<p>Unfortunately, galaxies don't have a neat edge (and the Moon is not at a fixed distance) so your question is difficult to answer in absolute terms.</p>
<p>However, a commonly quoted diameter for the disk of the Andromeda galaxy is something like 70 kpc (about 220,000 light years), which at a distance of 750 kpc (about 2.4 million light years, e.g. see <a href="https://en.wikipedia.org/wiki/Andromeda_Galaxy" rel="noreferrer">Andromeda galaxy</a>) gives an angular diameter of about  5 degrees, which is about 10 times the diameter of the full Moon.</p>
<p>Thus I think the picture is reasonably justified.</p>
<p>On the other hand, is it realistic to expect to be able to see Andromeda this big? I'm not so sure and have asked a <a href="https://astronomy.stackexchange.com/questions/49370/how-could-i-see-andromeda-as-large-as-in-this-picture?noredirect=1&amp;lq=1">linked question</a> about this.
For example, this (well-known) picture from the digitised sky survey shows an Andromeda disk that is less than 3 degrees across!</p>
<p><a href="https://i.sstatic.net/4P1mp.png" rel="noreferrer"><img src="https://i.sstatic.net/4P1mp.png" alt="M31 picture" /></a></p>

CC BY-SA 4.0
2022-05-18T16:13:16.680Z
thank you so very much for calculation. If I had my wits about me this morning I could have done the simple math myself. (embarrassed emoji here).  I am constantly being amazed by the cosmos around us that is unseen by the unaided eye!!
BradV
CC BY-SA 4.0
2022-05-22T01:18:20.027Z
For reference, the Moon is less than a degree wide in angular diameter, making the scaling in the original picture perfectly plausible
adrian
CC BY-SA 4.0
2022-08-24T01:55:47.690Z
@pela and ProfRob ... care to take up the task??
BradV
CC BY-SA 4.0
2023-06-28T21:38:27.560Z
FWIW, in my own shot here Andromeda fills the vast majority of an (astrometically determined) 3.8º wide shot
https://flickr.com/photos/raybellis/52647556258/in/album-72177720304516440/
Alnitak
CC BY-SA 4.0
2022-05-18T14:44:18.700Z
2022-05-18T19:06:54.000Z
2022-05-18T19:06:54.000Z
ProfRob
<p>SEE BJV EDIT BELOW.</p>
<p>I came across this image which purports to show what the size of the Andromeda galaxy would be in the night sky if we could see its dim stars with the naked eye. Is this image reasonably correct with respect to size?
<a href="https://i.sstatic.net/gMZb3.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/gMZb3.jpg" alt="enter image description here" /></a></p>
<p>EDIT BJV 08-23-2022: A friend asked me 'So... where is Andromeda anyway and what would it look like if I could see it all?&quot; I was at a loss to explain in terms he would totally grasp.</p>
<p>AND SO MY ASK... could somebody with the right combination of competencies create an image showing Andromeda (in its entirety) in the night sky in its correct location between Cassiopeia and Pegasus with trees/hills/buildings/whatever in the image to give a real sense of normal viewing scale??</p>
<p>Extra credit... provide same image with 'unaided human eye under IDEAL conditions'.</p>

CC BY-SA 4.0
2022-05-18T14:40:17.620Z
Seems Andromeda is too big.  The Andromeda Galaxy is about 3deg wide, the moon is about 1/2deg wide.  So it would be 6 times the width of the moon.  Here it looks like it's more like 8.
Greg Miller
CC BY-SA 4.0
2022-05-18T16:05:29.643Z
@Greg miller so... in your estimation image is NOT off by a factor of 100, or 10, or even a factor of 2
BradV
CC BY-SA 4.0
2022-05-18T19:04:40.037Z
No, it's certainly in the ballpark.  But like ProfRob said, it would depend exactly on where you define the edge of the galaxy.  I think a more precise method would be to compare the image above to other images of it of known scale.  But if you're just looking for ballpark estimates, it's certainly in it.
Greg Miller
<observational-astronomy>
Is the scaling in this composite photo of the moon and the Andromeda galaxy reasonably correct?
BradV
# Is the scaling in this composite photo of the moon and the Andromeda galaxy reasonably correct?

(Asked by: username_0 on 2022-05-18)

SEE BJV EDIT BELOW.

I came across this image which purports to show what the size of the Andromeda galaxy would be in the night sky if we could see its dim stars with the naked eye. Is this image reasonably correct with respect to size?
[![enter image description here](https://i.sstatic.net/gMZb3.jpg)](https://i.sstatic.net/gMZb3.jpg)

EDIT BJV 08-23-2022: A friend asked me 'So... where is Andromeda anyway and what would it look like if I could see it all?" I was at a loss to explain in terms he would totally grasp.

AND SO MY ASK... could somebody with the right combination of competencies create an image showing Andromeda (in its entirety) in the night sky in its correct location between Cassiopeia and Pegasus with trees/hills/buildings/whatever in the image to give a real sense of normal viewing scale??

Extra credit... provide same image with 'unaided human eye under IDEAL conditions'.

**Tags:** observational-astronomy
> **Comments:**
> **username_1**: Seems Andromeda is too big.  The Andromeda Galaxy is about 3deg wide, the moon is about 1/2deg wide.  So it would be 6 times the width of the moon.  Here it looks like it's more like 8.  
> **username_0**: @Greg miller so... in your estimation image is NOT off by a factor of 100, or 10, or even a factor of 2  
> **username_1**: No, it's certainly in the ballpark.  But like ProfRob said, it would depend exactly on where you define the edge of the galaxy.  I think a more precise method would be to compare the image above to other images of it of known scale.  But if you're just looking for ballpark estimates, it's certainly in it.  

---

## 1 answers

### Answer by username_2 on 2022-05-18. Score: 17 | Accepted answer

Unfortunately, galaxies don't have a neat edge (and the Moon is not at a fixed distance) so your question is difficult to answer in absolute terms.

However, a commonly quoted diameter for the disk of the Andromeda galaxy is something like 70 kpc (about 220,000 light years), which at a distance of 750 kpc (about 2.4 million light years, e.g. see [Andromeda galaxy](https://en.wikipedia.org/wiki/Andromeda_Galaxy)) gives an angular diameter of about 5 degrees, which is about 10 times the diameter of the full Moon.

Thus I think the picture is reasonably justified.

On the other hand, is it realistic to expect to be able to see Andromeda this big? I'm not so sure and have asked a [linked question](https://astronomy.stackexchange.com/questions/49370/how-could-i-see-andromeda-as-large-as-in-this-picture?noredirect=1&lq=1) about this.
For example, this (well-known) picture from the digitised sky survey shows an Andromeda disk that is less than 3 degrees across!

[![M31 picture](https://i.sstatic.net/4P1mp.png)](https://i.sstatic.net/4P1mp.png)

> **Comments:**
> **username_0**: thank you so very much for calculation. If I had my wits about me this morning I could have done the simple math myself. (embarrassed emoji here).  I am constantly being amazed by the cosmos around us that is unseen by the unaided eye!!  
> **username_3**: For reference, the Moon is less than a degree wide in angular diameter, making the scaling in the original picture perfectly plausible  
> **username_0**: @pela and ProfRob ... care to take up the task??  
> **username_4**: FWIW, in my own shot here Andromeda fills the vast majority of an (astrometically determined) 3.8º wide shot https://flickr.com/photos/raybellis/52647556258/in/album-72177720304516440/  

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