SLA_GMSTA - UT to GMST (extra precision)   

ACTION:

Conversion from universal time UT1 to Greenwich Mean sidereal time, with rounding errors minimized.

CALL:

D = sla_GMSTA (DATE, UT1)

GIVEN:

DATE UT1 date as Modified Julian Date (integer part of JD-2400000.5)

UT1 D UT1 time (fraction of a day)

RETURNED:

sla_GMST Greenwich mean sidereal time (radians)

NOTES:

1.

The algorithm is derived from the IAU 1982 expression (see page S15 of the 1984 Astronomical Almanac).

2.

There is no restriction on how the UT is apportioned between the DATE and UT1 arguments. Either of the two arguments could, for example, be zero and the entire date+time supplied in the other. However, the routine is designed to deliver maximum accuracy when the DATE argument is a whole number and the UT1 argument lies in the range $[\,0,\,1\,]$, or vice versa.

3.

See also the routine sla_GMST, which accepts the UT1 as a single argument. Compared with sla_GMST, the extra numerical precision delivered by the present routine is unlikely to be important in an absolute sense, but may be useful when critically comparing algorithms and in applications where two sidereal times close together are differenced.