FACT++  1.0
eors.c
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1 #include "erfa.h"
2 
3 double eraEors(double rnpb[3][3], double s)
4 /*
5 ** - - - - - - - -
6 ** e r a E o r s
7 ** - - - - - - - -
8 **
9 ** Equation of the origins, given the classical NPB matrix and the
10 ** quantity s.
11 **
12 ** Given:
13 ** rnpb double[3][3] classical nutation x precession x bias matrix
14 ** s double the quantity s (the CIO locator)
15 **
16 ** Returned (function value):
17 ** double the equation of the origins in radians.
18 **
19 ** Notes:
20 **
21 ** 1) The equation of the origins is the distance between the true
22 ** equinox and the celestial intermediate origin and, equivalently,
23 ** the difference between Earth rotation angle and Greenwich
24 ** apparent sidereal time (ERA-GST). It comprises the precession
25 ** (since J2000.0) in right ascension plus the equation of the
26 ** equinoxes (including the small correction terms).
27 **
28 ** 2) The algorithm is from Wallace & Capitaine (2006).
29 **
30 ** References:
31 **
32 ** Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855
33 **
34 ** Wallace, P. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
35 **
36 ** Copyright (C) 2013-2015, NumFOCUS Foundation.
37 ** Derived, with permission, from the SOFA library. See notes at end of file.
38 */
39 {
40  double x, ax, xs, ys, zs, p, q, eo;
41 
42 /* Evaluate Wallace & Capitaine (2006) expression (16). */
43  x = rnpb[2][0];
44  ax = x / (1.0 + rnpb[2][2]);
45  xs = 1.0 - ax * x;
46  ys = -ax * rnpb[2][1];
47  zs = -x;
48  p = rnpb[0][0] * xs + rnpb[0][1] * ys + rnpb[0][2] * zs;
49  q = rnpb[1][0] * xs + rnpb[1][1] * ys + rnpb[1][2] * zs;
50  eo = ((p != 0) || (q != 0)) ? s - atan2(q, p) : s;
51 
52  return eo;
53 
54 }
55 /*----------------------------------------------------------------------
56 **
57 **
58 ** Copyright (C) 2013-2015, NumFOCUS Foundation.
59 ** All rights reserved.
60 **
61 ** This library is derived, with permission, from the International
62 ** Astronomical Union's "Standards of Fundamental Astronomy" library,
63 ** available from http://www.iausofa.org.
64 **
65 ** The ERFA version is intended to retain identical functionality to
66 ** the SOFA library, but made distinct through different function and
67 ** file names, as set out in the SOFA license conditions. The SOFA
68 ** original has a role as a reference standard for the IAU and IERS,
69 ** and consequently redistribution is permitted only in its unaltered
70 ** state. The ERFA version is not subject to this restriction and
71 ** therefore can be included in distributions which do not support the
72 ** concept of "read only" software.
73 **
74 ** Although the intent is to replicate the SOFA API (other than
75 ** replacement of prefix names) and results (with the exception of
76 ** bugs; any that are discovered will be fixed), SOFA is not
77 ** responsible for any errors found in this version of the library.
78 **
79 ** If you wish to acknowledge the SOFA heritage, please acknowledge
80 ** that you are using a library derived from SOFA, rather than SOFA
81 ** itself.
82 **
83 **
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eraEors
double eraEors(double rnpb[3][3], double s)
Definition: eors.c:3