# Converting HJD to BJD: Equation and Code Solutions | Natski

• natski
In summary, Natski has created a calculator to convert heliocentric Julian dates to barycentric Julian dates. This is useful for high precision studies, but is not accurate to seconds. There are two time standards for barycentric Julian dates, coordinate and dynamical. The most robust time standard to use is the Barycentric Dynamical Time.
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

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

What's this for? Pulsar timing?

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

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.

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...

natski said:
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.

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.

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## 1. What is the difference between HJD and BJD?

The Heliocentric Julian Date (HJD) is the time measurement system based on the position of the Earth in its orbit around the Sun, while Barycentric Julian Date (BJD) is based on the center of mass of the solar system. This difference is important for precise measurements of astronomical events, as it accounts for the Earth's motion.

## 2. Why is it necessary to convert HJD to BJD?

Converting HJD to BJD is necessary because HJD is based on the Earth's position, which can change over time due to the Earth's orbit and rotation. This variation can lead to errors in astronomical data, so converting to BJD, which is based on the center of mass of the solar system, results in more accurate and consistent measurements.

## 3. How is the conversion from HJD to BJD done?

The conversion from HJD to BJD is done using software or algorithms that take into account the Earth's position and motion. It involves calculating the difference between the two time systems and applying a correction factor to account for the change in the Earth's position.

## 4. Is there a standard conversion method for HJD to BJD?

Yes, there are several standard methods for converting HJD to BJD, such as the Eastman, Siverd, and Gaudi method or the Eastman and Agol method. These methods are commonly used in astronomy research and are based on calculations that account for the Earth's motion and the time difference between the two systems.

## 5. How important is it to accurately convert HJD to BJD?

Accurately converting HJD to BJD is crucial for precise astronomical measurements and research. It ensures that data is consistent and comparable across different observations and eliminates errors that may result from variations in the Earth's position. Therefore, it is essential to use reliable and standardized methods for converting HJD to BJD in scientific studies.

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