A new report in a scholarly journal supports the long-held theory by many experts that Emperor Hadrian designed his famous circular, domed Pantheon temple to serve as a solar calendar.
Built by the Emperor Hadrian and completed in 128 AD, the classic structure features the largest unsupported concrete dome in the world which is punctured by a 10-meter-wide (30 ft) hole called the oculus.
This opening is the temple's only source of natural light. It has long been speculated that rays of sunlight shining through the oculus had some symbolic nature. The new report supports that theory.
The report published in the journal Numen by Giulio Magli, an historian of ancient architecture from Milan Polytechnic, and Robert Hannah, a classics scholar from the University of Otago in New Zealand, claims that at precisely midday during the March Equinox, a circular shaft of light shines through the oculus and illuminates the Pantheon's entrance.
The size and shape of the beam of light matches, down to the last inch, a semicircular stone arch above the doorway.
A similar effect is seen on April 21st, which the Romans celebrated as the founding date of their city.
At midday on April 21st the sun strikes a metal grill 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 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."
He explained that during the darker winter months the beam illuminates only the vaulted dome.
However, at noon on the equinoxes it reaches the floor and on April 21 the beam fully illuminates the entrance at midday.
Construction of the Pantheon was started in 27 BC by Agrippa and his name is still visible over the entrance, although it was not finished until 128 AD by Hadrian, who is believed to be directly responsible for the unprecedented circular dome — an architectural innovation which lives on in such structures as St Peter's Cathedral and the U.S. Capitol.
The word Pantheon means "to every god" and the circular interior has niches for all the major deities of Rome.
The two experts first proposed their theory in 2009, but the published report is the first written evidence of their claims.
Giulio Magli is a self-styled expert in "ancient archaeo-astronomy" and seeks to prove that structures built by the Egyptians, Mayans, Incas, Aztecs and other ancient cultures were often aligned with the stars. Research by another expert recently claimed that portions of Hadrian's Villa are also aligned to celestial events.