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<H3><A NAME="SECTION000515300000000000000">
Sidereal Time: GMST, LAST</A>
</H3>
Sidereal Time is the ``time of day'' relative to the
stars rather than to the Sun. After
one sidereal day the stars come back to the same place in the
sky, apart from sub-arcsecond precession effects. Because the Earth
rotates faster relative to the stars than to the Sun by one day
per year, the sidereal second is shorter than the solar
second; the ratio is about 0.9973.
<P>
The <I>Greenwich Mean Sidereal Time</I> GMST is
linked to UT1 by a numerical formula which
is implemented in the SLALIB routines
sla_GMST
and
sla_GMSTA.
There are, of course, no leap seconds in GMST, but the second
changes in length along with the UT1 second, and also varies
over long periods of time because of slow changes in the Earth's
orbit. This makes the timescale unsuitable for everything except
predicting the apparent directions of celestial sources.
<P>
The <I>Local Apparent Sidereal Time</I> LAST is the apparent right
ascension of the local meridian, from which the hour angle of any
star can be determined knowing its <IMG WIDTH="13" HEIGHT="14" ALIGN="BOTTOM" BORDER="0"
SRC="img24.gif"
ALT="$\alpha$">. It can be obtained from the
GMST by adding the east longitude (corrected for polar motion
in precise work) and the <I>equation of the equinoxes</I>. The
latter, already described, is an aspect of the nutation effect
and can be predicted by calling the SLALIB routine
sla_EQEQX
or, neglecting certain very small terms, by calling
sla_NUTC
and using the expression <IMG WIDTH="61" HEIGHT="27" ALIGN="MIDDLE" BORDER="0"
SRC="img310.gif"
ALT="$\Delta\psi\cos\epsilon$">.<P>
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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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