<p>If the cluster is symmetric take the difference in declination between the galaxies with the highest and lowest declinations<span class="math-container">$^{*}$</span>. This is the angular size.</p>
<p><span class="math-container">$*$</span> Assuming the cluster does not span the celestial north or south pole.</p>

CC BY-SA 4.0
2024-07-29T23:49:16.577Z
* assuming the cluster doesn't cross one of the poles.
Greg Miller
CC BY-SA 4.0
2024-07-26T12:13:08.330Z
2024-07-30T06:59:15.147Z
2024-07-30T06:59:15.147Z
ProfRob
<p>I have the RA  and Dec values of all the galaxies in a cluster and I need to find its angular size. I do not know how to calculate it</p>

CC BY-SA 4.0
2024-07-30T12:58:10.773Z
One method would be to compute the angles between every one and use the largest angle.  Here is an algorithm for computing the angle between RA/Dec points: https://www.celestialprogramming.com/snippets/anglebetweenpoints.html
Greg Miller
CC BY-SA 4.0
2024-12-27T18:18:31.347Z
What is the purpose of getting the radius? There are several cluster radii in use scientifically: Devaucouleur’s Law $R_e, R_{virial}, R_{200}, R_{200\Omega_m^{-1}}$, Harmonic Radius, etc.  Are all background galaxies removed?
eshaya
<galaxy><galaxy-cluster><galaxy-center><active-galaxy>
Finding the angular size of a galaxy cluster when given the RA and Dec cordinates of all the galaxies in the cluster
Meredith Grey
# Finding the angular size of a galaxy cluster when given the RA and Dec cordinates of all the galaxies in the cluster

(Asked by: username_0 on 2024-05-08)

I have the RA and Dec values of all the galaxies in a cluster and I need to find its angular size. I do not know how to calculate it

**Tags:** galaxy, galaxy-cluster, galaxy-center, active-galaxy

> **Comments:**
> **username_1**: One method would be to compute the angles between every one and use the largest angle.  Here is an algorithm for computing the angle between RA/Dec points: https://www.celestialprogramming.com/snippets/anglebetweenpoints.html  
> **username_2**: What is the purpose of getting the radius? There are several cluster radii in use scientifically: Devaucouleur’s Law $R_e, R_{virial}, R_{200}, R_{200\Omega_m^{-1}}$, Harmonic Radius, etc.  Are all background galaxies removed?  

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## 1 answers

### Answer by username_3 on 2024-07-26. Score: 0

If the cluster is symmetric take the difference in declination between the galaxies with the highest and lowest declinations$^{\*}$. This is the angular size.

$\*$ Assuming the cluster does not span the celestial north or south pole.

> **Comments:**
> **username_1**: * assuming the cluster doesn't cross one of the poles.  

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