Sep 3, 2011
On June 30, 2011, rhe cloud at the righthand side of the sky in this picture cast a big shadow over the West Arm of Grand Traverse Bay, near Traverse City, Michigan. That's because the sun had already set, and its last rays were hitting the cloud from a very low angle, well below the horizon.
The top of the shadow looks curved, I'm told, because of the extremely wide angle of vision here. It's a perspective thing–we think of the horizon line off in the distance as a straight line, but in a wide-angled scene like this we can see that it's actually a curved arc. For a few minutes, the shadow darkens a wedge of the celestial sphere; then this part of the world turns away and the scene is in serious earth shadow, not just cloud shadow, till morning.