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

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    $\begingroup$ 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. $\endgroup$ May 18 at 14:40
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    $\begingroup$ @Greg miller so... in your estimation image is NOT off by a factor of 100, or 10, or even a factor of 2 $\endgroup$
    – BradV
    May 18 at 16:05
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    $\begingroup$ 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. $\endgroup$ May 18 at 19:04

1 Answer 1


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) 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 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

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    $\begingroup$ 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!! $\endgroup$
    – BradV
    May 18 at 16:13
  • $\begingroup$ For reference, the Moon is less than a degree wide in angular diameter, making the scaling in the original picture perfectly plausible $\endgroup$
    – adrian
    May 22 at 1:18

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