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)