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I've obtained right ascension / declination information about some of stars in the Orion constellation from a few different sources and imported this into an open source video game I'm making. Using (my own possibly wonky) Ra/Dec/Parallax to x,y,z methods, Orion looks like this:

enter image description here

Notice Alnitak in red. It's visibly wrong.

I then manually adjust the declination of Alnitak by 1.5 degrees, and get a more believable result:

enter image description here

To give some background, I initially pulled data for these stars from the SIMBAD Astronomical Database but found that Alnitak had no parallax information. After manually looking up and adding in parallax info based on known distance, I realised the position looked wrong. I checked multiple sources - Star facts, Astro Pixels Wikipedia, and others - they all report similar declinations (well within 1 degree).

I find it strange that one star would be off and the others not, so normally I'd blame the data source. However it seems incredibly unlikely so many (likely peer reviewed) sources would simply be wrong, especially with such a well known star triple star system.

How I create my x,y,z positions:

  • I create an object in a 3D scene with world coords 0,0,0 and global rotation 0,0,0.
  • I set its x rotation to exactly match right ascension (converted to decimal degrees).
  • I then rotate the object along the y axis by the declination amount (in degrees).
  • I then move the object 'forward' by any arbitrary amount. Doing this for all stars, say, at a fixed distance of 1.5 parsecs, the result is as follows for BSC5P:

enter image description here

But now I don't know if my way of determining position is flawed.

Anyone have any ideas on where I might be going wrong?

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  • $\begingroup$ As I understand, you say that you convert ra/dec/parallax to x/y/z, but then you say that you position every star at z=1.5 pc, so you don't really use the parallax information. This makes the value of parallax that you have manually inserted for Alnitak irrelevant, am I correct? $\endgroup$
    – Prallax
    Aug 29 at 6:27
  • $\begingroup$ Hi @Prallax, yes you're right - I guess I should have been a bit clearer, the manually setting to 1.5 is just for demonstration purposes. The final objective is to actually use 1/parallax to set distance. In any case, I'm more concerned about the visuals from the center of the ecliptic - distance does not affect their apparent positions from there. $\endgroup$ Aug 29 at 6:30
  • $\begingroup$ Once all visual errors are gone, the player will have the option to fly to the actual star (it's a 3D space sim). This is why real distance is important once the visual problems have been figured out. $\endgroup$ Aug 29 at 6:45
  • $\begingroup$ Position on the sky has nothing to do with parallax. You say you calculate X,y,z, but then you've shown us 2D plots What are on the axes? $\endgroup$
    – ProfRob
    Aug 29 at 7:33
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    $\begingroup$ It should be obvious that astronomers have known the position of Alnitak ever since they produced the first star catalogues thousands of years ago. Either you have found one particular catalog with a typo (unlikely, for such a commonly known star), or your conversion is wrong. My guess would be that your code somehow interprets "−01° 56'" as (−01 + 56/60)° not (−01 − 56/60)°. $\endgroup$
    – alephzero
    Aug 29 at 14:27
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I tried to reproduce your result. I took the values of RA and DEC from Wikipedia: Alnitak, Alnilam, Mintaka.

The values are the following:

Alnitak: Ra = 05h 40m 45.52666s, Dec = −01° 56′ 34.2649″

Alnilam: Ra = 05h 36m 12.8s, Dec = −01° 12′ 06.9″

Mintaka: Ra = 05h 32m 00.40009s, Dec = −00° 17′ 56.7424″

Please, check whether these are about the same values that you have used. I plotted them and this is the result

position of the three stars on the celestial sphere

Looks ok to me, so the problem is not in the public known coordinates of the star. Please, check that you have entered the correct values for Alnitak.

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  • $\begingroup$ Thank you for the answer. Out of interest, did you use an application for plotting those? I so I think it might help if I had additional tools to troubleshoot with. $\endgroup$ Aug 29 at 7:48
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    $\begingroup$ I used python. I converted the wikipedia values to degrees with astropy.coordinates.SkyCoord to avoid making mistakes in a manual conversion. I plotted the result with matplotlib. $\endgroup$
    – Prallax
    Aug 29 at 7:51
  • $\begingroup$ Thanks for the info! Let me investigate a bit further on my end $\endgroup$ Aug 29 at 7:54

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