HELIO_JD

      Convert geocentric (reduced) Julian date to heliocentric Julian date

      This procedure correct for the extra light travel time between the Earth
      and the Sun.

       jdhelio = HELIO_JD( date, ra, dec, /B1950, /TIME_DIFF)

       date - reduced Julian date (= JD - 2400000), scalar or vector, MUST
               be double precision
       ra,dec - scalars giving right ascension and declination in DEGREES
               Equinox is J2000 unless the /B1950 keyword is set

       jdhelio - heliocentric reduced Julian date.  If /TIME_DIFF is set, then
                 HELIO_JD() instead returns the time difference in seconds
                 between the geocentric and heliocentric Julian date.

       /B1950 - if set, then input coordinates are assumed to be in equinox
                B1950 coordinates.
       /TIME_DIFF - if set, then HELIO_JD() returns the time difference
                (heliocentric JD - geocentric JD ) in seconds

       What is the heliocentric Julian date of an observation of V402 Cygni
       (J2000: RA = 20 9 7.8, Dec = 37 09 07) taken June 15, 1973 at 11:40 UT?
       IDL> juldate, [1973,6,15,11,40], jd      ;Get geocentric Julian date
       IDL> hjd = helio_jd( jd, ten(20,9,7.8)*15., ten(37,9,7) )
       ==> hjd = 41848.9881
 Wayne Warren (Raytheon ITSS) has compared the results of HELIO_JD with the
 FORTRAN subroutines in the STARLINK SLALIB library (see
 http://star-www.rl.ac.uk/).
                                                  Time Diff (sec)
      Date               RA(2000)   Dec(2000)  STARLINK      IDL
 1999-10-29T00:00:00.0  21 08 25.  -67 22 00.  -59.0        -59.0
 1999-10-29T00:00:00.0  02 56 33.4 +00 26 55.  474.1        474.1
 1940-12-11T06:55:00.0  07 34 41.9 -00 30 42.  366.3        370.2
 1992-02-29T03:15:56.2  12 56 27.4 +42 10 17.  350.8        350.9
 2000-03-01T10:26:31.8  14 28 36.7 -20 42 11.  243.7        243.7
 2100-02-26T09:18:24.2  08 26 51.7 +85 47 28.  104.0        108.8

       bprecess, xyz, zparcheck

       Algorithm from the book Astronomical Photometry by Henden, p. 114
       Written,   W. Landsman       STX     June, 1989
       Make J2000 default equinox, add B1950, /TIME_DIFF keywords, compute
       variation of the obliquity      W. Landsman   November 1999

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