;+
; NAME:
;  rd2ang
; PURPOSE: (one line)
;  pairs of ra,dec coordinates to great-circle angle
; DESCRIPTION:
;  
; CATEGORY:
;  Astronomy
; CALLING SEQUENCE:
;  ang = rd2ang(A, D, ra, dec)
; INPUTS:
;  A - ra of point 1 (radians)
;  D - dec of point 1 (radians)
;  ra - ra of point 2 (radians)
;  dec -dec of point 2 (radians) 
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  none
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
;  ang - angle between two points (radians)
; COMMON BLOCKS:
; SIDE EFFECTS:
; RESTRICTIONS:
; PROCEDURE:
; MODIFICATION HISTORY:
;  Written 2006 Mar 5, by Leslie Young, SwRI
;-

function rd2ang, A, D, ra, dec

    return, acos( cos(A-ra)*cos(D)*cos(dec) + sin(D)*sin(dec) < 1.d > (-1.d))

end