idlastro / Astronomical Utilities: HELIO_JD

[Source code]

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)
RLINK      IDL
1999-10-29T000000.0 21 08 25. -67 22 00. -59.0 -59.0
1999-10-29T000000.0 02 56 33.4 +00 26 55. 474.1 474.1
1940-12-11T065500.0 07 34 41.9 -00 30 42. 366.3 370.2
1992-02-29T031556.2 12 56 27.4 +42 10 17. 350.8 350.9
2000-03-01T102631.8 14 28 36.7 -20 42 11. 243.7 243.7
2100-02-26T091824.2 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