A Solar Analemma is a graph or plot that shows the position of the Sun in the sky at a single location and at the same time of the day throughout the year. Here in Bergamo, a hole in a metal disk causes a dot of sunlight to be projected on the ground and the path of the Sun throughout the year is recorded by keeping track of the position of the Sun at local noon, thus creating the analemma.
A north-south meridian line (blue arrow) in white marble is inlaid into the flag-stone floor with an analemna marked for solar noon on every day of the year permanently engraved in the stone work.
At the top of an arch at the southern end of the meridian line is the gnomon (red arrow), a bronze disk that is perforated with a small hole near the center. On any day of the year, at noon (and if the sky is clear) sunlight will pass through the hole and project a dot of light on the ground (yellow arrow). At the precice moment the center of the Sun intersects the meridian line, it is local noon.
On some days of the year, i.e. around the time of the winter solstice, the placement of this particular analemna is important, as the sunlight must pass the gap between the two large church buildings.
An excellent explanation of analemmas in general and specifically the Bergamo analemma can be found at the website of the Coventry Astronomical Society.
Some beautiful photographs of solar analemmas (the actual images of the sun at the same time of day over a year, not the sundial) can be viewed at Greek Astrophotographer Anthony Ayiomamitis’s website.