diff options
Diffstat (limited to 'ast/erfa/tdbtcb.c')
| -rw-r--r-- | ast/erfa/tdbtcb.c | 146 |
1 files changed, 146 insertions, 0 deletions
diff --git a/ast/erfa/tdbtcb.c b/ast/erfa/tdbtcb.c new file mode 100644 index 0000000..c12f807 --- /dev/null +++ b/ast/erfa/tdbtcb.c @@ -0,0 +1,146 @@ +#include "erfa.h" + +int eraTdbtcb(double tdb1, double tdb2, double *tcb1, double *tcb2) +/* +** - - - - - - - - - - +** e r a T d b t c b +** - - - - - - - - - - +** +** Time scale transformation: Barycentric Dynamical Time, TDB, to +** Barycentric Coordinate Time, TCB. +** +** Given: +** tdb1,tdb2 double TDB as a 2-part Julian Date +** +** Returned: +** tcb1,tcb2 double TCB as a 2-part Julian Date +** +** Returned (function value): +** int status: 0 = OK +** +** Notes: +** +** 1) tdb1+tdb2 is Julian Date, apportioned in any convenient way +** between the two arguments, for example where tdb1 is the Julian +** Day Number and tdb2 is the fraction of a day. The returned +** tcb1,tcb2 follow suit. +** +** 2) The 2006 IAU General Assembly introduced a conventional linear +** transformation between TDB and TCB. This transformation +** compensates for the drift between TCB and terrestrial time TT, +** and keeps TDB approximately centered on TT. Because the +** relationship between TT and TCB depends on the adopted solar +** system ephemeris, the degree of alignment between TDB and TT over +** long intervals will vary according to which ephemeris is used. +** Former definitions of TDB attempted to avoid this problem by +** stipulating that TDB and TT should differ only by periodic +** effects. This is a good description of the nature of the +** relationship but eluded precise mathematical formulation. The +** conventional linear relationship adopted in 2006 sidestepped +** these difficulties whilst delivering a TDB that in practice was +** consistent with values before that date. +** +** 3) TDB is essentially the same as Teph, the time argument for the +** JPL solar system ephemerides. +** +** Reference: +** +** IAU 2006 Resolution B3 +** +** Copyright (C) 2013-2016, NumFOCUS Foundation. +** Derived, with permission, from the SOFA library. See notes at end of file. +*/ +{ + +/* 1977 Jan 1 00:00:32.184 TT, as two-part JD */ + static const double t77td = ERFA_DJM0 + ERFA_DJM77; + static const double t77tf = ERFA_TTMTAI/ERFA_DAYSEC; + +/* TDB (days) at TAI 1977 Jan 1.0 */ + static const double tdb0 = ERFA_TDB0/ERFA_DAYSEC; + +/* TDB to TCB rate */ + static const double elbb = ERFA_ELB/(1.0-ERFA_ELB); + + double d, f; + + +/* Result, preserving date format but safeguarding precision. */ + if ( tdb1 > tdb2 ) { + d = t77td - tdb1; + f = tdb2 - tdb0; + *tcb1 = tdb1; + *tcb2 = f - ( d - ( f - t77tf ) ) * elbb; + } else { + d = t77td - tdb2; + f = tdb1 - tdb0; + *tcb1 = f + ( d - ( f - t77tf ) ) * elbb; + *tcb2 = tdb2; + } + +/* Status (always OK). */ + return 0; + +} +/*---------------------------------------------------------------------- +** +** +** Copyright (C) 2013-2016, 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. IN NO EVENT SHALL THE +** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, +** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, +** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; +** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER +** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN +** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE +** POSSIBILITY OF SUCH DAMAGE. +** +*/ |