The Meridian Line of San Petronio

Piazza Maggiore in the centre of Bologna is dominated by the massive unfinished Basilica of San Petronio, the male patron saint of Bologna. It’s the tenth biggest and the largest brick gothic style church in the world. Construction commenced in 1388 and ground to a halt sometime in the 16th century.

On any day around midday you’re likely to see a group of people intently looking at the floor of the church. What are they up to?

Before the invention of the electric telegraph there was no concept of standard time or time zones. There was no need for such a concept as the fastest form of communication was a galloping horse. With the construction of railway lines between cities and their associated timetables, standard time synchronised by telegraph messages became the norm.

Before then, each town had its own time, usually determined from when the sun was at its highest point each day. The sundial is the traditional way of measuring this.  A meridian line is a far more accurate instrument to determine solar noon as well as to measure progress through the months of the year and to ascertain important days of the solar year such as equinoxes and solstices.

Also the length of the Bologna meridian line, the longest internal functioning one in the world, is such that at the time of its construction it was a very important instrument for astronomical research.

High above the floor of the church there’s a hole in the ceiling of the left nave.

For about ten minutes either side of local noon, a circle of light is visible on the floor, as long as the sun is not obscured by cloud. Depending on the time of the year, this occurs in Bologna sometime between a few minutes before and almost a half an hour after Central European Standard Time 12pm noon. During summer time this is an extra hour later again around 1pm.

As the sun is at different heights during the year , the circle of  light appears at differing sizes along a line from almost directly below the hole in the ceiling in summer to close to the front wall of the church in winter. The corresponding month and dates are shown on the line.

The meridian line was constructed by Gian Domenico Cassini in 1655. It was rebuilt by Eustachio Zanotti in 1776 and its high accuracy has only diminished slightly since then due to changes in the earth’s orbit and some settling of the floor. Great trouble was taken to maximise the accuracy of the installation, particularly to ensure the line on the floor was a close as possible to horizontal.

A meridian line needs to run exactly north/south. Unlike most churches which are aligned east/west, the basilica is aligned to Piazza Maggiore on which it stands and a bit off north/ south. Luckily the required length of the line could be achieved avoiding the church’s many columns.

Measurements using the meridian line helped Cassini confirm the need toomit a leap year in 1700, as proposed in the Gregorian  calendar and to verify Kepler’s second law regarding the speed of planets in elliptical orbit.  Galileo’s idea from some thirty years earlier that the earth rotated around the Sun and not vice versa was also confirmed.  All this was possible due to the 67 metre length of the meridian line allowing much more accurate measurements than was possible using the telescopes of the time.

Finally, as you turn to leave the Basilica, look up at the window high in the front wall above the main door. You’ll see a little seat at an angle to the window. In fact, it’s aligned to the meridian line. From this vantage point, Cassini planned his creation.

If you’re in Bologna in the middle of a sunny day, don’t miss a visit to watch the sun’s image cross the line!

Winter Solstice 2024

In 2024, I made it to San Petronio on 22 December, the day after the Winter Solstice. Looking up at the ceiling hole, the appearance was quite different from some other times of the year.

A light cloud cover had cleared mid-morning, and as the sun’s image approached the meridian line, it looked like we would get an excellent display. A few seconds before noon, someone thrust their phone out creating a shadow and spoiling things for everyone, including themselves! People were calling out “telefono, teléfono!” but it was too late.

Outside the basilica in Piazza Maggiore there are more fascinating things to see, as described in my post, Curiosities of Bologna’s Piazza Maggiore .

You can find further astronomical detail at this site.

For your visit, you can find when solar noon will occur here.