#include "erfa.h" double eraS06a(double date1, double date2) /* ** - - - - - - - - ** e r a S 0 6 a ** - - - - - - - - ** ** The CIO locator s, positioning the Celestial Intermediate Origin on ** the equator of the Celestial Intermediate Pole, using the IAU 2006 ** precession and IAU 2000A nutation models. ** ** Given: ** date1,date2 double TT as a 2-part Julian Date (Note 1) ** ** Returned (function value): ** double the CIO locator s in radians (Note 2) ** ** Notes: ** ** 1) The TT date date1+date2 is a Julian Date, apportioned in any ** convenient way between the two arguments. For example, ** JD(TT)=2450123.7 could be expressed in any of these ways, ** among others: ** ** date1 date2 ** ** 2450123.7 0.0 (JD method) ** 2451545.0 -1421.3 (J2000 method) ** 2400000.5 50123.2 (MJD method) ** 2450123.5 0.2 (date & time method) ** ** The JD method is the most natural and convenient to use in ** cases where the loss of several decimal digits of resolution ** is acceptable. The J2000 method is best matched to the way ** the argument is handled internally and will deliver the ** optimum resolution. The MJD method and the date & time methods ** are both good compromises between resolution and convenience. ** ** 2) The CIO locator s is the difference between the right ascensions ** of the same point in two systems. The two systems are the GCRS ** and the CIP,CIO, and the point is the ascending node of the ** CIP equator. The CIO locator s remains a small fraction of ** 1 arcsecond throughout 1900-2100. ** ** 3) The series used to compute s is in fact for s+XY/2, where X and Y ** are the x and y components of the CIP unit vector; this series is ** more compact than a direct series for s would be. The present ** function uses the full IAU 2000A nutation model when predicting ** the CIP position. ** ** Called: ** eraPnm06a classical NPB matrix, IAU 2006/2000A ** eraBpn2xy extract CIP X,Y coordinates from NPB matrix ** eraS06 the CIO locator s, given X,Y, IAU 2006 ** ** References: ** ** Capitaine, N., Chapront, J., Lambert, S. and Wallace, P., ** "Expressions for the Celestial Intermediate Pole and Celestial ** Ephemeris Origin consistent with the IAU 2000A precession- ** nutation model", Astron.Astrophys. 400, 1145-1154 (2003) ** ** n.b. The celestial ephemeris origin (CEO) was renamed "celestial ** intermediate origin" (CIO) by IAU 2006 Resolution 2. ** ** Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855 ** ** McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003), ** IERS Technical Note No. 32, BKG ** ** Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981 ** ** Copyright (C) 2013-2017, NumFOCUS Foundation. ** Derived, with permission, from the SOFA library. See notes at end of file. */ { double rnpb[3][3], x, y, s; /* Bias-precession-nutation-matrix, IAU 20006/2000A. */ eraPnm06a(date1, date2, rnpb); /* Extract the CIP coordinates. */ eraBpn2xy(rnpb, &x, &y); /* Compute the CIO locator s, given the CIP coordinates. */ s = eraS06(date1, date2, x, y); return s; } /*---------------------------------------------------------------------- ** ** ** Copyright (C) 2013-2017, NumFOCUS Foundation. ** All rights reserved. ** ** This library is derived, with permission, from the International ** Astronomical Union's "Standards of Fundamental Astronomy" library, ** available from http://www.iausofa.org. ** ** The ERFA version is intended to retain identical functionality to ** the SOFA library, but made distinct through different function and ** file names, as set out in the SOFA license conditions. The SOFA ** original has a role as a reference standard for the IAU and IERS, ** and consequently redistribution is permitted only in its unaltered ** state. The ERFA version is not subject to this restriction and ** therefore can be included in distributions which do not support the ** concept of "read only" software. ** ** Although the intent is to replicate the SOFA API (other than ** replacement of prefix names) and results (with the exception of ** bugs; any that are discovered will be fixed), SOFA is not ** responsible for any errors found in this version of the library. ** ** If you wish to acknowledge the SOFA heritage, please acknowledge ** that you are using a library derived from SOFA, rather than SOFA ** itself. ** ** ** TERMS AND CONDITIONS ** ** Redistribution and use in source and binary forms, with or without ** modification, are permitted provided that the following conditions ** are met: ** ** 1 Redistributions of source code must retain the above copyright ** notice, this list of conditions and the following disclaimer. ** ** 2 Redistributions in binary form must reproduce the above copyright ** notice, this list of conditions and the following disclaimer in ** the documentation and/or other materials provided with the ** distribution. ** ** 3 Neither the name of the Standards Of Fundamental Astronomy Board, ** the International Astronomical Union nor the names of its ** contributors may be used to endorse or promote products derived ** from this software without specific prior written permission. ** ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS ** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 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