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REAL FUNCTION sla_SEP (A1, B1, A2, B2)
*+
* - - - -
* S E P
* - - - -
*
* Angle between two points on a sphere (single precision)
*
* Given:
* A1,B1 real spherical coordinates of one point
* A2,B2 real spherical coordinates of the other point
*
* (The spherical coordinates are RA,Dec, Long,Lat etc, in radians.)
*
* The result is the angle, in radians, between the two points. It
* is always positive.
*
* Called: sla_CS2C
*
* P.T.Wallace Starlink April 1985
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
REAL A1,B1,A2,B2
INTEGER I
REAL V1(3),V2(3),W
* Convert coordinates from spherical to Cartesian
CALL sla_CS2C(A1,B1,V1)
CALL sla_CS2C(A2,B2,V2)
* Modulus squared of half the difference vector
W=0.0
DO I=1,3
W=W+(V1(I)-V2(I))**2
END DO
W=W/4.0
* Angle between the vectors
sla_SEP=2.0*ATAN2(SQRT(W),SQRT(MAX(0.0,1.0-W)))
END
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