The Pantheon has the largest unreinforced dome in the world. The dome has a hole in it, called an oculus, which along with the open doors are the only sources of light for the church. This oculus also served as a kind of sundial:
It is one of the best preserved buildings from the Roman world, a 2,000-year-old testament to the immense power and wealth of the empire.Because the oculus and the doors are not the best at providing interior light, it is very difficult to take pictures inside the Pantheon. This was the best I could do for the oculus:
But mystery has always surrounded what lies behind the unusual design of the Pantheon, a giant temple in the heart of Rome that was built by the Emperor Hadrian.
Now experts have come up with an intriguing theory – that the temple acted as a colossal sun dial, with a beam of light illuminating its enormous entrance at the precise moment that the emperor entered the building.
Constructed on Hadrian's orders and completed in AD128, the Pantheon's hemispherical dome is punctured by a 30ft-wide circular hole known as the 'oculus'.
It provides the interior of the building with its only source of natural light and allows in rain and – on rare occasions – snow.
Giulio Magli, a historian of ancient architecture from Milan Polytechnic, Italy, and Robert Hannah, a classics scholar from the University of Otago in New Zealand, have discovered that at precisely midday during the March equinox, a circular shaft of light shines through the oculus and illuminates the Pantheon's imposing entrance.
They have been working on the theory since 2009 but recently brought together all their latest research in a paper published in a scholarly journal, Numen.
The precise calculations made in the positioning and construction of the Pantheon mean that the size and shape of the beam perfectly matches, down to the last inch, a semicircular stone arch above the doorway.
A similar effect is seen on April 21, which the Romans celebrated as the founding date of their city, when at midday the sun beam strikes a metal grille above the doorway, flooding the colonnaded courtyard outside with light.
The dramatic displays would have been seen by the Romans as elevating an emperor into the realm of the gods – a cosmological affirmation of his divine power as he entered the building, which was used as an audience hall as well as a place of worship.
He was in effect being "invited" by the sun to enter the Pantheon, which as its name suggests was dedicated to the most important deities of the Roman world.
"The emperor would have been illuminated as if by film studio lights," said Professor Magli. "The Romans believed the relationship between the emperor and the heavens was at its closest during the equinoxes.
It would have been a glorification of the power of the emperor, and of Rome itself." The sun had a special significance for the Romans, as it did for the ancient Egyptians. The god Apollo was associated with the sun, and the emperor Nero was depicted as the Greek sun god Helios in a giant statue called the Colossus, which gave its name to the Colosseum.