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?

"At an apparent magnitude of 3.4, the Andromeda Galaxy is one of the brightest Messier objects,[15] making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope"




1 Answer 1


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: http://apod.nasa.gov/apod/ap061228.html


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