<p>I remember <a href="https://en.wikipedia.org/wiki/Stokes_parameters" rel="nofollow noreferrer">Stokes parameters</a> as a way to completely define the polarization state of a well-defined beam of light. However several questions here refer to Stokes parameters used in characterizing the morphology of galaxies.</p>

<p>In Optics it's <span class="math-container">$S_0, S_1, S_2, S_3$</span> and in Astronomy it's <span class="math-container">$Q, U$</span>.</p>

<p>I'm guessing that these are not exactly the same set of parameters. but there might (or might not) be some connection either historically or in the way some observations were made.</p>

<ol>
<li>How did they both end up being called Stokes parameters? Is there a relationship?</li>
<li>How are they calculated (equation and/or short explanation please, not just a link)</li>
</ol>

<p>Related questions:</p>

<ul>
<li><a href="https://astronomy.stackexchange.com/q/13786/7982">What methods exist to calculate the ellipticity of galaxies</a></li>
<li><a href="https://astronomy.stackexchange.com/q/13755/7982">How to determine the ellipticity of galaxies in SDSS</a></li>
<li><a href="https://astronomy.stackexchange.com/q/33085/7982">What is the meaning of ellipticity of a galaxy being more than 1?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/31772/7982">How to get ellipticities of galaxies from SDSS using casjobs?</a></li>
</ul>

CC BY-SA 4.0
2020-03-01T05:22:17.303Z
There doesn't seem to be an appropriate morphology tag
uhoh
CC BY-SA 4.0
2020-03-01T06:32:38.063Z
Can you give a reference for Stokes parameters (which refer to polarisation) being used to characterise galaxy shapes? Ah ok, I see one. It is just a way of characterising ellipticity (in both cases).
ProfRob
CC BY-SA 4.0
2020-03-01T08:43:18.920Z
@RobJeffries I thought that might be the case; it would be quite a coincidence if two different people named Stokes worked on two different ellipticity problems and came up with two different sets of parameters which were both independently named after them.
uhoh
<observational-astronomy><galaxy><galactic-dynamics><star-catalogues>
What are Stokes parameters for galaxy shapes, how do they compare to Stokes parameters in optics?
uhoh
# What are Stokes parameters for galaxy shapes, how do they compare to Stokes parameters in optics?

(Asked by: username_0 on 2020-03-01)

I remember [Stokes parameters](https://en.wikipedia.org/wiki/Stokes_parameters) as a way to completely define the polarization state of a well-defined beam of light. However several questions here refer to Stokes parameters used in characterizing the morphology of galaxies.

In Optics it's $S\_0, S\_1, S\_2, S\_3$ and in Astronomy it's $Q, U$.

I'm guessing that these are not exactly the same set of parameters. but there might (or might not) be some connection either historically or in the way some observations were made.

1. How did they both end up being called Stokes parameters? Is there a relationship?
2. How are they calculated (equation and/or short explanation please, not just a link)

Related questions:

* [What methods exist to calculate the ellipticity of galaxies](https://astronomy.stackexchange.com/q/13786/7982)
* [How to determine the ellipticity of galaxies in SDSS](https://astronomy.stackexchange.com/q/13755/7982)
* [What is the meaning of ellipticity of a galaxy being more than 1?](https://astronomy.stackexchange.com/q/33085/7982)
* [How to get ellipticities of galaxies from SDSS using casjobs?](https://astronomy.stackexchange.com/q/31772/7982)

**Tags:** observational-astronomy, galaxy, galactic-dynamics, star-catalogues
> **Comments:**
> **username_0**: There doesn't seem to be an appropriate morphology tag  
> **username_1**: Can you give a reference for Stokes parameters (which refer to polarisation) being used to characterise galaxy shapes? Ah ok, I see one. It is just a way of characterising ellipticity (in both cases).  
> **username_0**: @RobJeffries I thought that might be the case; it would be quite a coincidence if two different people named Stokes worked on two different ellipticity problems and came up with two different sets of parameters which were both independently named after them.  

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## 0 answers

