**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 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.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk