05 February 2009

Time Gazing at the Pantheon in Rome

What's the time now?

An expert in ancient timekeeping thinks we can tell the time from this sunbeam entering the Pantheon in Rome. The building is, he says, a colossal sundial.

The Pantheon, a Roman temple dedicated to "all the gods" , was finished by the Emperor Hadrian in 125 CE. It consists of a cylindrical chamber topped by a domed roof with a big open hole in the top (the oculus) which lets through a dramatic shaft of sunlight -- bang, as you see on the left.

Paranormal blogs have got quite excited by this news ... but what is it all about?

Robert Hannah, author of Time in Antiquity, argues that the Pantheon is just a vastly larger version of the sundial used by the Greeks and Romans -- a hollowed-out hemisphere with a hole in the top -- to tell the time as well as seasonal information (solstices and equinoxes). The Roman comic poet Plautus (250-185 BCE) complained bitterly that the town was already full of those accursed sun-dials:
The gods confound the man who first found out
How to distinguish hours! Confound him, too,
Who in this place set up a sun-dial,
To cut and hack my days so wretchedly
Into small portions.
Like those unutterably smaller dials, the Pantheon's dome forms a perfect hemisphere inside. Hannah reckons that this is no coincidence but, he says, "a deliberate design feature."

You enter through gigantic bronze doors – the originals. There were once veneers of precious marbles within, pure gold tiles on the roof, and the bronze doors, weighing twenty tons each, were themselves once covered with plates of beaten gold. Inside the temple is one immense circular room. The interior is a cylinder above which rises the hemispherical dome, constructed of stepped rings of solid concrete with less and less density as lighter aggregate (pumice) is used as it rises. The only natural light enters through the oculus at the centre of the dome and through the bronze doors to the portico.

As the sun moves, striking patterns of light illuminate the walls and floors of porphyry, granite and yellow marbles.

Its roof represented the dome of the sky -- where Romans believed the gods resided. Cassius Dio (who saw it in its glory days) says as much:

Also [Hadrian] completed the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens.(53.27)

At equinox, the sun is on the celestial equator -- where Earth's equator would lie if projected into space -- which was seen as the most stable part of the sky, a perfect eternal home for the gods.


The inscription on the architrave reads “Marcus Agrippa, son of Lucius, third time consul, made this temple.” Originally constructed in 27 B.C. by Agrippa, the Pantheon was rebuilt by the Emperor Hadrian in 118-125 CE to reflect the terrestrial and cosmic order.

The interior space was designed around a perfect sphere 43.3m (142 feet), in diameter, the largest dome ever constructed in masonry. Its mathematical perfection is underscored by its symmetry: the width and the height of the dome are equal. A whole sphere can be inscribed in the interior volume, with the diameter at the floor of the cylinder of 43.3 m (143 feet) equalling the interior height.

The sphere (or orb) was a symbol of the world and universe and consequently doubled up as a symbol of the Emperor's power over the world. It's not far-fetched to see the Pantheon as also a political message marking the symbolic connections between the cosmos and the empire, and between the sun and the emperor.

There is a progressive narrowing of the five coffered rows of the cupola’s interior, so that the eye is drawn to the centre. And up, up, up to the oculus. The 9 m. (30' ) hole in the roof allows in light (and rain!). When the sun is shining, the shaft of light travels across the interior of the building. It takes little imagination to conclude that a cosmic effect was being sought rather like being in a large scale observatory where the heavens are turned outside-in.

In her Memoirs of Hadrian, Marguerite Yourcenar speaks of
"this open and secret temple, conceived as a sundial. The hours were to circle the center of its carefully polished pavement where the disk of the day was supposed to rest like a golden buckler; there the rain would make a limpid pool from which prayer could transpire like smoke toward the void where we place the gods.”
All other Roman temples before and since cope perfectly well without the oculus solution. Even Hadrian himself designed the enormous temple to Venus and Rome without such a device. Hannah thinks that by marking the equinoxes, the Pantheon was designed to elevate emperors who worshipped there into the realm of the gods.

How It Could Have Worked

During the six months of summer the noon sun falls on the walls and floor of the temple, and in winter (when the sun is lower in the sky) it falls onto the inside of the roof itself. But at the exact moment of the two equinoxes, in March and September, the sun falls at the junction between the roof and the walls, directly above the northern doorway, and shines through a grill there onto the porch floor beyond.

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. So, it seems certain that the building served some sort of important function on the equinoxes. Extinguish the torches ... and it's easy to imagine that, twice a year, the emperor would get to stand in that ray of light.

The Pantheon probably doesn't work as a sun-dial (the necessary markings are not there; or, at least, have not survived) but imagine the impact of the emperor's entrance. And, then, his prayers rising through the open oculus to the heavens, a union of earth and sky, from the god on earth to all the gods above.


Top: From Great Buildings.com

Upper left: From The Monolithic Dome

Middle left: From Roman Pantheon

Below: No place like dome: the Emperor Hadrian's connection with the Pantheon at TimesOnLine

And with thanks to Dea Adria Mallin at The Cultured Traveller: the Oculus, and to MariaMilani, Antiquities of Rome: Purpose of the Roman Pantheon


  1. A great post, Judith. I'm going to put a link to it on my old post on the Pantheon. Making the concrete less dense at the top of the dome by adding pumice was somebody's brainstorm. I wonder if that's in Vitruvius. I know they used puzzoli to waterproof their concrete for bridge piles, so I guess the engineers all knew that it also lightened the mix.

    Hey--I had never seen the building from the side and am puzzled by the rows of arches. Aren't they unsupported just where you'd expect them to be? Shouldn't the feet of the arches rest on the top, the keystones, of the arches under them and not in the "air"?

  2. I think,100swallows, that it might have something to do with the lighter load as the rings rise:

    Deeper analysis of the Pantheon's building materials has shown it to be made up of 6 annular sections of which the lowest is the foundation made in cement mixed with travertine stone to a thickness of 4.5m. As the rings reach the oculus at the top the materials are progressively lightened: Travertine is replaced by porous Tufa, brick and finally pumice (volcanic ash).

    ...The exterior of the perimeter wall is entirely faced in brickwork with inbuilt arches aimed at channeling the weight of the dome down to the ground. The ancient architect Piranesi suggests that similar weight shedding arches were also introduced into the dome itself.

    From Antiquities of Rome

  3. Nothing of value to add except to say that the photos are gorgeous and that thinking of all this going on - so very many years ago - really puts 'life' into a different perspective.

    Very wonderful post. :)


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