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82 lines
2.2 KiB
Fortran
82 lines
2.2 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_DPAV ( V1, V2 )
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*+
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* - - - - -
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* D P A V
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* - - - - -
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*
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* Position angle of one celestial direction with respect to another.
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*
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* (double precision)
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*
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* Given:
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* V1 d(3) direction cosines of one point
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* V2 d(3) direction cosines of the other point
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*
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* (The coordinate frames correspond to RA,Dec, Long,Lat etc.)
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*
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* The result is the bearing (position angle), in radians, of point
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* V2 with respect to point V1. It is in the range +/- pi. The
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* sense is such that if V2 is a small distance east of V1, the
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* bearing is about +pi/2. Zero is returned if the two points
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* are coincident.
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*
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* V1 and V2 need not be unit vectors.
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*
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* The routine sla_DBEAR performs an equivalent function except
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* that the points are specified in the form of spherical
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* coordinates.
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*
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* Last revision: 16 March 2005
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*
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* Copyright P.T.Wallace. All rights reserved.
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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 V1(3),V2(3)
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DOUBLE PRECISION X1,Y1,Z1,W,X2,Y2,Z2,SQ,CQ
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* The unit vector to point 1.
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X1 = V1(1)
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Y1 = V1(2)
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Z1 = V1(3)
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W = SQRT(X1*X1+Y1*Y1+Z1*Z1)
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IF (W.NE.0D0) THEN
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X1 = X1/W
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Y1 = Y1/W
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Z1 = Z1/W
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END IF
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* The vector to point 2.
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X2 = V2(1)
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Y2 = V2(2)
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Z2 = V2(3)
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* Position angle.
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SQ = Y2*X1-X2*Y1
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CQ = Z2*(X1*X1+Y1*Y1)-Z1*(X2*X1+Y2*Y1)
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IF (SQ.EQ.0D0.AND.CQ.EQ.0D0) CQ=1D0
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sla_DPAV = ATAN2(SQ,CQ)
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END
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