| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaEtrms ( double ep, double ev[3] )
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| 4 | /*
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| 5 | ** - - - - - - - - -
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| 6 | ** s l a E t r m s
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| 7 | ** - - - - - - - - -
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| 8 | **
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| 9 | ** Compute the e-terms (elliptic component of annual aberration)
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| 10 | ** vector.
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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 | ** ep double Besselian epoch
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| 16 | **
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| 17 | ** Returned:
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| 18 | ** ev double[3] e-terms as (dx,dy,dz)
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| 19 | **
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| 20 | ** References:
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| 21 | **
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| 22 | ** 1 Smith, C.A. et al, 1989. "The transformation of astrometric
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| 23 | ** catalog systems to the equinox J2000.0". Astron.J. 97, 265.
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| 24 | **
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| 25 | ** 2 Yallop, B.D. et al, 1989. "Transformation of mean star places
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| 26 | ** from FK4 B1950.0 to FK5 J2000.0 using matrices in 6-space".
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| 27 | ** Astron.J. 97, 274.
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| 28 | **
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| 29 | ** Note the use of the J2000 aberration constant (20.49552 arcsec).
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| 30 | ** This is a reflection of the fact that the e-terms embodied in
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| 31 | ** existing star catalogues were computed from a variety of
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| 32 | ** aberration constants. Rather than adopting one of the old
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| 33 | ** constants the latest value is used here.
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| 34 | **
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| 35 | ** Defined in slamac.h: DAS2R
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| 36 | **
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| 37 | ** Last revision: 31 October 1993
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| 38 | **
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| 39 | ** Copyright P.T.Wallace. All rights reserved.
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| 40 | */
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| 41 | {
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| 42 | double t, e, e0, p, ek, cp;
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| 43 |
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| 44 | /* Julian centuries since B1950 */
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| 45 | t = ( ep - 1950.0 ) * 1.00002135903e-2;
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| 46 |
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| 47 | /* Eccentricity */
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| 48 | e = 0.01673011 - ( 0.00004193 + 0.000000126 * t ) * t;
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| 49 |
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| 50 | /* Mean obliquity */
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| 51 | e0 = ( 84404.836 -
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| 52 | ( 46.8495 + ( 0.00319 + 0.00181 * t ) * t ) * t ) * DAS2R;
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| 53 |
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| 54 | /* Mean longitude of perihelion */
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| 55 | p = ( 1015489.951 +
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| 56 | ( 6190.67 + ( 1.65 + 0.012 * t ) * t ) * t ) * DAS2R;
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| 57 |
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| 58 | /* E-terms */
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| 59 | ek = e * 20.49552 * DAS2R;
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| 60 | cp = cos ( p );
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| 61 | ev[0] = ek * sin ( p );
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| 62 | ev[1] = -ek * cp * cos ( e0 );
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| 63 | ev[2] = -ek * cp * sin ( e0 );
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| 64 | }
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