Not as elaborate or elegant as Walter Murch's notion of the Pantheon as a repository for ancient <a href="http://bldgblog.blogspot.com/2007/04/heliocentric-pantheon-interview-with.html" target="_blank">heliocentric astronomical knowledge</a>, but still plenty interesting:

<em>During the six months of winter, the light of the noon sun traces a path across the inside of the Pantheon's domed roof. During summer, with the sun higher in the sky, the shaft shines onto the lower walls and floor. At the two equinoxes, in March and September, the sunlight coming in through the hole strikes the junction between the roof and wall, above the Pantheon's grand northern doorway (pictured). A grille above the door allows a sliver of light through to the front courtyard - the only moment in the year that it sees sunlight if its main doors are closed </em><em>(see diagram)</em><em>.</em>

<em>Robert Hannah reckons this is no coincidence. A hollowed-out hemisphere with a hole in the top was a type of sundial used in Roman times, albeit on a much smaller scale, to show the time of year. While the Pantheon's dome is quite flat on the outside, it forms a perfect hemisphere inside. "This is quite a deliberate design feature," says Hannah.</em>

Full story at <a href="During the six months of winter, the light of the noon sun traces a path across the inside of the domed roof. During summer, with the sun higher in the sky, the shaft shines onto the lower walls and floor. At the two equinoxes, in March and September, the sunlight coming in through the hole strikes the junction between the roof and wall, above the Pantheon's grand northern doorway (pictured). A grille above the door allows a sliver of light through to the front courtyard - the only moment in the year that it sees sunlight if its main doors are closed (see diagram)." target="_blank">New Scientist</a>