| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | double slaGmsta ( double date, double ut )
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| 4 | /*
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| 5 | ** - - - - - - - - -
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| 6 | ** s l a G m s t a
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| 7 | ** - - - - - - - - -
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| 8 | **
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| 9 | ** Conversion from Universal Time to Greenwich mean sidereal time,
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| 10 | ** with rounding errors minimized.
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| 11 | **
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| 12 | ** (double precision)
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| 13 | **
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| 14 | ** Given:
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| 15 | * date double UT1 date (MJD: integer part of JD-2400000.5))
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| 16 | ** ut double UT1 time (fraction of a day)
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| 17 | **
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| 18 | ** The result is the Greenwich Mean Sidereal Time (double precision,
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| 19 | ** radians, in the range 0 to 2pi).
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| 20 | **
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| 21 | ** There is no restriction on how the UT is apportioned between the
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| 22 | ** date and ut1 arguments. Either of the two arguments could, for
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| 23 | ** example, be zero and the entire date+time supplied in the other.
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| 24 | ** However, the routine is designed to deliver maximum accuracy when
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| 25 | ** the date argument is a whole number and the ut argument lies in
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| 26 | ** the range 0 to 1, or vice versa.
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| 27 | **
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| 28 | ** The algorithm is based on the IAU 1982 expression (see page S15 of
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| 29 | ** the 1984 Astronomical Almanac). This is always described as giving
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| 30 | ** the GMST at 0 hours UT1. In fact, it gives the difference between
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| 31 | ** the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
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| 32 | ** ST with respect to UT. When whole days are ignored, the expression
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| 33 | ** happens to equal the GMST at 0 hours UT1 each day.
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| 34 | **
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| 35 | ** In this routine, the entire UT1 (the sum of the two arguments date
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| 36 | ** and ut) is used directly as the argument for the standard formula.
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| 37 | ** The UT1 is then added, but omitting whole days to conserve accuracy.
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| 38 | **
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| 39 | ** See also the routine slaGmst, which accepts the UT1 as a single
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| 40 | ** argument. Compared with slaGmst, the extra numerical precision
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| 41 | ** delivered by the present routine is unlikely to be important in
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| 42 | ** an absolute sense, but may be useful when critically comparing
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| 43 | ** algorithms and in applications where two sidereal times close
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| 44 | ** together are differenced.
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| 45 | **
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| 46 | ** Called: slaDranrm
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| 47 | **
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| 48 | ** Defined in slamac.h: DS2R, dmod
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| 49 | **
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| 50 | ** Last revision: 13 April 1998
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| 51 | **
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| 52 | ** Copyright P.T.Wallace. All rights reserved.
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| 53 | */
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| 54 | {
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| 55 | double d1, d2, t;
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| 56 |
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| 57 | /* Julian centuries since J2000. */
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| 58 | if ( date < ut ) {
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| 59 | d1 = date;
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| 60 | d2 = ut;
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| 61 | } else {
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| 62 | d1 = ut;
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| 63 | d2 = date;
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| 64 | }
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| 65 | t = ( d1 + ( d2 - 51544.5 ) ) / 36525.0;
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| 66 |
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| 67 | /* GMST at this UT1. */
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| 68 | return slaDranrm ( DS2R * ( 24110.54841
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| 69 | + ( 8640184.812866
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| 70 | + ( 0.093104
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| 71 | - 6.2e-6 * t ) * t ) * t
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| 72 | + 86400.0 * ( dmod ( d1, 1.0 ) +
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| 73 | dmod ( d2, 1.0 ) ) ) );
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| 74 | }
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