Gnomon

Another great day in the Duomo! Today I attended an observation of the transit of the sun on the meridian plane (I’ll have to study more to understand that fully!). What happens is that on a few days in June, the sun’s rays are channeled through a bushing under one of the windows of the lantern atop the dome. The resulting disc of light covers a disc of marble set into the floor (the ‘solstitial marble’) on the northern side of the cathedral.

The bushing is the summit of a gnomon (an object that by the position or length of its shadow serves as an indicator especially of the hour of the day). Documents in the archive of the Opera di Santa Maria del Fiore suggest that Florentine mathematician Paolo dal Pozzo Toscanelli was the designer of the gnomon and that it was operational by 1475. Professor Alberto Righini, who gave us a presentation, also noted that this is the largest and most beautiful pinhole camera in the world!

When the disc of light first appeared, it was several feet away from the marble disc in the floor. I have never been more aware of the fact that it is not, in fact, the sun that moves, but the earth that rotates, as when Professor Righini said, as we watched the disc of light move, ‘it is the image of our motion.’

I came away having witnessed a fascinating phenomenon, with a book to add to my reading list (The Sun in the Church: Cathedrals as Solar Observatories by J. L. Heilbron), and with a potential obsession with seeking out other churches with gnomons! 

“Gnomon.” Merriam-Webster.com. Merriam-Webster, n.d. Web. 17 June 2016.

Astronomy in the Cathedral:  Opera di Santa Maria del Fiore.

Images © Melissa Corcoran

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