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.help gmsta Jun99 "Slalib Package"
.nf
DOUBLE PRECISION FUNCTION slGMSA (DATE, UT)
- - - - - -
G M S A
- - - - - -
Conversion from Universal Time to Greenwich mean sidereal time,
with rounding errors minimized.
double precision
Given:
DATE d UT1 date (MJD: integer part of JD-2400000.5))
UT d UT1 time (fraction of a day)
The result is the Greenwich mean sidereal time (double precision,
radians, in the range 0 to 2pi).
There is no restriction on how the UT is apportioned between the
DATE and UT 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 UT lies in the range
0 to 1 (or vice versa).
The algorithm is based on the IAU 1982 expression (see page S15 of
the 1984 Astronomical Almanac). This is always described as giving
the GMST at 0 hours UT1. In fact, it gives the difference between
the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
ST with respect to UT. When whole days are ignored, the expression
happens to equal the GMST at 0 hours UT1 each day.
In this routine, the entire UT1 (the sum of the two arguments DATE
and UT) is used directly as the argument for the standard formula.
The UT1 is then added, but omitting whole days to conserve accuracy.
See also the routine slGMST, which accepts the UT as a single
argument. Compared with slGMST, 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.
Called: slDA2P
P.T.Wallace Starlink 13 April 1998
Copyright (C) 1998 Rutherford Appleton Laboratory
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
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