Name

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      XYZ

Purpose

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

Explanation

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      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

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

Input

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

Output

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      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

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      EQUINOX: equinox of output. Default is 1950.

Example

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      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

Procedure Calls

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      PRECESS_XYZ

Revision History

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      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