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I am trying to understand something I read on wikipedia about M31.

Wikipedia says that M31 appears more than six times as wide as the full Moon.

But I remember that when I watched it naked eye it seamed a lot smaller, even considering that I was able to see only the central region. For this reason I also used the software Stellarium to check it and setting the same magnification for the two objects (see pictures), the wikipedia information still seems wrong ... or most likely I am wrong!

Where is that I fail?




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up vote 6 down vote accepted

The answer is right there in your screenshots. The 'dimensione' or angular diameter on the sky is 3 degrees for M31, whereas the moon has an angular diameter of only 0.5 degrees. In other words, M31 is about 6 times as wide as the moon.

As wikipedia correctly states, you need a large telescope to see the full extend of M31. With the naked eye of even a small telescope you simply cannot see the spiral arms.

I think this image shows it very neatly:

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