The clock that was built into a city's streets

'Half-past mausoleum' could have been a Roman's response to a request for the time.

We've talked a bit before about how the ancients knew what time it was. In Athens a water clock dripped out the seconds, but just how your average Roman knew when to knock off for lunch was rather more of a mystery. In fact, the answer was engineered into the streets of Rome itself.

The Romans, we know, divided the day into 12 equal portions of daylight regardless of the time of year, so the hours expanded and contracted in length with the seasons. But for a long time no one was quite sure how this was measured, and how a vast empire was run from Rome on such an elastic timescale. Clues have emerged from excavations in the city and in the works of ancient writers, but it's only with the aid of modern CGI reconstructions that the answer has begun to emerge, and it's startlingly simple – the centre of their city was one vast sundial.

Just whose idea it was to turn the monumental architecture of Augustan period Rome into a timepiece remains unknown but, perhaps not surprisingly, the finished marvel was known as the 'Horologium Augusti'. To call it just a sundial seems a bit churlish when you consider its scale and sophistication. The plan behind the Horologium was to create a device on such a scale that the inhabitants of the city could walk through it rather than up to it. It would have to be the largest sundial ever built.

A sundial consists of two fundamental components. A gnomon, which casts the shadow, and a dial plate against which it is read. If the dial plate was to be the pavements and buildings of Rome, then the gnomon was going to have to be pretty huge and so the Romans turned to a country famous for building on a rather epic scale – Egypt. Here at Heliopolis, in their newly conquered territory, they found a 22m-high red granite obelisk, formerly the property of Pharaoh Psammetichus II (595-589 BC), which they had to transport to Rome.

As we know from the trouble the British had getting 'Cleopatra's Needle' to London, moving obelisks is tricky. Pliny the Elder was impressed by Roman engineering expertise: "Transporting the obelisk to Rome by sea was a more difficult task by far. The ships attracted much interest. The late emperor Augustus dedicated the ship that carried the first obelisk and preserved it in a permanent dock at Puteoli to mark this marvelous feat. Cement caissons were installed on board at Puteoli. The vessel was then towed to Ostia and scuttled to help construct the port."

This obelisk was then set up on a 160m by 75m pavement in the Campus Martius, into which a meridian, worked out by the mathematician Facondius Novus, was placed and a quadrant marked out with the hours, months, seasons and signs of the zodiac in Greek. This dealt with the timekeeping, but the Horologium was intended to do more. The buildings around it were placed so that the shadow of the obelisk would touch them on significant days.

On one day of the year the tip of the shadow passed over the place where deceased members of Augustus's family were ceremonially cremated. On another, it touched the centre of Augustus's mausoleum. And on 23 September, the Emperor Augustus's birthday, the shadow reached the bottom of the steps of the Ara Pacis Augustae, the 'Altar of Augustan Peace' commissioned by the Roman Senate for the triumphal return of the emperor from Hispania and Gaul. This was to remind the people of Rome of the gifts that the emperor had given the city.

Of course there are problems with setting your timepiece in stone, even in Pliny the Elder's day, and despite his enthusiasm for the Horologium he has to note that: "These measurements, however, have not agreed with the calendar for some 30 years. Either the sun itself is out of phase or has been altered by some change in the behaviour of the heavens, or the whole earth has moved slightly off centre."

Eventually a new pavement was laid to correct the timekeeping. The gnomon continued to stand until at least the 8th century although, thanks to earthquakes, its timekeeping become yet more erratic. At some point the obelisk fell and was only rediscovered in 1512. In 1789 it was set up again in the Piazza di Montecitorio.

In 1998 a new meridian was traced on the pavement. Sadly, the shadow of the gnomon doesn't point precisely in the same direction, so Augustus's clock is once again telling the wrong time. 

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