Andromeda is wider in the sky than the Moon

The disk of the Andromeda galaxy spans about three degrees, roughly six times the Moon’s width. The eye only picks out the bright centre.

Andromeda sits about 2.5 million light-years away, which is why a spiral that dwarfs the Milky Way covers a patch of sky instead of a point. The full disk, including the faint outer light, is about three degrees long and one degree wide. The Moon is about half a degree across. Six Moons, side by side, would fit along Andromeda’s length.

What the unaided eye sees, on a dark night, is the central bulge, a soft smudge about the size of the Moon or a little larger. The rest of the disk is there and is lost in the sky’s own glow. A long photograph is not inventing a size. It is collecting the light the eye gives up on. The galaxy is also coming this way, which is a different entry. The surprise here is only the angle.