A slightly shaky photo taken inside the Pantheon with the light coming through the oculus striking one of the niches on the wall. With the doors closed, this oculus is the only source of natural light in building. Watching the ellipse of light slowly track across the floor and up the wall is endless fascinating. It is one of the few places where you can feel that the earth is rotating relative to the sun, not that the sun is moving around the earth.
Photo by Mark Gerwing, 1988.