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include <math.h>
# AST_ARCSEP -- Arc distance (arcsec) between two spherical coordinates
# (hours,deg).
double procedure ast_arcsep (ra1, dec1, ra2, dec2)
double ra1 # Right ascension (hours)
double dec1 # Declination (degrees)
double ra2 # Right ascension (hours)
double dec2 # Declination (degrees)
double a1, b1, c1, a2, b2, c2
begin
# Direction cosines
a1 = cos (DEGTORAD (15D0 * ra1)) * cos (DEGTORAD (dec1))
b1 = sin (DEGTORAD (15D0 * ra1)) * cos (DEGTORAD (dec1))
c1 = sin (DEGTORAD (dec1))
a2 = cos (DEGTORAD (15D0 * ra2)) * cos (DEGTORAD (dec2))
b2 = sin (DEGTORAD (15D0 * ra2)) * cos (DEGTORAD (dec2))
c2 = sin (DEGTORAD (dec2))
# Modulus squared of half the difference vector.
a1 = ((a1-a2)**2 + (b1-b2)**2 + (c1-c2)**2) / 4
# Angle
a1 = 2D0 * atan2 (sqrt (a1), sqrt (max (0D0, 1D0-a1)))
a1 = RADTODEG (a1) * 3600D0
return (a1)
end
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