The Meridian Passage of the Sun at Bergamo

 

At exactly Latitude 48° 42’ 11” North, Longitude 9° 39’ 16” East, you are in the portico of the Palazzo della Ragione (The Palace of Reason) in Pergamo. At your feet is a masterwork of the Age of Reason. You are standing on a compass rose where the Latitude and Longitude has been carved into the stone. Heading due north along the meridian is a long white line of marble set into grey flagstones. It is marked with days and months of the year. It extends about ten metres into the shadows under the building.  The line is capped at each end by the summer and winter solstices, when the Sun is at its most northerly and southerly points in the sky giving us our seasons.

What you are looking at under the shuffling feet of uninterested tourists, is the analemmatic sundial of Bergamo. It was created by the mathematician Giovanni Albrici in 1798 to provide a standard for setting the city’s clocks. It is a remarkable and precise scientific instrument.

In Pergamo on 5 April 2025, sidereal or true noon falls at 1:25pm by your watch. As the time approaches, a small circle of sunlight, beamed through a hole in a metal plate high in the arch, moves slowly eastward towards the meridian line. The circle of light crosses a thin black line right by the 5 April mark at the moment the Sun reaches its zenith. At that instant, the observer knows that it is true noon and knows the date — no mean feat in 1798.

Why did this happen at 1:25pm on your watch rather than 12:00 pm? The answer lies in the fact that the human contrivances of time and longitude are interchangeable. At 15° East, noon happens one hour earlier than it does at the Greenwich Meridian. So, at 9° 39’ 16” East, noon comes a little over half an hour earlier: 11:25 am. Adding an hour for a time zone change and another for Daylight Saving Time gets you to 1:25pm.

What the row of dates along the line measures is the declination of the Sun; that is its apparent rise and fall in the sky at noon as the Earth, with its 23° tilt wobbles its way round its orbit. It is also marked by an elegant, elongated figure of eight engraved in black and interwoven with the meridian line in Albrici’s design. It is this black line that the disc of light crossed at noon.  It is called the Analemma and has been known since ancient times.

Mariners carry out the same calculation in reverse. They know the date and a nautical almanac gives them the declination. They watch the sun through a sextant and as it reaches its highest point, they note its angle above the horizon. By subtracting that from 90° and making a clever adjustment for declination, they have their exact latitude.

Although guides point out Albrici’s analemmatic sundial to their tourists, few take any interest beyond noticing the zodiacal symbols that indicate the progress of the Earth round its solar orbit. Today, you are the only keen observer of the meridian passage of the Sun.

Beneath the arches of the Palace of Reason, we have lost our connection to the Age of Reason.

I travelled with the excellent Great Rail Journeys