# Homework Help: Barycentric Julian Date (astronomy)

1. Oct 29, 2007

### big man

1. The problem statement, all variables and given/known data
I'm wanting to convert barycentric julian date to the equivalent UT time. Here is a description of the barycentric julian date given to us:

Barycentric Julian Date (TT) – Observation epoch, given in terrestrial time, that has been corrected for light propagation time to the solar system barycentre.

3. The attempt at a solution
Now I know how to convert Julian date to UT and I know that $$Terrestrial Time = UT + \delta T$$, where the value dT can be obtained from the web. But I really wouldn't say that I understand or have found a definition of TT that makes me understand exactly what it is. Is the correction for light propagation time to the solar system barycentre just terrestrial time?

Thanks

2. Sep 12, 2010

### jdeast

My apologies for reviving such a long-dead thread, but as it's still one of the top hits for Barycentric Julian Date, some may benefit from a belated answer, if not the original poster.

UT is an ambiguous term and may mean UTC, UT1, or many other variants, all of which may vary by up to 0.9 seconds from one another. As you've described it, you're talking about UT1. However, you probably actually want UTC (if you care about the difference at all), since UTC is the international standard, and what your clock is most likely to use. It's also much easier to calculate, as it depends on discrete leap seconds, not the continuously variable quantity $$\delta t$$.

If I'm right about that, or you don't care about the 0.9 second difference, you can use my online calculator (http://astroutils.astronomy.ohio-state.edu/time/bjd2utc.html), which converts from Barycentric Julian Date in Barycentric Dynamical Time (BJD_TDB) to the Julian Date in Coordinated Universal Time (JD_UTC) for objects outside of the solar system.

Note: BJD_TDB is the same as BJD in Terrestial Time (BJD_TT) to with +/- 1.7 ms.

Doing the precise conversion yourself is extremely involved (see the paper linked at the end of my response for the steps if you really want to...).

No, the observed time corrected for the light propagation time from the observer to the Solar System Barycenter is the Barycentric Julian Date (BJD). However, the BJD can be specified in any time standard (eg, TT, TDB, UTC, etc).

Terrestrial Time (TT) is a time standard. Wikipedia has an excellent article describing it if you care about the details. The important thing is that it is commonly used in astronomical timing and can differ from UTC by more than 1 minute.

A more thorough explanation of the BJD, time standards, and more than you ever wanted to know about astronomical timing can be found in my paper:
http://arxiv.org/abs/1005.4415