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/*
*+
* Name:
* palEtrms
* Purpose:
* Compute the E-terms vector
* Language:
* Starlink ANSI C
* Type of Module:
* Library routine
* Invocation:
* void palEtrms ( double ep, double ev[3] );
* Arguments:
* ep = double (Given)
* Besselian epoch
* ev = double [3] (Returned)
* E-terms as (dx,dy,dz)
* Description:
* Computes the E-terms (elliptic component of annual aberration)
* vector.
*
* Note the use of the J2000 aberration constant (20.49552 arcsec).
* This is a reflection of the fact that the E-terms embodied in
* existing star catalogues were computed from a variety of
* aberration constants. Rather than adopting one of the old
* constants the latest value is used here.
*
* See also:
* - Smith, C.A. et al., 1989. Astr.J. 97, 265.
* - Yallop, B.D. et al., 1989. Astr.J. 97, 274.
* Authors:
* PTW: Pat Wallace (STFC)
* TIMJ: Tim Jenness (JAC, Hawaii)
* {enter_new_authors_here}
* History:
* 2012-02-12 (TIMJ):
* Initial version with documentation taken from Fortran SLA
* Adapted with permission from the Fortran SLALIB library.
* {enter_further_changes_here}
* Copyright:
* Copyright (C) 1996 Rutherford Appleton Laboratory
* Copyright (C) 2012 Science and Technology Facilities Council.
* All Rights Reserved.
* Licence:
* This program is free software: you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation, either
* version 3 of the License, or (at your option) any later
* version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General
* License along with this program. If not, see
* <http://www.gnu.org/licenses/>.
* Bugs:
* {note_any_bugs_here}
*-
*/
#include "pal.h"
#include "palmac.h"
void palEtrms ( double ep, double ev[3] ) {
/* Use the J2000 aberration constant */
const double ABCONST = 20.49552;
double t, e, e0, p, ek, cp;
/* Julian centuries since B1950 */
t = (ep - 1950.) * .0100002135903;
/* Eccentricity */
e = .01673011 - (t * 1.26e-7 + 4.193e-5) * t;
/* Mean obliquity */
e0 = (84404.836 - ((t * .00181 + .00319) * t + 46.8495) * t) *
PAL__DAS2R;
/* Mean longitude of perihelion */
p = (((t * .012 + 1.65) * t + 6190.67) * t + 1015489.951) *
PAL__DAS2R;
/* E-terms */
ek = e * ABCONST * PAL__DAS2R;
cp = cos(p);
ev[0] = ek * sin(p);
ev[1] = -ek * cp * cos(e0);
ev[2] = -ek * cp * sin(e0);
}
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