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mapguy
2012-Jul-23, 01:24 PM
Clear skies this weekend, so we drove 30 miles out of the city, and enjoyed a good view of the Milky Way. One thing that confused me, though: the band seemed to have a slight curve to it as I followed it across the sky. Since the plane of the galaxy is flat, I would've expected the Milky Way to appear as a straight line across the sky when viewed with the naked eye. Is there something about our atmosphere that causes it to refract into a curve? Or was I experiencing some kind of optical illusion, perhaps having to do with the curvature of the Earth?

ngc3314
2012-Jul-23, 02:53 PM
The midplane of the Milky Way does appear very close to a great circle (as shown on celestial maps). In my experience, the eye doesn't do a very good job of telling us that - my worst example is seeing the phase of the Moon with the Sun in the sky. I know perfectly well that the crescent must point exactly along a great circle to the Sun, but my perception is pretty much always that it angles one way or the other.

When the Milky Way goes exactly overhead, I do see it as a great circle, when the visual perception is pretty much "that's vertical".

utesfan100
2012-Jul-24, 07:00 PM
Clear skies this weekend, so we drove 30 miles out of the city, and enjoyed a good view of the Milky Way. One thing that confused me, though: the band seemed to have a slight curve to it as I followed it across the sky. Since the plane of the galaxy is flat, I would've expected the Milky Way to appear as a straight line across the sky when viewed with the naked eye. Is there something about our atmosphere that causes it to refract into a curve? Or was I experiencing some kind of optical illusion, perhaps having to do with the curvature of the Earth?

A simple way to rule out an optical illusion would have been to hold up a straight edge to the Milky Way. If the Milky Way really was curved, the straight edge should appear as a tangent line to the curve.

Our brains tend to see the sky as a flat screen, rather than a spherical dome. I would expect the Milky Way to physiologically appear to curve away from you as you move your eye away from the highest point in the sky due to this effect, but a straight edge would show that it is indeed a line.

John Mendenhall
2012-Jul-24, 07:20 PM
Plenty of obscuring dust, also. Wiki has good photos, article is 'Milky Way'.

mapguy
2013-Jul-15, 08:34 PM
A simple way to rule out an optical illusion would have been to hold up a straight edge to the Milky Way. If the Milky Way really was curved, the straight edge should appear as a tangent line to the curve.
Our brains tend to see the sky as a flat screen, rather than a spherical dome. I would expect the Milky Way to physiologically appear to curve away from you as you move your eye away from the highest point in the sky due to this effect, but a straight edge would show that it is indeed a line.

Apologies for resurrecting an old thread, but it's taken this long for me to have a chance to use utesfan's idea. Last week I was at about 10,000 feet with a clear, moonless sky, and a spectacular view of the summer Milky Way. To me it appeared to have a very pronounced curve. So I took the edge of a blanket, stretched it out taut, and held it up... and sure enough, the "curve" is completely an optical illusion. I was relieved to get this result, but I wondered if anyone has more info on what causes our brains to grossly mis-perceive straight lines in the sky?

speedfreek
2013-Jul-15, 09:20 PM
No need to apologise, especially when you are resurrecting your own thread to give us an update like that! :)

I think the reason is the same as for any optical illusion - our brain can be fooled as to the nature of the information it receives from the eyes. There are classic optical illusions where straight lines appear curved. Also, the moon can appear larger near the horizon than when it is up in the sky, and this is due to our having other familiar objects to compare it to near the horizon which we lack when it is up in the sky, and I think this might have a bearing on it.

grapes
2013-Jul-15, 10:00 PM
Yes, thanks mapguy, we've gone round and round with this one. :)

Another "good" example is crepuscular rays that develop into anti-crepuscular rays. They're straight, but many people cannot even understand the geometry, even when *not* looking at them. :)

ETA: http://cosmoquest.org/forum/showthread.php?39035-Why-does-the-moon-s-terminator-not-appear-orthogonal-to-the-direction-of-the-sun