HELIO

      Compute (low-precision) heliocentric coordinates for the planets.

      The mean orbital elements for epoch J2000 are used.   These are derived
      from a 250 yr least squares fit of the DE 200 planetary ephemeris to a
      Keplerian orbit where each element is allowed to vary linearly with
      time.  For dates between 1800 and 2050, this solution fits the
      terrestrial planet orbits to ~25" or better, but achieves only ~600"
      for Saturn.
      Use PLANET_COORDS (which calls HELIO) to get celestial (RA, Dec)
      coordinates of the planets

       HELIO, JD, LIST, HRAD, HLONG, HLAT, [/RADIAN]

       JD = Julian date, double precision scalar or vector
       LIST = List of planets array.  May be a single number.
               1 = merc, 2 = venus, ... 9 = pluto.

       HRAD = array of Heliocentric radii (A.U).
       HLONG = array of Heliocentric (ecliptic) longitudes (degrees).
       HLAT = array of Heliocentric latitudes (degrees).
             These output parameters will be dimensioned Nplanet by Ndate,
             where Nplanet is the number of elements of list, and Ndate is
             the number of elements of JD.

       /RADIAN - If set, then the output longitude and latitude are given in
                 radians.

       (1) Find the current heliocentric positions of all the planets
        IDL> GET_JULDATE, jd      ;Get current Julian date
        IDL> HELIO,jd,indgen(9)+1,hrad,hlong,hlat  ;Get radius, long, and lat
       (2) Find heliocentric position of Mars on August 23, 2000
         IDL> JDCNV, 2000,08,23,0,jd
         IDL> HELIO,JD,2,HRAD,HLONG,HLAT
                  ===> hrad = 1.6407 AU hlong = 124.3197 hlat = 1.7853
         For comparison, the JPL ephemeris gives
                       hrad = 1.6407 AU hlong = 124.2985 hlat = 1.7845
       (3) Find the heliocentric positions of Mars and Venus for every day in
           November 2000
        IDL> JDCNV, 2000, 11, 1, 0, jd    ;Julian date of November 1, 2000
        IDL> helio, jd+indgen(30), [4,2], hrad,hlong,hlat   ;Mars=4, Venus=2
                   hrad, hlong, and hlat will be dimensioned [2,30]
                   first column contains Mars data, second column Venus

       None

       CIRRANGE - force angle between 0 and 2*!PI

       (1) The calling sequence for this procedure was changed in August 2000
       (2) This program is based on the two-body model and thus neglects
           interactions between the planets.   This is why the worst results
           are for Saturn.  Use the procedure JPLEPHINTERp for more accurate
           positions using the JPL ephemeris.   Also see
           http://ssd.jpl.nasa.gov/cgi-bin/eph for a more accurate ephemeris
           generator online.
       (3) The coordinates are given for equinox 2000 and *not* the equinox
           of the supplied date(s)

       R. Sterner.  20 Aug, 1986.
       Code cleaned up a bit      W. Landsman             December 1992
       Converted to IDL V5.0   W. Landsman   September 1997
       Major rewrite, use modern orbital elements, vectorize, more accurate
         solution to Kepler's equation          W. Landsman August 2000
       Wasn't working for planet vectors        W. Landsman August 2000

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