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Well, Rome is celebrating a big birthday this week. The city was founded 2,764 years ago. 

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Well, according to legend, Remus and his twin brother, Romulus, decided to found Rome at the place where they were fed by a she-wolf after being orphaned as infants. 

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It would be hundreds more years before one of the city's most famous structures, the Pantheon, was rebuilt by the emperor Hadrian. Well, Nick Glass takes us inside.

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You look and you marvel. 

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The Pantheon is one of the great buildings from antiquity and one of the best preserved, a triumph of Roman architecture. 

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The idea was to strike awe, and that's what it's been doing for the best part of 2,000 years.

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The emperor Hadrian, who built it, spent extravagantly; just look at the columns in the portico. 

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White marble bases come from Greece, the marble capitals right at the top are from northern Italy, 

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and the shafts themselves are granite, and they come from Egypt from the imperial quarries. 

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Just imagine how they got here across the desert, across the sea and up the river Tiber. 

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They come from single blocks of stone. They each weigh about 100 tons.

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A magnificent if conventional temple on the outside, but that said, nothing prepares you for the grandeur of what's inside. 

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It's taken weeks to get permission to film, 

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but it's inside where our story really gets intriguing, where we get to rethink what the Pantheon is about.

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The eye is immediately drawn to the domed roof and to another eye, the so-called oculus, some nine meters across and the only light source. 

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Scholars have long struggled to explain why it's there, but now we've been offered a stunning new theory. 

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It comes from a relatively new field of science -- archeoastronomy, the study of ancient monuments and the heavens.

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Giulio Magli is a professor at Milan Polytechnic. One thing about the building struck him immediately.

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You will never see the facade of the Pantheon illuminated by the sun.

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That's extraordinary.

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It's extraordinary. It's extraordinary. It gives you a sense of cold. It's like the monument is cold. 

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The monument is, in principle, unfriendly. The key to this unfriendly behavior of the monument is precisely the role of the sun inside the monument.

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The fact is, unusually for a Roman temple, the Pantheon faces north. 

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The tourists will always be better lit than the shadowy monument behind them.

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Professor Magli and the classics professor from New Zealand's Otago University, Robert Hannah, 

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analyzed the movement of the light beam inside, and they concluded that the Pantheon acted for ancient Romans as a sort of colossal sun dial. 

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It's self-evident that the Pantheon was exactingly engineered.

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Having just discovered concrete, the Romans used it to construct the largest dome known to man. 

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The hole in the roof had a purpose. The architect was orchestrating the light. 

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In autumn and winter, it illuminated upper parts of the dome. In spring and summer, it moved down to the floor.

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But what really excited our two professors was what happened at the March equinox. 

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At noon precisely, the beam hit the grille above the entrance door and lit up the portico. 

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The equinox was when the Roman emperor was perceived to join the gods himself. 

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We don't believe anyone has ever filmed this phenomenon before -- the beam gradually migrating across the dome to the arch above the door. 

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For a fleeting moment, it's like a spotlight. The arch is precisely outlined. 

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The light then filters through the grille, and ultimately on April 21st, the traditional date for the foundation of Rome, it floods the portico.

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The sun's cycle was heavy with symbolism for ancient Romans. 

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It seems the Pantheon with its shallow dome, was designed to follow that cycle, not for precise timekeeping in any modern sense, 

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but to mark key dates in the Roman calendar.