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      Calculate geocentric X,Y, and Z and velocity coordinates of the Sun


      Calculates geocentric X,Y, and Z vectors and velocity coordinates
      (dx, dy and dz) of the Sun. (The positive X axis is directed towards
      the equinox, the y-axis, towards the point on the equator at right
      ascension 6h, and the z axis toward the north pole of the equator).
      Typical position accuracy is <1e-4 AU (15000 km).

Calling Sequence

      XYZ, date, x, y, z, [ xvel, yvel, zvel, EQUINOX = ]


      date: reduced julian date (=JD - 2400000), scalar or vector


      x,y,z: scalars or vectors giving heliocentric rectangular coordinates
                (in A.U) for each date supplied. Note that sqrt(x^2 + y^2
                + z^2) gives the Earth-Sun distance for the given date.
      xvel, yvel, zvel: velocity vectors corresponding to X, Y and Z.

Optional Keyword Input

      EQUINOX: equinox of output. Default is 1950.


      What were the rectangular coordinates and velocities of the Sun on
      Jan 22, 1999 0h UT (= JD 2451200.5) in J2000 coords? NOTE:
      Astronomical Almanac (AA) is in TDT, so add 64 seconds to
      UT to convert.
      IDL> xyz,51200.5+64.d/86400.d,x,y,z,xv,yv,zv,equinox = 2000
      Compare to Astronomical Almanac (1999 page C20)
                  X (AU) Y (AU) Z (AU)
      XYZ: 0.51456871 -0.76963263 -0.33376880
      AA: 0.51453130 -0.7697110 -0.3337152
      abs(err): 0.00003739 0.00007839 0.00005360
          (km): 5609 11759 8040
      NOTE: Velocities in AA are for Earth/Moon barycenter
            (a very minor offset) see AA 1999 page E3
                  X VEL (AU/DAY) YVEL (AU/DAY) Z VEL (AU/DAY)
      XYZ: -0.014947268 -0.0083148382 -0.0036068577
      AA: -0.01494574 -0.00831185 -0.00360365
      abs(err): 0.000001583 0.0000029886 0.0000032077
        (km/sec): 0.00265 0.00519 0.00557

Procedure Calls


Revision History

      Original algorithm from Almanac for Computers, Doggett et al. USNO 1978
      Adapted from the book Astronomical Photometry by A. Henden
      Written W. Landsman STX June 1989
      Correct error in X coefficient W. Landsman HSTX January 1995
      Added velocities, more terms to positions and EQUINOX keyword,
          some minor adjustments to calculations
          P. Plait/ACC March 24, 1999

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