<p>I understand that ellipticity is a very important concept in weak gravitational lensing because it describes the shape properties of the transformed galaxy. Shear, on the other hand, also describes the shape properties of the transformed galaxy. Since shear can also describe the shape properties of the transformed galaxy, I don't see why ellipticity is so important. It is not used to calculate shear. It is not used to figure out the matter power spectrum. I don't see why we want to measure ellipticity. I am confused.</p>
<p>Can anyone explain this for me? Thanks so much!</p>

CC BY-SA 4.0
2024-02-10T07:35:49.653Z
*Welcome to Stack Exchange!* For ellipticity or shear, does either one alone describe ***some** of the*, or ***all** of the* shape properties? What does "the shape properties" even mean? How many are they? What are they called? When premising a question it's best to cite, link to and quote the source the basis of the premise.
uhoh
CC BY-SA 4.0
2024-02-10T16:42:04.990Z
Shear does not describe the "shape property" of a galaxy; it describes *how the intrinsic shape is modified* (or "transfomred"). You need to combine that with (some assumption about) the intrinsic ellipticity of the galaxy in order to end up with the *observed* ellipticity of the galaxy.
Peter Erwin
CC BY-SA 4.0
2024-02-12T15:01:28.317Z
Thank you. I now understand. @PeterErwin
Gene
<gravitational-lensing>
Cosmic shear: what is ellipticity for?
Gene
# Cosmic shear: what is ellipticity for?

(Asked by: username_0 on 2024-02-10)

I understand that ellipticity is a very important concept in weak gravitational lensing because it describes the shape properties of the transformed galaxy. Shear, on the other hand, also describes the shape properties of the transformed galaxy. Since shear can also describe the shape properties of the transformed galaxy, I don't see why ellipticity is so important. It is not used to calculate shear. It is not used to figure out the matter power spectrum. I don't see why we want to measure ellipticity. I am confused.

Can anyone explain this for me? Thanks so much!

**Tags:** gravitational-lensing

> **Comments:**
> **username_1**: *Welcome to Stack Exchange!* For ellipticity or shear, does either one alone describe ***some** of the*, or ***all** of the* shape properties? What does "the shape properties" even mean? How many are they? What are they called? When premising a question it's best to cite, link to and quote the source the basis of the premise.  
> **username_2**: Shear does not describe the "shape property" of a galaxy; it describes *how the intrinsic shape is modified* (or "transfomred"). You need to combine that with (some assumption about) the intrinsic ellipticity of the galaxy in order to end up with the *observed* ellipticity of the galaxy.  
> **username_0**: Thank you. I now understand. @PeterErwin  

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