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## SLA_GMST - UT to GMST

ACTION:
Conversion from universal time UT1 to Greenwich mean sidereal time.

CALL:
D = sla_GMST (UT1)

GIVEN:

 D UT1 universal time (strictly UT1) expressed as modified Julian Date (JD-2400000.5)

RETURNED:

 D sla_GMST 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 UT; in fact, it gives the difference between the GMST and the UT, which happens to equal the GMST (modulo 24 hours) at 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 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.

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SLALIB --- Positional Astronomy Library