Talk:Heliocentric Julian Day

Comment
Due to a possible conflict of interest (self-promotion), it may not be appropriate for me to add to this article. However, I think it may be useful for the editors to look at my paper:

http://arxiv.org/abs/1005.4415

and my online BJD_TDB calculator (which probably belongs in a separate article for BJD, but as there is none at the moment...)

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

In particular, I think there are several very important points not addressed in this article:


 * 1) the necessity of specifying the time standards in which one quotes the HJD (which can cause ambiguities of up to one minute)
 * 2) note that the HJD has been deprecated by the IAU in favor of the BJD -- IAU Resolution A4 (1991)
 * 3) the equation is not applicable inside the solar system, as it assumes the wave fronts are plane-parallel, and can be off by as much as 1000 seconds for the Moon, 100 seconds for the Main asteroid belt, and 5 seconds for the Kuiper Belt.
 * 4) A more natural form of the equation (still plane-parallel) is:

$$ HJD = JD + \frac{\vec{r} \cdot \hat{n}}{c} $$

where $$\vec{r}$$ is the the vector from the origin of the new reference frame (in the case of the HJD, the heliocenter) to the observer and $$\hat{n}$$ is the unit vector from the observer to the object which can be written in terms of its right ascension ($$\alpha$$) and declination ($$\delta$$):

$$ \hat{n} = \begin{pmatrix} \cos(\delta)\cos(\alpha) \\ \cos(\delta)\sin(\alpha) \\ \sin(\delta) \end{pmatrix} $$

I have the exact, spherical, equation in my paper too (equation 7).

Jdeast (talk) 19:20, 26 May 2010 (UTC)


 * I'm not a seasoned Wikipedian, but it seems to me that being too scrupulous about self-promotion can lead to articles having to be written by those who are lesser experts, and then probably ignoring the best references. Your paper will go through a peer review at PASP, so it's far from advertising. Although I love cartesian coordinates, the average sidewalk astronomer may find the scalar product a bit offputting. As for the page title, perhaps a redirect page from Barycentric Julian Date is in order. Should "Time scales" be added to the categories? Chi And H (talk) 13:00, 27 May 2010 (UTC)


 * This talk page is my first addition to wikipedia, so I want to make sure I'm playing by the rules...


 * I'd say "Time Standards" should be added, but not without clarifying that to be precise, the HJD must also have a time standard associated with it. It is not, in and of itself, a time standard.


 * Perhaps the "sidewalk astronomer" would prefer the expanded notation:



HJD = JD + \frac{1}{c} \left( X\cos(\delta)\cos(\alpha) + Y\cos(\delta)\sin(\alpha) + Z\sin(\delta) \right) $$


 * where $$X$$, $$Y$$, and $$Z$$ are the Cartesian coordinates of the observer with the Sun at the origin and ($$\alpha,\delta$$) are the target's right ascension and declination, respectively.


 * I don't think a redirect from Barycentric Julian Date to here is a good idea, they already run the risk of being confused with one another. I think it deserves a separate page, with a link in each to the other.


 * Can I suggest an alternate notation? Rather than $$\lambda, \phi$$ for right ascension/declination, use $$\alpha, \delta$$, and use the solar symbol as a subscript:

HJD = JD - (d/c) \left[\sin(\delta)\sin(\delta_{\odot}) + \cos(\delta)\cos(\delta_{\odot})\cos(\alpha-\alpha_{\odot})\right] $$


 * Jdeast (talk) 22:54, 27 May 2010 (UTC)


 * I've reviewed the page. Go ahead and edit it if there's something wrong or could be better. Chi And H (talk) 19:02, 30 May 2010 (UTC)