Looking out of the window of commercial airliners I've always felt that the horizon was not entirely straight, in other words, that you could see the curvature of the surface of the earth. However, I was never sure if I was just imagining it. On a recent flight I snapped a photo with a wide-angle lens with a horizontal field of view of 90 degrees. Sure enough, the horizon came out curved. (The lens was rectilinear, not a fish-eye).
If you think about it, the curvature of the horizon isn't all that surprising. Looking at the world from an altitude of 11km (36,000 feet) above sea level your horizon is about 375km away, and the distance between two points at the horizon that are 90 degrees apart from your point of view (left and right edge of the photo) is 530km. The direction that is "up" at those two points differs by about 4.8 degrees. A difference of almost five degrees in the direction of the horizon should be clearly visible.
Curvature of the Earth by fksr. Click on the image for a larger version.
I also shot an anaglyph 3D version of this photo, where can also see the surface of the earth curve away from you in the distance:
Curvature of the Earth in 3D by fksr. Click on the image for a larger version.
