Name

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      POSANG

Purpose

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      Computes rigorous position angle of source 2 relative to source 1
     

Explanation

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      Computes the rigorous position angle of source 2 (with given RA, Dec)
      using source 1 (with given RA, Dec) as the center.
 

Calling Sequence

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      POSANG, U, RA1, DC1, RA2, DC2, ANGLE

Inputs

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      U -- Describes units of inputs and output:
              0: everything radians
              1: RAx in decimal hours, DCx in decimal
                      degrees, ANGLE in degrees
      RA1 -- Right ascension of point 1
      DC1 -- Declination of point 1
      RA2 -- Right ascension of point 2
      DC2 -- Declination of point 2

Outputs

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      ANGLE-- Angle of the great circle containing [ra2, dc2] from
              the meridian containing [ra1, dc1], in the sense north
              through east rotating about [ra1, dc1]. See U above
              for units.

Procedure

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      The "four-parts formula" from spherical trig (p. 12 of Smart's
      Spherical Astronomy or p. 12 of Green' Spherical Astronomy).

Example

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      For the star 56 Per, the Hipparcos catalog gives a position of
      RA = 66.15593384, Dec = 33.94988843 for component A, and
      RA = 66.15646079, Dec = 33.96100069 for component B. What is the
      position angle of B relative to A?
      IDL> RA1 = 66.15593384/15.d & DC1 = 33.95988843
      IDL> RA2 = 66.15646079/15.d & DC2 = 33.96100069
      IDL> posang,1,ra1,dc1,ra2,dc2, ang
            will give the answer of ang = 21.4 degrees

Notes

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      (1) If RA1,DC1 are scalars, and RA2,DC2 are vectors, then ANGLE is a
      vector giving the position angle between each element of RA2,DC2 and
      RA1,DC1. Similarly, if RA1,DC1 are vectors, and RA2, DC2 are scalars,
      then DIS is a vector giving the position angle of each element of RA1,
      DC1 and RA2, DC2. If both RA1,DC1 and RA2,DC2 are vectors then ANGLE
      is a vector giving the position angle between each element of RA1,DC1
      and the corresponding element of RA2,DC2. If then vectors are not the
      same length, then excess elements of the longer one will be ignored.
      (2) Note that POSANG is not commutative -- the position angle between
        A and B is theta, then the position angle between B and A is 180+theta

Procedure Calls

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        ISARRAY()

History

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      Modified from GCIRC, R. S. Hill, RSTX, 1 Apr. 1998
      Use V6.0 notation W.L. Mar 2011