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      Calculates heliocentric and barycentric velocity components of Earth.


      BARYVEL takes into account the Earth-Moon motion, and is useful for
      radial velocity work to an accuracy of ~1 m/s.

Calling Sequence

      BARYVEL, dje, deq, dvelh, dvelb, [ JPL = ]


      DJE - (scalar) Julian ephemeris date.
      DEQ - (scalar) epoch of mean equinox of dvelh and dvelb. If deq=0
              then deq is assumed to be equal to dje.


      DVELH: (vector(3)) heliocentric velocity component. in km/s
      DVELB: (vector(3)) barycentric velocity component. in km/s
      The 3-vectors DVELH and DVELB are given in a right-handed coordinate
      system with the +X axis toward the Vernal Equinox, and +Z axis
      toward the celestial pole.
      JPL - if /JPL set, then BARYVEL will call the procedure JPLEPHINTERP
            to compute the Earth velocity using the full JPL ephemeris.
            The JPL ephemeris FITS file JPLEPH.405 must exist in either the
            current directory, or in the directory specified by the
            environment variable ASTRO_DATA. Alternatively, the JPL keyword
            can be set to the full path and name of the ephemeris file.
            A copy of the JPL ephemeris FITS file is available in

Procedures Called

      Function PREMAT() -- computes precession matrix
      JPLEPHREAD, JPLEPHINTERP, TDB2TDT - if /JPL keyword is set


      Algorithm taken from FORTRAN program of Stumpff (1980, A&A Suppl, 41,1)
      Stumpf claimed an accuracy of 42 cm/s for the velocity. A
      comparison with the JPL FORTRAN planetary ephemeris program PLEPH
      found agreement to within about 65 cm/s between 1986 and 1994
      If /JPL is set (using JPLEPH.405 ephemeris file) then velocities are
      given in the ICRS system; otherwise in the FK4 system.


      Compute the radial velocity of the Earth toward Altair on 15-Feb-1994
          using both the original Stumpf algorithm and the JPL ephemeris
      IDL> jdcnv, 1994, 2, 15, 0, jd ;==> JD = 2449398.5
      IDL> baryvel, jd, 2000, vh, vb ;Original algorithm
              ==> vh = [-17.07243, -22.81121, -9.889315] ;Heliocentric km/s
              ==> vb = [-17.08083, -22.80471, -9.886582] ;Barycentric km/s
      IDL> baryvel, jd, 2000, vh, vb, /jpl ;JPL ephemeris
              ==> vh = [-17.07236, -22.81126, -9.889419] ;Heliocentric km/s
              ==> vb = [-17.08083, -22.80484, -9.886409] ;Barycentric km/s
      IDL> ra = ten(19,50,46.77)*15/!RADEG ;RA in radians
      IDL> dec = ten(08,52,3.5)/!RADEG ;Dec in radians
      IDL> v = vb[0]*cos(dec)*cos(ra) + $ ;Project velocity toward star
              vb[1]*cos(dec)*sin(ra) + vb[2]*sin(dec)

Revision History

      Jeff Valenti, U.C. Berkeley Translated BARVEL.FOR to IDL.
      W. Landsman, Cleaned up program sent by Chris McCarthy (SfSU) June 1994
      Converted to IDL V5.0 W. Landsman September 1997
      Added /JPL keyword W. Landsman July 2001
      Documentation update W. Landsman Dec 2005

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