Heliocentric Julian date

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  • #573519
    James Screech
    Participant
    What is the algorithm for calculating Heliocentric Julian date from a given Julian date and object RA/Dec?
    I would like this for a variable star application I’m writing.
    James
    #577213
    Len Entwisle
    Participant

    There is a calculator on the BAA Computing Section site here http://britastro.org/computing/applets_dt.html

    #577216
    Nick James
    Participant

    There are quite a few examples of code around that convert between different timescales. Probably the best examples are the IDL source code here. Note that BJD is barycentric JD which, for most VS purposes, is the same as HJD. I think the difference only matters if you are trying to detect planets going around pulsars…

    #577217
    James Screech
    Participant

    I’ve now compared 3 different resources for the calculation and they all come out different! I’ve tried the BAA applet, a sppreadsheet I found on the web (HeliocentricJulianDay.xls) and the ohio-state.edu web site. For a test I’ve used an observation of NR Cam at JD 2457388.40539.

    BAA app -> 4.609 min (276.54 sec)
    Spreadsheet -> 281.3 sec
    ohio-state.edu -> 344.98 sec

    The first two are less than 5sec different however the ohio site is over a minute off from the others. It would appear that they all use different methods of calculation that are not consistent! I’m wondering if there are issues with floating point rounding due to the number of significant figures required.

    James

    #577219
    Nick James
    Participant

    I think the reason for the difference is that the two values of JD that you calculated are in different timescales. I assume that the BAA calculator calculates HJD in the UTC timescale whereas the Ohio site calculates it in TDB. At present TDB is around 68.2s ahead of UTC so a BJD(TDB) will be around 68s later than HJD(UTC). When talking about JD it is important to specify the timescale used. The difference between the Ohio and BAA times is 68.44s which agrees well.

    #579270
    John Mallett
    Participant

    I need an application (exe) that will convert JD(UTC) to BJD.  For very short period binary systems that may have other variations due to bodies in orbit.

    The trouble is that the variations are hidden by the errors in time shift to BJD

    John

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