### SLA_GMST

UT to GMST

ACTION:
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.