds <IMG WIDTH="17" HEIGHT="17" ALIGN="BOTTOM" BORDER="0"
SRC="img140.gif"
ALT="$1\hspace{-0.05em}^{'\hspace{-0.1em}'}$">/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.
<P>
Mean <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> and rate of change are not sufficient to pin
down a star; the epoch at which the <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> 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 <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">. The full specification for a star
is <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">, proper motions, equinox and epoch (plus something to
identify which set of models for the precession <I>etc.</I> 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.
<P>
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
<IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$"> component is in the form <IMG WIDTH="13" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img115.gif"
ALT="$\dot{\alpha}$"> (<I>i.e.</I> big
numbers near the poles).
Some star catalogues have proper motion per century, and
in some catalogues the <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$"> component is in the form
<IMG WIDTH="48" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
SRC="img116.gif"
ALT="$\dot{\alpha}\cos\delta$"> (<I>i.e.</I> angle on the sky).
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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