Constellation boundaries app

[Demonperformer]

I know I can get a list of constellation boundaries here: http://www.iau.org/public/constellations/

But as the boundaries don't seem to follow RA & Dec, it can be a pain working out exactly where one constellation becomes another along the line.

For example: the first listed boundary in Andromeda runs from 22h57m51.6729s RA 35.1682358 degrees dec to 22h 57m 22.2843s RA 53.1680298 degrees dec, so is 22h57m41.5s RA 41.9531 degrees dec in Andromeda or across the boundary?

I therefore started thinking that an appl that could take as input the RA & Dec of an object and output the correct constellation would make life a lot easier. I personally think it would also make life easier for any set of co-ordinates rather than having to check the atlas, but I would accept that for an object well within the boundary it is not such an issue.

Before I start the laborious task of doing all the math to try and do this, I wonder if anyone knows of such an app that is already in existence/available? If there is, this could save me a lot of unnecessary work.

Thanks.

[umadog]

I know that the Argo Navis (and I think also the Inteliscope) controllers return the current constellation. Other than that, I didn't know so I looked on Google:

Someone has asked your question on SGL before, but I don't see a clear answer:

Someone on CN has written a program in C. Here's his post: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3567032/Main/3537329

The same question, again, on spacebanter: http://www.spacebanter.com/showthread.php?t=128155

Another option is to see if an existing planetarium program is scriptable and will produce the output you want. Stellarium might fit the bill. http://www.stellarium.org/wiki/index.php/Advanced_Use#Writing_Scripts

Maybe pyephem will do you want? http://rhodesmill.org/pyephem/quick.html

[umadog]

Hmm... maybe Python will work:

import ephem

ephem.constellation((0,90))

('UMa', 'Ursa Major')

Edit: I haven't figured out the units, but in principle this will work. For known stars, it works:

ald=ephem.star('Alderamin')

ald.compute()

ephem.constellation(ald)

('Cep', 'Cepheus')

Conceivably the units are radians because that would go from 0 to 6.28:

ald.ra

5.580407930728714

print ald.ra

21:18:56.12

[Demonperformer]

Thanks, guys.

That c-program looks the business - will give it a go.

[umadog]

I think the Python library might be easier to work with. I don't think the poster on CN put his C code on-line.

[Demonperformer]

This link in the last post http://cdsarc.u-strasbg.fr/viz-bin/Cat?target=http&cat=VI%2F42&

I haven't actually given it a try yet, but the read-me file makes it look fairly straight forward: http://cdsarc.u-strasbg.fr/vizier/ftp/cats/VI/42/ReadMe Of course, it could all depend on "the format in which the positions must be entered"!

[umadog]

Ah, yes, I see!

[umadog]

Ah! I think I figured out the syntax for the Python library. Here are two examples. I looked up coords from my star atlas an pyephem gives me the constellation correctly:

In [27]: ephem.constellation((ephem.hours('1:10:29.41'), ephem.degrees('51.5')))

Out[27]: ('Cas', 'Cassiopeia')

In [28]: ephem.constellation((ephem.hours('6:30:29.41'), ephem.degrees('35.5')))

Out[28]: ('Aur', 'Auriga')

So it's RA then DEC and you appear to need different conversion functions for each.

[Demonperformer]

Thanks for that Umadog. I got rather 'locked in' on the c prog, 'cos I already have c installed on my computer, but having looked at the PyEphem, it looks v good. Another bonus is that it will automatically precess co-ordinates, which is another thing I have been battling with. Will definitely give this prog a go.

[umadog]

Yeah, Python's worth a go if it has a suitable library because it's a lot less effort to code in than C is (particularly for something small like this job). It's also very easy to make nice plots should you need that (e.g. see Matplotlib, Chaco, and PyQtGraph).

[Demonperformer]

OK ... now I know I'm being greedy, but ...

Got PyEthem working and producing constellations accurately - excellent. Because the input line is quite long, and I am just changing the co-ordinates each time I run it, I am using the 'F3' command to re-insert the line. When changing the co-ordinates, it is self-evidently faster to overwrite, but every time I press F3, it reverts the insert key to INSERT rather than OVERWRITE. Not a major problem, but it does rather irritate me having to (remember to!) press the insert key before I start typing in the new co-ordinates. So is there any way of preventing the machine from doing that?

As I said ... being greedy ...

Thanks.

[Demonperformer]

OK, after some initial teething-troubles, I have now got PyEthem working very nicely: single input line [so I can use F3 to obviate the need to type in the whole input every time] and I just need to adjust the RA/Dec figures, giving me as output the constellation and the precessed co-ordinates for 2025. Having to do each object individually, means that this is not the most exciting part of the whole exercise (IMO) but considerably easier than having to adjust all the settings in Cdc for each object.

All in all, I give this program the 'thumbs-up'. Thanks for the recommendation.