The Vatican obelisk, from Caligula's circus to the middle of St Peter's Square

Most people treat the obelisk in St Peter’s Square like a lamp post, something glimpsed from a distance on the way to the queue. I think it’s the oldest and most travelled object in the whole Vatican, older than the basilica, the colonnade and Christian Rome itself. If Pliny’s clue is right it’s close to four thousand years old, it has changed cities three times and it ended up working as a sundial. Below is what the Holy See’s fact sheet tells us, and what to look for around the base.

Three cities before this square

Its origins are uncertain, and the Holy See says so plainly. In his Natural History, Pliny the Elder describes an obelisk at Heliopolis raised by a pharaoh he calls Nencoreus, perhaps Amenemhat II. Between 30 and 28 BC Cornelius Gallus used it to decorate the Forum Julium in Alexandria. Then Caligula, emperor from AD 37 to 41, had it shipped to Rome for his private circus, on the very ground the state now occupies.

That circus is why the story matters. Tradition says Peter was martyred beside it, and both basilicas rose over his grave nearby. So the obelisk is the only survivor still standing from the landscape around that death. The story of this hill before it became the Vatican is told in my page on Vatican history.

How do you move 330 tonnes in 1586?

With engineering, patience and a small army. Sixtus V wanted the obelisk at the centre of the square and gave the job to Domenico Fontana, his trusted architect. The official sheet puts the operation at 907 men, 75 horses and 44 winches. You’ll see other figures online, 800 men and 160 horses among them, but I’m sticking with the Holy See’s.

The stone deserved the effort. The red granite shaft is 25.36 metres, the base 8.25, and the whole thing weighs 330 tonnes. It’s the second tallest obelisk in Rome and the only one without hieroglyphs, which to me is its oddest feature, because in a city full of carved obelisks this one is smooth.

Sixtus V also Christianised it. He removed the bronze ball from the tip and put up the cross, and commissioned the bronze lions and inscriptions at the base. Later, in 1723, Innocent XIII added bronze festoons and eagles on each face.

What’s written on the base

This is where most visitors walk straight past, and they shouldn’t. The inscriptions on the north and south faces record the move. Those on the east and west carry lines from the exorcism spoken when the obelisk was consecrated. A pagan monument turned round with words, in full public view. Cardinal Silvio Antoniano wrote the texts. For me it’s the detail that best sums up the late 16th-century Vatican, which didn’t destroy the ancient world so much as reuse it with a new story attached.

Since 1982, at John Paul II’s wish, the square’s nativity scene has been set up at the foot of the obelisk. If you’re in Rome in December, it all comes together, as I explain in my page on Christmas at the Vatican.

A seventy-metre sundial

In 1817 a Vatican astronomer, Mgr Filippo Luigi Gilj, made the obelisk into a gnomon. He laid a straight line of grey granite 70.79 metres long, set with marble discs, running from the base towards the right-hand colonnade. At noon the shadow of the cross falls on it and shows the sun’s declination. The two end discs mark the solstices on 22 June and 22 December, and five more mark the signs of the zodiac in pairs.

The same year Gilj set out the wind rose, sixteen marble discs around the obelisk naming the compass points and their winds. It has a slip the Vatican’s own website mentions with some amusement. On the basilica side it reads ‘west’, in English, while the opposite side reads ‘est’ rather than ‘east’. There are no other foreign words. Finding it is a great game with children. And if the astronomy hooks you, the Vatican later had an observatory of its own, which you can now visit at the Vatican Observatory in Castel Gandolfo.

How I’d look at it

The obelisk has no ticket office and no hours. You’ll see it free at any time, and at night, with the basilica lit up behind it, it’s even better. I’d do three things. First, stand on one of the two discs marking the centre of each curve, to see how Bernini’s colonnade arranges itself around the obelisk. Then walk up to the base to read the inscriptions and find the lions. Finally, follow the meridian out to whichever solstice disc is closer.

On audience Wednesdays and at the Sunday Angelus the base disappears behind barriers and crowds, so that’s not the moment to study it. The fountains, the travertine strip marking the border and the square’s weekly rhythm all sit on the main page for St Peter’s Square.