We've talked a bit before about how the ancients knew what time of day 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 for a long time rather more of a mystery. In fact the answer was engineered into the very streets of Rome itself.

We know that the Romans divided the day into twelve equal portions of daylight regardless of the time of year so the hours expanded and contracted in length with the seasons. But how this was measured, and how a vast empire was run from Rome on such an elastic timescale was for a long time a bit of a mystery. Clues have emerged over the years from excavations in the city and in the works of ancient writers, but its only been with the aid of modern CGI reconstructions that the answer to how Romans told the time 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. To achieve this it would have to be the largest sundial ever built.

A sundial, as you know consists of two fundamental components. A gnomon which casts the shadow and a dial plate against which that shadow is read. If the dial plate was to be the very pavements and buildings of Rome itself, then the gnomon was obviously 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 handy 22 metres high red granite obelisk, formerly the property of Pharaoh Psammetichus II (595-589 BC), which they then had to transport to Rome. As we know from the trouble the British had getting their similar 'Cleopatra's needle' to London, moving obelisks is a tricky business. Pliny the Elder, who later recorded the event, was certainly 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."

In Rome this obelisk was then set up on a 160 by 75 metre pavement in the Campus Martius, into which a meridian, worked out by the mathematician Facondius Novus, was placed and a quadrant marked out with massive bronze letters, with indications of the hours, months, seasons and signs of the zodiac in Greek. This dealt with the timekeeping but the Horologium was intended to do more than mark that. The buildings around it were placed so that the shadow of the obelisk would touch them on significant days. On just one day of the year the tip of the shadow passed over the place where deceased members of Augustus' family were ceremonially cremated. On another it touched the very centre of Augustus' mausoleum. And on 23rd September, the Emperor Augustus' 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 a device to remind the people of Rome of the gifts, both physical and political that the emperor had given the city, and demonstrate that the world and the heavens followed a divine order in which the Gods had placed the Imperial family above ordinary men and women.

Of course there are problem associated with setting you timepiece in stone and even in Pliny the Elder's day, and despite his enthusiasm for the horologium and the Imperial family, 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 behavior of the heavens, or the whole earth has moved slightly off centre. I hear this phenomenon has been observed in other places."

Eventually a new pavement was laid to correct the timekeeping, although the buildings surrounding the gnomon could not, of course, be moved. The gnomon itself continued to stand however until at least the 8th century although, thanks to earthquakes, its time keeping become yet more erratic. After that the records fall silent and at some point the obelisk fell. It was only rediscovered in 1512 and had to wait until 1789 before being set up again in its present location in the Piazza di Montecitorio by Pope Pius VI. In 1998, during a redesign of the Piazza, a new meridian was traced on the pavement pointing towards the Palazzo Montecitorio. Sadly the shadow of the gnomon doesn't point precisely in that direction, so Augustus's clock is once again telling the wrong time.