<p>I tried to reproduce your result. I took the values of RA and DEC from Wikipedia: <a href="https://en.wikipedia.org/wiki/Alnitak" rel="noreferrer">Alnitak</a>, <a href="https://en.wikipedia.org/wiki/Alnilam" rel="noreferrer">Alnilam</a>, <a href="https://en.wikipedia.org/wiki/Mintaka" rel="noreferrer">Mintaka</a>.</p>
<p>The values are the following:</p>
<p>Alnitak: Ra = 05h 40m 45.52666s, Dec = −01° 56′ 34.2649″</p>
<p>Alnilam: Ra = 05h 36m 12.8s,     Dec = −01° 12′ 06.9″</p>
<p>Mintaka: Ra = 05h 32m 00.40009s, Dec = −00° 17′ 56.7424″</p>
<p>Please, check whether these are about the same values that you have used.
I plotted them and this is the result</p>
<p><a href="https://i.sstatic.net/MnC3s.png" rel="noreferrer"><img src="https://i.sstatic.net/MnC3s.png" alt="position of the three stars on the celestial sphere" /></a></p>
<p>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.</p>

CC BY-SA 4.0
2021-08-29T07:48:13.320Z
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.
aggregate1166877
CC BY-SA 4.0
2021-08-29T07:51:09.203Z
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.
Prallax
CC BY-SA 4.0
2021-08-29T07:54:46.640Z
Thanks for the info! Let me investigate a bit further on my end
aggregate1166877
CC BY-SA 4.0
2021-08-29T07:44:40.017Z
2021-08-29T07:44:40.017Z
Prallax
<p>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 <code>x,y,z</code> methods, Orion looks like this:<br><br>
<a href="https://i.sstatic.net/vjkra.jpg" rel="noreferrer"><img src="https://i.sstatic.net/vjkra.jpg" alt="enter image description here" /></a></p>
<p>Notice Alnitak in red. It's visibly wrong.</p>
<p>I then manually adjust the declination of Alnitak by 1.5 degrees, and get a more believable result:<br><br>
<a href="https://i.sstatic.net/0O5xF.jpg" rel="noreferrer"><img src="https://i.sstatic.net/0O5xF.jpg" alt="enter image description here" /></a></p>
<p>To give some background, I initially pulled data for these stars from the <a href="https://simbad.u-strasbg.fr/simbad/" rel="noreferrer">SIMBAD Astronomical Database</a> 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 - <a href="https://www.star-facts.com/alnitak/" rel="noreferrer">Star facts</a>, <a href="http://astropixels.com/stars/Alnitak-01.html" rel="noreferrer">Astro Pixels</a> <a href="https://en.wikipedia.org/wiki/Alnitak" rel="noreferrer">Wikipedia</a>, and others - they all report similar declinations (well within  1 degree).</p>
<p>I find it strange that <em>one</em> 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.</p>
<p><strong>How I create my x,y,z positions:</strong><br></p>
<ul>
<li>I create an object in a 3D scene with world coords <code>0,0,0</code> and global rotation <code>0,0,0</code>.</li>
<li>I set its <code>x</code> rotation to exactly match right ascension (converted to decimal degrees).</li>
<li>I then rotate the object along the <code>y</code> axis by the declination amount (in degrees).</li>
<li>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 <a href="https://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/bsc5p.html" rel="noreferrer">BSC5P</a>:</li>
</ul>
<p><a href="https://i.sstatic.net/vsuBd.gif" rel="noreferrer"><img src="https://i.sstatic.net/vsuBd.gif" alt="enter image description here" /></a></p>
<p>But now I don't know if my way of determining position is flawed.</p>
<p>Anyone have any ideas on where I might be going wrong?</p>

