function helio_jd,date,ra,dec, B1950 = B1950, TIME_DIFF = time_diff ;+ ; 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 ;- On_error,2 If N_params() LT 3 then begin print,'Syntax - jdhelio = HELIO_JD( date, ra, dec, /B1950, /TIME_DIFF)' print,' date - reduced Julian date (= JD - 2400000)' print,' Ra and Dec must be in degrees' endif ;Because XYZ uses default B1950 coordinates, we'll convert everything to B1950 if not keyword_set(B1950) then bprecess,ra,dec,ra1,dec1 else begin ra1 = ra dec1 = dec endelse radeg = 180.0d/!DPI zparcheck,'HELIO_JD',date,1,[3,4,5],[0,1],'Reduced Julian Date' delta_t = (double(date) - 33282.42345905d)/36525.0d epsilon_sec = poly( delta_t, [44.836d, -46.8495, -0.00429, 0.00181]) epsilon = (23.433333d0 + epsilon_sec/3600.0d)/radeg ra1 = ra1/radeg dec1 = dec1/radeg xyz, date, x, y, z ;Find extra distance light must travel in AU, multiply by 1.49598e13 cm/AU, ;and divide by the speed of light, and multiply by 86400 second/year time = -499.00522d*( cos(dec1)*cos(ra1)*x + \$ (tan(epsilon)*sin(dec1) + cos(dec1)*sin(ra1))*y) if keyword_set(TIME_DIFF) then return, time else \$ return, double(date) + time/86400.0d end