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HELIO_JD

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)
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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