We walk the streets of Rome and know the best-known sights, which do not escape the eye. But if we pay closer attention, we discover new stories and unexpected tools hidden among Rome’s buildings. Like the Roman meridian, or “Prime Meridian of Italy.”
It is a geographical reference line that runs through the city of Rome and has had important scientific, political and symbolic functions throughout history.
What is the Roman meridian?
Originally, the meridian of Rome was the reference point for measuring longitudes in Italy, before the universal adoption of the Greenwich meridian as the international standard (1884). The Roman meridian passes at 12°27’08.4″ east from Greenwich and historically passed through St. Peter’s Dome, Vatican City, Monte Mario (where a memorial plaque stands today) and, symbolically, also the Roman Forum, considered the navel of the Empire.
In the 19th century, after the unification of Italy, there was a need to establish an official point from which to start the geographical measurements of the state. Participating in a major European geodetic operation, Italy chose Rome itself, and more specifically the Monte Mario observatory, as the origin of its longitudes.
With the advent of GPS systems and the universal adoption of the Greenwich meridian as the world “zero” of longitudes, the Roman meridian has lost its practical function in measurement and navigation systems, but it remains a valuable historical and identity record.
Watch out for yellow medallions on sidewalks
The tower indicating the Monte Mario point is located in a military zone (visible only from the outside), but since 2008 the meridian has also been celebrated with 39 yellow medallions on sidewalks in various neighborhoods, showing citizens and visitors the line’s path on maps and in the city.
For those who want to scout them out: they represent an S that stands for the name of Secchi (astronomer who had the First Meridian tower built), with a vertical line to indicate the direction of the meridian and the dot to refer to North. One medallion is found on Candia Street, others can be found on Portuense Street, Cortina d’Ampezzo Street, Prospero Colonna Avenue, etc. The streets touched by the meridian’s passage affect municipalities XV, I, XIII, XII, XI and IX.
The sundial at St. Peter’s
Below the obelisk in St. Peter’s Square, on the right side, is a sundial (built in 1817 by the astronomer Gilj). With the shadow of the obelisk, it indicates solstices and equinoxes. And the Roman meridian passed through here.