
ACTION:
 Conversion from universal time UT1 to Greenwich mean sidereal time, with
rounding errors minimized.

CALL:
D = sla_GMSTA (DATE, UT1)
GIVEN:
DATE  D  UT1 date as Modified Julian Date (integer part of
JD$$2400000.5) 

UT1  D  UT1 time (fraction of a day) 

RETURNED:
sla_GMST  D  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 $\left[\phantom{\rule{0.3em}{0ex}}0,\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\right]$,
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.