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60 lines
1.8 KiB
Fortran
60 lines
1.8 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_DBEAR (A1, B1, A2, B2)
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*+
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* - - - - - -
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* D B E A R
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* - - - - - -
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*
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* Bearing (position angle) of one point on a sphere relative to another
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* (double precision)
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*
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* Given:
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* A1,B1 d spherical coordinates of one point
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* A2,B2 d spherical coordinates of the other point
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*
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* (The spherical coordinates are RA,Dec, Long,Lat etc, in radians.)
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*
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* The result is the bearing (position angle), in radians, of point
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* A2,B2 as seen from point A1,B1. It is in the range +/- pi. If
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* A2,B2 is due east of A1,B1 the bearing is +pi/2. Zero is returned
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* if the two points are coincident.
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*
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* P.T.Wallace Starlink 23 March 1991
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*
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* Copyright (C) 1995 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION A1,B1,A2,B2
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DOUBLE PRECISION DA,X,Y
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DA=A2-A1
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Y=SIN(DA)*COS(B2)
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X=SIN(B2)*COS(B1)-COS(B2)*SIN(B1)*COS(DA)
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IF (X.NE.0D0.OR.Y.NE.0D0) THEN
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sla_DBEAR=ATAN2(Y,X)
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ELSE
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sla_DBEAR=0D0
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END IF
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END
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