siderealtime( jd [, /atzerotime] [, /mean])
Returns the sidereal time (relative to the mean equinox of the date)
for 0 degrees longitude, measured in hours (between 0 and 24), for
Julian Day numbers jd (a numerical scalar or array). If
/atzerotime is specified, then the sidereal time is returned
for the beginning (0:00 UT) of the Julian Day that jd indicates.
Otherwise, the mean sidereal time at the exact date and time indicated
by jd is returned. If
/mean is specified, then the mean
sidereal time is returned, otherwise the true sidereal time.
See also: calendar