The Pantheon

The Pantheon is easy to look at and hard to write about accurately. It has been continuously in use for nineteen centuries, which means almost everything about it has been described many times, and a fair amount of what has been described is wrong.

These eight pages take the building apart: how the dome works, where the light goes, what happened to it over nineteen hundred years, who is actually buried in it, and which of the famous stories hold up. Where the sources disagree we have shown the disagreement instead of picking the tidier number.

If you read nothing else here, read this. It is the whole building, in the order you meet it.

You arrive at a temple front that does not belong to the building behind it. Sixteen granite columns, eight across the face and eight more forming a central passage and two aisles, holding up a pediment that once carried a bronze relief nobody can now reconstruct. The columns came from Egypt, each one a single stone about 11.7 metres long weighing something like fifty tonnes, and the eight at the front are grey while the eight behind them are pink, from a quarry six hundred kilometres further up the Nile. The inscription across the top credits Marcus Agrippa, who had nothing to do with it.

Then the doorway, and the trick. The bronze doors are 7.53 metres high and possibly the oldest thing in the building. Behind them the porch is dark and low. You take four or five steps through it and the room opens above you all at once, which is not an accident of construction: the porch exists to make the rotunda happen suddenly.

The room is a sphere sitting on the floor. Its diameter and its height are the same, about 43.3 metres, so the dome you are under is the top half of a ball whose bottom half would fit exactly into the cylinder holding it up. The cornice running round the wall marks the halfway line precisely. Nothing else in the room competes with that geometry: seven recesses, eight small shrines between them, coloured marble underfoot, and then the eye goes up because there is nowhere else for it to go.

The dome is thicker at the bottom and lighter at the top, and both of those are deliberate. The concrete thins from roughly 5.9 metres where it springs off the wall to about 1.5 at the rim, and the aggregate inside it changes as it rises, from travertine at floor level to broken brick to volcanic scoria near the crown. Somebody planned that in advance and supervised it for years. The 140 coffers, arranged in five rings of 28, remove under five per cent of the weight: they are pattern, not structure.

And at the top there is a hole. Nine metres across, never glazed, the only source of daylight in the building. Rain comes through it, which four centuries of popes tried and failed to prevent, and a disc of sunlight moves across the interior all day and all year, landing on the dome in winter, on the wall in spring, on the floor for about a hundred days in summer, and never once in the middle.

Rome is full of Roman buildings that no longer exist. They came down because an unowned, unguarded structure full of cut marble and bronze fittings is a supply of building material, and for a thousand years that is exactly how they were treated.

Two things saved this one, and neither is engineering. It acquired an owner: consecrated as a church in 609 or 610, it had a community that used it weekly and objected when people turned up with carts. And it acquired a roof: after the emperor Constans II stripped the gilded bronze tiles in 663, the structure stood exposed for seventy years until Pope Gregory III covered it in lead, somewhere between 731 and 741. Mark Wilson Jones calls that by far the most important protective measure in the building's history, and the margin was thinner than it sounds: seventy winters of standing water on an unprotected dome, in a city that gets around 800 millimetres of rain a year.

The concrete gets the credit in most accounts. The lead and the congregation deserve it.