DOUBLE PRECISION FUNCTION iau_HD2PA ( HA, DEC, PHI ) *+ * - - - - - - - - - - * i a u _ H D 2 P A * - - - - - - - - - - * * Parallactic angle for a given hour angle and declination. * * Given: * HA d hour angle * DEC d declination * PHI d site latitude * * Returned: * iau_HD2PA d parallactic angle * * Notes: * * 1) All the arguments are angles in radians. * * 2) The parallactic angle at a point in the sky is the position angle * of the vertical, i.e. the angle between the directions to the * north celestial pole and to the zenith respectively. * * 3) The result is returned in the range -pi to +pi. * * 4) At the pole itself a zero result is returned. * * 5) The latitude PHI is pi/2 minus the angle between the Earth's * rotation axis and the adopted zenith. In many applications it * will be sufficient to use the published geodetic latitude of the * site. In very precise (sub-arcsecond) applications, PHI can be * corrected for polar motion. * * 6) Should the user wish to work with respect to the astronomical * zenith rather than the geodetic zenith, PHI will need to be * adjusted for deflection of the vertical (often tens of * arcseconds), and the zero point of HA will also be affected. * * Reference: * Smart, W.M., "Spherical Astronomy", Cambridge University Press, * 6th edition (Green, 1977), p49. * * This revision: 2017 September 12 * * SOFA release 2021-05-12 * * Copyright (C) 2021 IAU SOFA Board. See notes at end. * *----------------------------------------------------------------------- IMPLICIT NONE DOUBLE PRECISION HA, DEC, PHI DOUBLE PRECISION CP, SQSZ, CQSZ * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - CP = COS(PHI) SQSZ = CP*SIN(HA) CQSZ = SIN(PHI)*COS(DEC) - CP*SIN(DEC)*COS(HA) IF ( SQSZ.EQ.0D0 .AND. CQSZ.EQ.0D0 ) CQSZ = 1D0 iau_HD2PA = ATAN2(SQSZ,CQSZ) * Finished. *+---------------------------------------------------------------------- * * Copyright (C) 2021 * Standards Of Fundamental Astronomy Board * of the International Astronomical Union. * * ===================== * SOFA Software License * ===================== * * NOTICE TO USER: * * BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND * CONDITIONS WHICH APPLY TO ITS USE. * * 1. The Software is owned by the IAU SOFA Board ("SOFA"). * * 2. Permission is granted to anyone to use the SOFA software for any * purpose, including commercial applications, free of charge and * without payment of royalties, subject to the conditions and * restrictions listed below. * * 3. You (the user) may copy and distribute SOFA source code to others, * and use and adapt its code and algorithms in your own software, * on a world-wide, royalty-free basis. That portion of your * distribution that does not consist of intact and unchanged copies * of SOFA source code files is a "derived work" that must comply * with the following requirements: * * a) Your work shall be marked or carry a statement that it * (i) uses routines and computations derived by you from * software provided by SOFA under license to you; and * (ii) does not itself constitute software provided by and/or * endorsed by SOFA. * * b) The source code of your derived work must contain descriptions * of how the derived work is based upon, contains and/or differs * from the original SOFA software. * * c) The names of all routines in your derived work shall not * include the prefix "iau" or "sofa" or trivial modifications * thereof such as changes of case. * * d) The origin of the SOFA components of your derived work must * not be misrepresented; you must not claim that you wrote the * original software, nor file a patent application for SOFA * software or algorithms embedded in the SOFA software. * * e) These requirements must be reproduced intact in any source * distribution and shall apply to anyone to whom you have * granted a further right to modify the source code of your * derived work. * * Note that, as originally distributed, the SOFA software is * intended to be a definitive implementation of the IAU standards, * and consequently third-party modifications are discouraged. 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The provision of any version of the SOFA software under the terms * and conditions specified herein does not imply that future * versions will also be made available under the same terms and * conditions. * * In any published work or commercial product which uses the SOFA * software directly, acknowledgement (see www.iausofa.org) is * appreciated. * * Correspondence concerning SOFA software should be addressed as * follows: * * By email: sofa@ukho.gov.uk * By post: IAU SOFA Center * HM Nautical Almanac Office * UK Hydrographic Office * Admiralty Way, Taunton * Somerset, TA1 2DN * United Kingdom * *----------------------------------------------------------------------- END