1 #include "erfa.h"
2 
eraS06a(double date1,double date2)3 double eraS06a(double date1, double date2)
4 /*
5 **  - - - - - - - -
6 **   e r a S 0 6 a
7 **  - - - - - - - -
8 **
9 **  The CIO locator s, positioning the Celestial Intermediate Origin on
10 **  the equator of the Celestial Intermediate Pole, using the IAU 2006
11 **  precession and IAU 2000A nutation models.
12 **
13 **  Given:
14 **     date1,date2  double    TT as a 2-part Julian Date (Note 1)
15 **
16 **  Returned (function value):
17 **                  double    the CIO locator s in radians (Note 2)
18 **
19 **  Notes:
20 **
21 **  1) The TT date date1+date2 is a Julian Date, apportioned in any
22 **     convenient way between the two arguments.  For example,
23 **     JD(TT)=2450123.7 could be expressed in any of these ways,
24 **     among others:
25 **
26 **            date1          date2
27 **
28 **         2450123.7           0.0       (JD method)
29 **         2451545.0       -1421.3       (J2000 method)
30 **         2400000.5       50123.2       (MJD method)
31 **         2450123.5           0.2       (date & time method)
32 **
33 **     The JD method is the most natural and convenient to use in
34 **     cases where the loss of several decimal digits of resolution
35 **     is acceptable.  The J2000 method is best matched to the way
36 **     the argument is handled internally and will deliver the
37 **     optimum resolution.  The MJD method and the date & time methods
38 **     are both good compromises between resolution and convenience.
39 **
40 **  2) The CIO locator s is the difference between the right ascensions
41 **     of the same point in two systems.  The two systems are the GCRS
42 **     and the CIP,CIO, and the point is the ascending node of the
43 **     CIP equator.  The CIO locator s remains a small fraction of
44 **     1 arcsecond throughout 1900-2100.
45 **
46 **  3) The series used to compute s is in fact for s+XY/2, where X and Y
47 **     are the x and y components of the CIP unit vector;  this series is
48 **     more compact than a direct series for s would be.  The present
49 **     function uses the full IAU 2000A nutation model when predicting
50 **     the CIP position.
51 **
52 **  Called:
53 **     eraPnm06a    classical NPB matrix, IAU 2006/2000A
54 **     eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
55 **     eraS06       the CIO locator s, given X,Y, IAU 2006
56 **
57 **  References:
58 **
59 **     Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
60 **     "Expressions for the Celestial Intermediate Pole and Celestial
61 **     Ephemeris Origin consistent with the IAU 2000A precession-
62 **     nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
63 **
64 **     n.b. The celestial ephemeris origin (CEO) was renamed "celestial
65 **          intermediate origin" (CIO) by IAU 2006 Resolution 2.
66 **
67 **     Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855
68 **
69 **     McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003),
70 **     IERS Technical Note No. 32, BKG
71 **
72 **     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
73 **
74 **  Copyright (C) 2013-2014, NumFOCUS Foundation.
75 **  Derived, with permission, from the SOFA library.  See notes at end of file.
76 */
77 {
78    double rnpb[3][3], x, y, s;
79 
80 
81 /* Bias-precession-nutation-matrix, IAU 20006/2000A. */
82    eraPnm06a(date1, date2, rnpb);
83 
84 /* Extract the CIP coordinates. */
85    eraBpn2xy(rnpb, &x, &y);
86 
87 /* Compute the CIO locator s, given the CIP coordinates. */
88    s = eraS06(date1, date2, x, y);
89 
90    return s;
91 
92 }
93 /*----------------------------------------------------------------------
94 **
95 **
96 **  Copyright (C) 2013-2014, NumFOCUS Foundation.
97 **  All rights reserved.
98 **
99 **  This library is derived, with permission, from the International
100 **  Astronomical Union's "Standards of Fundamental Astronomy" library,
101 **  available from http://www.iausofa.org.
102 **
103 **  The ERFA version is intended to retain identical functionality to
104 **  the SOFA library, but made distinct through different function and
105 **  file names, as set out in the SOFA license conditions.  The SOFA
106 **  original has a role as a reference standard for the IAU and IERS,
107 **  and consequently redistribution is permitted only in its unaltered
108 **  state.  The ERFA version is not subject to this restriction and
109 **  therefore can be included in distributions which do not support the
110 **  concept of "read only" software.
111 **
112 **  Although the intent is to replicate the SOFA API (other than
113 **  replacement of prefix names) and results (with the exception of
114 **  bugs;  any that are discovered will be fixed), SOFA is not
115 **  responsible for any errors found in this version of the library.
116 **
117 **  If you wish to acknowledge the SOFA heritage, please acknowledge
118 **  that you are using a library derived from SOFA, rather than SOFA
119 **  itself.
120 **
121 **
122 **  TERMS AND CONDITIONS
123 **
124 **  Redistribution and use in source and binary forms, with or without
125 **  modification, are permitted provided that the following conditions
126 **  are met:
127 **
128 **  1 Redistributions of source code must retain the above copyright
129 **    notice, this list of conditions and the following disclaimer.
130 **
131 **  2 Redistributions in binary form must reproduce the above copyright
132 **    notice, this list of conditions and the following disclaimer in
133 **    the documentation and/or other materials provided with the
134 **    distribution.
135 **
136 **  3 Neither the name of the Standards Of Fundamental Astronomy Board,
137 **    the International Astronomical Union nor the names of its
138 **    contributors may be used to endorse or promote products derived
139 **    from this software without specific prior written permission.
140 **
141 **  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
142 **  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
143 **  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
144 **  FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
145 **  COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
146 **  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
147 **  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
148 **  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
149 **  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
150 **  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
151 **  ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
152 **  POSSIBILITY OF SUCH DAMAGE.
153 **
154 */
155