PLANET_COORDS

    Find low or high precision RA and DEC for the planets given a date

    For low precision this routine uses HELIO to get the heliocentric ecliptic
    coordinates of the planets at the given date, then converts these to
    geocentric ecliptic coordinates ala "Astronomical Algorithms" by Jean
    Meeus (1991, p 209). These are then converted to RA and Dec using EULER.
    The accuracy between the years 1800 and 2050 is better than 1 arcminute
    for  the terrestial planets, but reaches 10 arcminutes for Saturn.
    Before 1850 or after 2050 the accuracy can get much worse.
    For high precision use the /JPL option ito use the full JPL ephemeris.

    PLANET_COORDS, DATE, RA, DEC, [ PLANET = , /JD, /JPL]

       DATE - If /JD is not set, then date is a 3-6 element vector containing
              year,month (1-12), day, and optionally hour, minute, & second.
              If /JD is set then DATE is a Julian date.   An advantage of the
              /JD option is that it allows the use of vector dates.

       RA - right ascension of planet(s), J2000 degrees, double precision
       DEC - declination of   planet(s), J2000 degrees, double precision

       PLANET - scalar string giving name of a planet, e.g. 'venus'. Default
               is to compute coords for all of them (except Earth).
       /JD - If set, then the date parameter should be supplied as Julian date
       JPL - if /JPL set, then PLANET_COORDS will call the procedure
             JPLEPHINTERP to compute positions 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 JPLEPH.405 is available in
                 http://idlastro.gsfc.nasa.gov/ftp/data/

    (1)  Find the RA, Dec of Venus on 1992 Dec 20
          IDL> planet_coords, [1992,12,20], ra,dec    ;Compute for all planets
          IDL> print,adstring(ra[1],dec[1],1)         ;Venus is second planet
     ====> RA = 21 05  2.66  Dec = -18 51 45.7
    This position is 37" from the full DE406 ephemeris position of
          RA = 21 05  5.24        -18 51 43.1
    (2) Return the current RA and Dec of all 8 planets using JPL ephemeris
          IDL> get_juldate, jd                 ;Get current Julian Date
          IDL> planet_coords,jd,ra,dec,/jd,/jpl  ;Find positions of all planets
          IDL> forprint,adstring(ra,dec,0)     ;Display positions
    (3) Plot the declination of Mars for every day in the year 2001
          IDL> jdcnv,2001,1,1,0,jd      ;Get Julian date of midnight on Jan 1
               Now get Mars RA,Dec for 365 consecutive days
          IDL> planet_coords,jd+indgen(365),ra,dec,/jd, planet = 'mars'
          IDL> plot,indgen(365)+1,dec

          HELIO is based on the two-body problem and neglects interactions
           between the planets.   This is why the worst results are for
           Saturn.   Use the /JPL option or the online ephemeris generator
           http://ssd.jpl.nasa.gov/cgi-bin/eph for more accuracy.
           The procedure returns astrometric coordinates, i.e. no correction
           for aberration.   A correction for light travel time is applied
           when /JPL is set, but not for the default low-precision calculation.

        JULDATE
        EULER, HELIO  - if /JPL is not set
        JPLEPHREAD, JPLEPHINTERP - if /JPL is set

        Written P.Plait & W. Landsman     August 2000
        Fixed Julian date conversion   W. Landsman August 2000
        Added /JPL keyword  W. Landsman   July 2001
        Allow vector Julian dates with JPL ephemeris W. Landsman December 2002

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