Name

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    GCIRC

Purpose

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    Computes rigorous great circle arc distances.

Explanation

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    Input position can either be either radians, sexagesimal RA, Dec or
    degrees. All computations are double precision.

Calling Sequence

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      GCIRC, U, RA1, DC1, RA2, DC2, DIS

Inputs

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      U -- integer = 0,1, or 2: Describes units of inputs and output:
              0: everything radians
              1: RAx in decimal hours, DCx in decimal
                      degrees, DIS in arc seconds
              2: RAx and DCx in degrees, DIS in arc seconds
      RA1 -- Right ascension or longitude of point 1
      DC1 -- Declination or latitude of point 1
      RA2 -- Right ascension or longitude of point 2
      DC2 -- Declination or latitude of point 2

Outputs

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      DIS -- Angular distance on the sky between points 1 and 2
              See U above for units; double precision

Procedure

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      "Haversine formula" see
      http://en.wikipedia.org/wiki/Great-circle_distance

Notes

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      (1) If RA1,DC1 are scalars, and RA2,DC2 are vectors, then DIS is a
      vector giving the distance of each element of RA2,DC2 to RA1,DC1.
      Similarly, if RA1,DC1 are vectors, and RA2, DC2 are scalars, then DIS
      is a vector giving the distance of each element of RA1, DC1 to
      RA2, DC2. If both RA1,DC1 and RA2,DC2 are vectors then DIS is a
      vector giving the distance of each element of RA1,DC1 to the
      corresponding element of RA2,DC2. If the input vectors are not the
      same length, then excess elements of the longer ones will be ignored.
      (2) The function SPHDIST provides an alternate method of computing
        a spherical distance.
      (3) The haversine formula can give rounding errors for antipodal
      points.

Procedure Calls

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      None

Modification History

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      Written in Fortran by R. Hill -- SASC Technologies -- January 3, 1986
      Translated from FORTRAN to IDL, RSH, STX, 2/6/87
      Vector arguments allowed W. Landsman April 1989
      Prints result if last argument not given. RSH, RSTX, 3 Apr. 1998
      Remove ISARRAY(), V5.1 version W. Landsman August 2000
      Added option U=2 W. Landsman October 2006
      Use double precision for U=0 as advertised R. McMahon/W.L. April 2007
      Use havesine formula, which has less roundoff error in the
            milliarcsecond regime W.L. Mar 2009