<p>If you mouse-over the words &quot;Angular size&quot; you see the description:</p>
<blockquote>
<p>Syntax of angular size is : &quot;maj-axis min-axis angle (wtype) quality bibcode&quot;</p>
<ul>
<li>maj-axis : major axis size (arc minutes)</li>
<li>min-axis : minor axis size (arc minutes)</li>
<li>angle : orientation angle (in degrees)</li>
<li>(wtype) : wavelength class for the origin of the angular size (Rad, mm, IR, Opt, UV, Xray, Gam)</li>
<li>quality : flag of quality of the angular size values ( A=best quality -&gt; E=worst quality, {� } =unknown quality)</li>
<li>bibcode : bibcode of the angular size reference</li>
</ul>
</blockquote>
<p>So NGC1792 is an oval shape, the length of the oval is 6.17 arcminutes and the width is 3.02.  The other numbers don't relate to the angular size of the galaxy.</p>
<p>You should use the length, not the width: 6.17 arcminutes.</p>
<p>There are 60 arc minutes in a degree and <span class="math-container">$180/\pi$</span> degrees in a radian . So to convert 6.17 arcminutes to radians you do <span class="math-container">$6.17 ÷ 60 ÷ \frac{180}{\pi} = 0.00179\ \text{radians}$</span>. (that's just high school maths).</p>
<p>If you use the formula in the other question, you get an answer measured in the same units that you used for the distance.  If you use &quot;megaparsecs&quot; then D is measured in megaparsecs.</p>

CC BY-SA 4.0
2021-11-13T14:22:32.277Z
Thanks for the help once again. I tested it with another galaxy, Little Sombrero, the calculation roughly matched the figure given apod.nasa.gov/apod/ap210722.html , i calculated 58.29 thousand whereas they said estimated 60 thousand.
MiscellaneousUser
CC BY-SA 4.0
2021-11-11T22:07:57.050Z
2021-11-11T22:07:57.050Z
James K
<p>With reference to this
<a href="https://astronomy.stackexchange.com/questions/7720/diameter-of-any-galaxy">Diameter of any galaxy</a> question.</p>
<p>I need some help to calculate an estimate for NGC 1792. I need to calculate a few galaxy diameters so looking for help on one to then work out the rest on the others. I know the megaparsec is  15.33 to 2 dp.  <a href="https://www.eso.org/public/images/eso0338b/" rel="nofollow noreferrer">https://www.eso.org/public/images/eso0338b/</a></p>
<p>What I don't know is the the radians.  I get Angular size from Simbad as</p>
<pre><code>6.17 3.02 136 (Opt) D 
</code></pre>
<p>but which is if any is the radian or how to calculate the radian.  In the answer, Dieudonné says &quot;you can get the actual diameter D from: D = d tan A&quot;.  What is D measured in?  Parsec etc?</p>

CC BY-SA 4.0
2021-11-11T21:18:40.957Z
The answer by Dieudonné explains what the units are.
Peter Erwin
CC BY-SA 4.0
2021-11-11T21:19:10.950Z
Also, what does "the megaparsec is 15.33 to 2 dp" mean?
Peter Erwin
CC BY-SA 4.0
2021-11-13T12:40:51.950Z
https://www.unitconverters.net/length/light-year-to-megaparsec.htm says 50 million light years is 15.33006969 Megaparsec.  So all I'm saying is I round up the value to 2dp (.33)
MiscellaneousUser
CC BY-SA 4.0
2021-11-13T14:00:07.037Z
Ah, OK. I’ve never seen “dp” used as an abbreviation for decimal places before.
Peter Erwin
<galaxy>
Calculate the diameter of a galaxy
MiscellaneousUser
# Calculate the diameter of a galaxy

(Asked by: username_0 on 2021-11-11)

With reference to this
Diameter of any galaxy question.
I need some help to calculate an estimate for NGC 1792. I need to calculate a few galaxy diameters so looking for help on one to then work out the rest on the others. I know the megaparsec is  15.33 to 2 dp.  https://www.eso.org/public/images/eso0338b/
What I don't know is the the radians.  I get Angular size from Simbad as
6.17 3.02 136 (Opt) D 

but which is if any is the radian or how to calculate the radian.  In the answer, Dieudonné says "you can get the actual diameter D from: D = d tan A".  What is D measured in?  Parsec etc?

**Tags:** galaxy
> **Comments:**
> **username_1**: The answer by Dieudonné explains what the units are.  
> **username_1**: Also, what does "the megaparsec is 15.33 to 2 dp" mean?  
> **username_0**: https://www.unitconverters.net/length/light-year-to-megaparsec.htm says 50 million light years is 15.33006969 Megaparsec.  So all I'm saying is I round up the value to 2dp (.33)  
> **username_1**: Ah, OK. I’ve never seen “dp” used as an abbreviation for decimal places before.  

---

## 1 answers

### Answer by username_2 on 2021-11-11. Score: 1 | Accepted answer

If you mouse-over the words "Angular size" you see the description:

> Syntax of angular size is : "maj-axis min-axis angle (wtype) quality bibcode"
>
> * maj-axis : major axis size (arc minutes)
> * min-axis : minor axis size (arc minutes)
> * angle : orientation angle (in degrees)
> * (wtype) : wavelength class for the origin of the angular size (Rad, mm, IR, Opt, UV, Xray, Gam)
> * quality : flag of quality of the angular size values ( A=best quality -> E=worst quality, {� } =unknown quality)
> * bibcode : bibcode of the angular size reference

So NGC1792 is an oval shape, the length of the oval is 6.17 arcminutes and the width is 3.02. The other numbers don't relate to the angular size of the galaxy.

You should use the length, not the width: 6.17 arcminutes.

There are 60 arc minutes in a degree and $180/\pi$ degrees in a radian . So to convert 6.17 arcminutes to radians you do $6.17 ÷ 60 ÷ \frac{180}{\pi} = 0.00179\ \text{radians}$. (that's just high school maths).

If you use the formula in the other question, you get an answer measured in the same units that you used for the distance. If you use "megaparsecs" then D is measured in megaparsecs.

> **Comments:**
> **username_0**: Thanks for the help once again. I tested it with another galaxy, Little Sombrero, the calculation roughly matched the figure given apod.nasa.gov/apod/ap210722.html , i calculated 58.29 thousand whereas they said estimated 60 thousand.  

---

