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HELIO_JD

HELIO_JD

Name


      HELIO_JD

Purpose


      Convert geocentric (reduced) Julian date to heliocentric Julian date

Explanation


      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


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

Inputs


      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


      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


      bprecess, xyz, zparcheck

Revision History


      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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