The Pantheon as Sun Dial

I've written before about the Pantheon. We recently learned about some special properties of the beam of light coming through the central oculus. Starting around the spring equinox, as the angle of the sun becomes longer each day, the light moves daily further down the wall. Today, the beam was supposed to lineup with and light the arch above the inside of the massive entry doors. So we walked over to have a look. It was impressive (and, as you can see from the photos, we were not the only interested parties). Thought you might find it interesting (the perfect alignment happened at about 1:01 pm; alas I wasn't standing exactly in the center, so my photo is a little bit off):

12:54

12:57

1:00

1:01 

1:03

Some theorize that the Pantheon was built to be a kind of sun dial because it faces north (not an orientation used by the ancient Romans for temples) and is designed so that the sun lights up the entrance fully on April 21, shooting out the front at the moment when the light beam swings around (as it did today) and reaches the doorway.   

April 21 is significant because, for centuries, it has been celebrated as the anniversary of the founding of Rome. This last photo is taken outside looking back in (the brightness over the door is the sun coming through the oculus inside). You can see how brightly it is already lighting up the entry. It is said that Hadrian, the emperor who built the Pantheon, designed it so the sun would spill out the entry on exactly April 21 to mark Rome's founding and to represent the emperor himself.