# HJD to BJD

1. Mar 24, 2008

### natski

Dear all,

I am looking for some code or some equations which I can sure to convert heliocentric julian dates (HJD) to barycentric julian dates (BJD). I know the level of difference is small but this is irrelevant, I still require the conversion none-the-less.

I am aware of a previous thread on this topic...

https://www.physicsforums.com/archive/index.php/t-194645.html

... but no answer was given to this problem. It seems Google cannot provide an answer to this problem either, so can anyone help me?

Natski

2. Mar 25, 2008

### FTL_Diesel

I think that SkyCalc might do this. Perhaps digging through the source code will provide some answers?

What's this for? Pulsar timing?

3. Apr 2, 2008

### natski

Hmmm, this problem remains unresolved. I couldn't find a way of doing it with SkyCalc.

4. Apr 2, 2008

### natski

One of the major problems I've encountered is that I am not sure as to the nature of BJD. Is BJD a coordinate or dynamical timescale?

On http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html it explains that there are two sorts of barycentric times, dynamical and coordinate. BJD is not even mentioned, so whether BJD is the former or latter remains unclear to me.

The difference between these two timescales seems to be on the order of seconds so it is critical even outside of high precision studies.

5. Sep 13, 2010

### jdeast

My apologies for reviving a long dead post, but as of now, this is the top google hit for "HJD to BJD," so some may benefit from an answer to this question...

The BJD can be stated in any time standard, dynamical or coordinate, so the time standard must be specified by the person quoting the BJD (they can differ by more than a minute!). Up until recently, the most commonly used time standard of BJD was UTC -- neither dynamical nor coordinate!

While the IAU recommends the use of the Barycentric Coordinate Time (TCB), if you're concerned about this rate change of 10^-8, you also have to be concerned with the similar rate changes caused by each of your observed targets. Therefore, I argue that the most robust time standard to use in practice is the Barycentric Dynamical Time (TDB).

Now, to answer your original question -- I have created an calculator that can do this conversion (HJD in TT or UTC -> BJD_TDB) here:

http://astroutils.astronomy.ohio-state.edu/time/hjd2bjd.html

However, most HJD calculators are only accurate to 1 second (5 decimal places in JD), so of course you can't get any better accuracy out of it. This is not much better than the +/- 4 second accuracy of the HJD (though converting to the TDB time standard may be a significant improvement).

If at all possible, you're far better off getting the original JD_UTC and using my JD_UTC -> BJD_TDB calculator (http://astroutils.astronomy.ohio-state.edu/time/utc2bjd.html), which is accurate to a few ms if given sufficiently precise inputs (or you can download the IDL source code which is accurate to ~1 us when used with great care -- there's a lot to worry about at that level).

My paper (http://arxiv.org/abs/1005.4415) goes into far more detail on the whole mess.

Last edited: Sep 13, 2010