Name

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      HELIO_JD

Purpose

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      Convert geocentric (reduced) Julian date to heliocentric Julian date

Explanation

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      This procedure correct for the extra light travel time between the Earth
      and the Sun.
      An online calculator for this quantity is available at
      http://www.physics.sfasu.edu/astro/javascript/hjd.html
      Users requiring more precise calculations and documentation should
      look at the IDL code available at
      http://astroutils.astronomy.ohio-state.edu/time/

Calling Sequence

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

Inputs

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      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

Outputs

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      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.
               

Optional Input Keywords

---

      /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

Example

---

      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

Procedures Called

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      bprecess, xyz, zparcheck

Revision History

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      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