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 $0^{\rm h}$UT; in fact, it gives the difference between the GMST and the UT, which happens to equal the GMST (modulo 24 hours) at $0^{\rm h}$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 $1.0027379\cdots$ 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.