Conversion from universal time UT1 to Greenwich mean sidereal time.
CALL:
D = sla_GMST (UT1)
GIVEN:
UT1
D
universal time (strictly UT1)
expressed as modified Julian Date
(JD$-$2400000.5)
RETURNED:
sla_GMST
D
Greenwich mean sidereal time (radians)
NOTES:
(1)
The IAU 1982 expression (see page S15 of the 1984 Astronomical Almanac) is used,
but rearranged to reduce rounding errors. This expression is always described as
giving the GMST at ${0}^{h}$UT;
in fact, it gives the difference between the GMST and the UT, which happens to equal
the GMST (modulo 24 hours) at ${0}^{h}$UT
each day. In sla_GMST, the entire UT is used directly as the argument for the canonical
formula, and the fractional part of the UT is added separately; note that the factor
$1.0027379\cdots \phantom{\rule{0.3em}{0ex}}$
does not appear.
(2)
See also the routine sla_GMSTA, which delivers better numerical precision by
accepting the UT date and time as separate arguments.