The *Greenwich Mean Sidereal Time* 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.

The *Local Apparent Sidereal Time* LAST is the apparent right
ascension of the local meridian, from which the hour angle of any
star can be determined knowing its . It can be obtained from the
GMST by adding the east longitude (corrected for polar motion
in precise work) and the *equation of the equinoxes*. 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 .

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk