Check out this amazing sunset phenomenon from the other day. Note how the cloud creates a strong shadow so that it's dark on one side and bright on the other:
Now here's the view you saw if you turned 180 degrees around and looked in the other direction. The shadow was so strong that it was dark on one side of the sky and bright on the other across to the other horizon. It brought to mind E.E. Cummings' line, "pulling all the sky over him with one smile."