The grand building that once housed Pennsylvania Railroad's Union Station in Pittsburgh features a majestic entrance rotunda lit during the day by a central dome skylight.
The skylight's rings of glass are divided into segments. The outer rings each have 28 identical sections. The innermost ring has a twelvefold symmetry. That combination is a bit puzzling: 28 and 12 don't appear to fit together. Moreover, 28 seems an unlikely divisor for a circle. I would expect multiples of four or six.
Indeed, the stained-glass dome skylight (below) in the 11-story central light well of the ornate Union Trust building in Pittsburgh has a pleasing 16-fold symmetry.
At the same time, the Rotunda at the University of Virginia, designed by Thomas Jefferson, has a dome skylight (below) divided into 28 sectors, demonstrating that the geometry of the Pennyslvanian dome skylight is not unique.
Other numbers come up, too. A dome skylight (below) in the Marriott Wardman Park Hotel in Washington, D.C., has 18 sectors.
And the skylight pattern (below) in the Priory Chapel of Saint Louis Abbey in Creve Coeur, Missouri, has a wonderful tenfold symmetry.
Photos by I. Peterson