Sunday, November 18, 2012

The Bergamo Analemma

While in Italy I briefly visited Bergamo and stumbled upon a wonderful projection of the Sun's analemma in the old town. 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. The path of the Sun is an analemma. In Bergamo the position of the dot has been permanently engraved in the stone work:

In the photograph you can see the curved line that shows the local noon position of the Sun as projected through a metal disk. At the top of the image you can just make out the metal disk that has the hole used to create the track of the Sun on the ground. The two ends of the analemma represent the summer and winter solstices. The central line is a meridian and the photograph above is facing south. Here's the compass rose at the end of the meridian line:

Here's the disk:

And the position of the solstices:

All along the meridian line are the days of the year laid out so that the date can be determined from the angle of the Sun at solar noon.

I didn't manage to get a good shot of the characteristic elongated figure-8 shape of the analemma itself. The ends of the 8 are the solstices. The two outer lines parallel to the meridian show 15 minutes before and after solar noon.

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