MERIDIANS AND OBELISKS IN ROME

Measuring time in the Eternal City

Rome has thirteen obelisks — more than survive in Egypt. We see them every day at the centre of the most famous squares, we photograph them, we walk past them. We rarely ask what they really are: scientific instruments. Each obelisk is a gnomon — the needle of a sundial — and its shadow has measured time for three thousand years, first for the pharaohs of Heliopolis, then for the emperors of Rome, then for the popes of the Counter-Reformation. This itinerary leads the visitor along the invisible line that crosses the city from the Field of Mars to the Vatican, touching the places where the shadow of a stone changed history. It begins at the obelisk of Augustus, whose colossal gnomon pointed every 23 September — the emperor's birthday — towards the Ara Pacis. It passes through the oculus of the Pantheon, which projects onto the floor a disc of moving light like a solar clock built into the dome. It arrives at the Clementine Meridian of Santa Maria degli Angeli — the longest in Europe — built in 1702 to verify that the Gregorian calendar reform was astronomically correct; and at the Tower of the Winds in the Vatican, where that reform had been worked out a century earlier.