XYZ

       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).

       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.

       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
       abs(err)
           (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
       abs(err)
        (km/sec): 0.00265        0.00519          0.00557

       PRECESS_XYZ

       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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