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New Scientist features some interesting speculation regarding astronomical alignments built into the Pantheon:

The imposing temple in Rome, completed in AD 128, is one of the most impressive buildings that survives from antiquity. It consists of a cylindrical chamber topped by a domed roof with an oculus in the top which lets through a dramatic shaft of sunlight. It boasts a colonnaded courtyard at the front.

When Robert Hannah of the University of Otago in Dunedin, New Zealand, visited the Pantheon in 2005, researching for a book, he realised that the Pantheon may have been more than just a temple. 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).

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.

The book Hannah was researching is Time in Antiquity (Amazon US and UK), which “explores the different perceptions of time from Classical antiquity, principally through the technology designed to measure, mark or tell time.” While this case is certainly still up in the air, it seems that more and more ancient structures were built with intentional alignments to the sun and heavens.