1 | #include "erfa.h"
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2 |
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3 | double eraS00b(double date1, double date2)
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4 | /*
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5 | ** - - - - - - - -
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6 | ** e r a S 0 0 b
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7 | ** - - - - - - - -
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8 | **
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9 | ** The CIO locator s, positioning the Celestial Intermediate Origin on
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10 | ** the equator of the Celestial Intermediate Pole, using the IAU 2000B
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11 | ** precession-nutation model.
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12 | **
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13 | ** Given:
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14 | ** date1,date2 double TT as a 2-part Julian Date (Note 1)
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15 | **
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16 | ** Returned (function value):
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17 | ** double the CIO locator s in radians (Note 2)
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18 | **
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19 | ** Notes:
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20 | **
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21 | ** 1) The TT date date1+date2 is a Julian Date, apportioned in any
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22 | ** convenient way between the two arguments. For example,
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23 | ** JD(TT)=2450123.7 could be expressed in any of these ways,
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24 | ** among others:
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25 | **
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26 | ** date1 date2
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27 | **
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28 | ** 2450123.7 0.0 (JD method)
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29 | ** 2451545.0 -1421.3 (J2000 method)
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30 | ** 2400000.5 50123.2 (MJD method)
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31 | ** 2450123.5 0.2 (date & time method)
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32 | **
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33 | ** The JD method is the most natural and convenient to use in
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34 | ** cases where the loss of several decimal digits of resolution
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35 | ** is acceptable. The J2000 method is best matched to the way
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36 | ** the argument is handled internally and will deliver the
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37 | ** optimum resolution. The MJD method and the date & time methods
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38 | ** are both good compromises between resolution and convenience.
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39 | **
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40 | ** 2) The CIO locator s is the difference between the right ascensions
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41 | ** of the same point in two systems. The two systems are the GCRS
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42 | ** and the CIP,CIO, and the point is the ascending node of the
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43 | ** CIP equator. The CIO locator s remains a small fraction of
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44 | ** 1 arcsecond throughout 1900-2100.
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45 | **
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46 | ** 3) The series used to compute s is in fact for s+XY/2, where X and Y
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47 | ** are the x and y components of the CIP unit vector; this series
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48 | ** is more compact than a direct series for s would be. The present
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49 | ** function uses the IAU 2000B truncated nutation model when
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50 | ** predicting the CIP position. The function eraS00a uses instead
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51 | ** the full IAU 2000A model, but with no significant increase in
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52 | ** accuracy and at some cost in speed.
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53 | **
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54 | ** Called:
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55 | ** eraPnm00b classical NPB matrix, IAU 2000B
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56 | ** eraBnp2xy extract CIP X,Y from the BPN matrix
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57 | ** eraS00 the CIO locator s, given X,Y, IAU 2000A
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58 | **
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59 | ** References:
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60 | **
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61 | ** Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
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62 | ** "Expressions for the Celestial Intermediate Pole and Celestial
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63 | ** Ephemeris Origin consistent with the IAU 2000A precession-
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64 | ** nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
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65 | **
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66 | ** n.b. The celestial ephemeris origin (CEO) was renamed "celestial
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67 | ** intermediate origin" (CIO) by IAU 2006 Resolution 2.
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68 | **
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69 | ** McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
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70 | ** IERS Technical Note No. 32, BKG (2004)
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71 | **
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72 | ** Copyright (C) 2013-2017, NumFOCUS Foundation.
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73 | ** Derived, with permission, from the SOFA library. See notes at end of file.
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74 | */
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75 | {
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76 | double rbpn[3][3], x, y, s;
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77 |
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78 |
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79 | /* Bias-precession-nutation-matrix, IAU 2000B. */
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80 | eraPnm00b(date1, date2, rbpn);
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81 |
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82 | /* Extract the CIP coordinates. */
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83 | eraBpn2xy(rbpn, &x, &y);
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84 |
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85 | /* Compute the CIO locator s, given the CIP coordinates. */
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86 | s = eraS00(date1, date2, x, y);
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87 |
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88 | return s;
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89 |
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90 | }
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91 | /*----------------------------------------------------------------------
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92 | **
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93 | **
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94 | ** Copyright (C) 2013-2017, NumFOCUS Foundation.
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95 | ** All rights reserved.
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96 | **
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97 | ** This library is derived, with permission, from the International
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98 | ** Astronomical Union's "Standards of Fundamental Astronomy" library,
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99 | ** available from http://www.iausofa.org.
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100 | **
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101 | ** The ERFA version is intended to retain identical functionality to
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102 | ** the SOFA library, but made distinct through different function and
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103 | ** file names, as set out in the SOFA license conditions. The SOFA
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104 | ** original has a role as a reference standard for the IAU and IERS,
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105 | ** and consequently redistribution is permitted only in its unaltered
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106 | ** state. The ERFA version is not subject to this restriction and
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107 | ** therefore can be included in distributions which do not support the
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108 | ** concept of "read only" software.
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109 | **
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110 | ** Although the intent is to replicate the SOFA API (other than
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111 | ** replacement of prefix names) and results (with the exception of
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112 | ** bugs; any that are discovered will be fixed), SOFA is not
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113 | ** responsible for any errors found in this version of the library.
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114 | **
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115 | ** If you wish to acknowledge the SOFA heritage, please acknowledge
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116 | ** that you are using a library derived from SOFA, rather than SOFA
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117 | ** itself.
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118 | **
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119 | **
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120 | ** TERMS AND CONDITIONS
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121 | **
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122 | ** Redistribution and use in source and binary forms, with or without
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123 | ** modification, are permitted provided that the following conditions
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124 | ** are met:
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125 | **
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126 | ** 1 Redistributions of source code must retain the above copyright
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127 | ** notice, this list of conditions and the following disclaimer.
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128 | **
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129 | ** 2 Redistributions in binary form must reproduce the above copyright
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130 | ** notice, this list of conditions and the following disclaimer in
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131 | ** the documentation and/or other materials provided with the
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132 | ** distribution.
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133 | **
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134 | ** 3 Neither the name of the Standards Of Fundamental Astronomy Board,
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135 | ** the International Astronomical Union nor the names of its
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136 | ** contributors may be used to endorse or promote products derived
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137 | ** from this software without specific prior written permission.
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138 | **
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139 | ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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140 | ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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141 | ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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142 | ** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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143 | ** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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144 | ** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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145 | ** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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146 | ** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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147 | ** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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148 | ** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
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149 | ** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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150 | ** POSSIBILITY OF SUCH DAMAGE.
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151 | **
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152 | */
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