CC BY-SA 4.0
2021-08-29T06:27:34.677Z
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?
Prallax
CC BY-SA 4.0
2021-08-29T06:30:10.073Z
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.
aggregate1166877
CC BY-SA 4.0
2021-08-29T06:45:32.627Z
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.
aggregate1166877
CC BY-SA 4.0
2021-08-29T07:33:29.793Z
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?
ProfRob
CC BY-SA 4.0
2021-08-29T07:41:44.723Z
Hi @ProfRob - I only added that I'm using parallax for informational purposes. The pictures of the Orion stars I posted are not 2D plots, they're actual 3D objects I can orbit around. The screenshots were taken from the center of the virtual universe I created, so a constellation forms.
aggregate1166877
CC BY-SA 4.0
2021-08-29T08:18:31.783Z
@ProfRob I'm making the assumption that we can think of stars as being projected from the 'center' of the ecliptic. I produce a star, place it at the center, rotate it to point towards known RA/Dec coordinates, then move it a predefined distance in that direction in a 3D space. So in a sense I guess I'm plotting from the center of the ecliptic out towards the location of the star. My objective is to create a 3-dimensional map of the stars in the Bright Star Catalog. You alluded to the possibility that there might be a problem in my code, you may be right. I'll investigate.
aggregate1166877
CC BY-SA 4.0
2021-08-29T14:27:10.593Z
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)°.
alephzero
CC BY-SA 4.0
2021-08-30T11:14:55.007Z
@alephzero Thank you thank you thank you!!! That turned out to be *exactly* the problem! I thought what I'd done was precisely follow the formula, but after carefully testing your hypotheses I discovered the `-` sign was being lost at some point. It's working now. Please post this as an answer if you have the time
aggregate1166877
<declination><right-ascension>
Is the Ra/Dec of Alnitak in Orion's Belt known to be correct?
aggregate1166877
# Is the Ra/Dec of Alnitak in Orion's Belt known to be correct?

(Asked by: username_0 on 2021-08-29)

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](https://i.sstatic.net/vjkra.jpg)](https://i.sstatic.net/vjkra.jpg)

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](https://i.sstatic.net/0O5xF.jpg)](https://i.sstatic.net/0O5xF.jpg)

To give some background, I initially pulled data for these stars from the [SIMBAD Astronomical Database](https://simbad.u-strasbg.fr/simbad/) 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](https://www.star-facts.com/alnitak/), [Astro Pixels](http://astropixels.com/stars/Alnitak-01.html) [Wikipedia](https://en.wikipedia.org/wiki/Alnitak), 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](https://heasarc.gsfc.nasa.gov/W3Browse/star-catalog/bsc5p.html):

[![enter image description here](https://i.sstatic.net/vsuBd.gif)](https://i.sstatic.net/vsuBd.gif)

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?

**Tags:** declination, right-ascension
> **Comments:**
> **username_1**: 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?  
> **username_0**: 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.  
> **username_0**: 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.  
> **username_2**: 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?  
> **username_0**: Hi @ProfRob - I only added that I'm using parallax for informational purposes. The pictures of the Orion stars I posted are not 2D plots, they're actual 3D objects I can orbit around. The screenshots were taken from the center of the virtual universe I created, so a constellation forms.  
> **username_0**: @ProfRob I'm making the assumption that we can think of stars as being projected from the 'center' of the ecliptic. I produce a star, place it at the center, rotate it to point towards known RA/Dec coordinates, then move it a predefined distance in that direction in a 3D space. So in a sense I guess I'm plotting from the center of the ecliptic out towards the location of the star. My objective is to create a 3-dimensional map of the stars in the Bright Star Catalog. You alluded to the possibility that there might be a problem in my code, you may be right. I'll investigate.  
> **username_3**: 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)°.  
> **username_0**: @alephzero Thank you thank you thank you!!! That turned out to be *exactly* the problem! I thought what I'd done was precisely follow the formula, but after carefully testing your hypotheses I discovered the `-` sign was being lost at some point. It's working now. Please post this as an answer if you have the time  

---

## 1 answers

### Answer by username_1 on 2021-08-29. Score: 6 | Accepted answer

I tried to reproduce your result. I took the values of RA and DEC from Wikipedia: [Alnitak](https://en.wikipedia.org/wiki/Alnitak), [Alnilam](https://en.wikipedia.org/wiki/Alnilam), [Mintaka](https://en.wikipedia.org/wiki/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](https://i.sstatic.net/MnC3s.png)](https://i.sstatic.net/MnC3s.png)

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.

> **Comments:**
> **username_0**: 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.  
> **username_1**: 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.  
> **username_0**: Thanks for the info! Let me investigate a bit further on my end  

---

