The Gnomon of Florence,
an ancient astronomical instrument
revived by the astronomers of Arcetri Observatory

by Silvia Messeri

The time is 12.15 and 58 seconds on the 21st of June when the Sun, passing through the bronze plate inside the cathedral dome, fits perfectly within the white marble circle 90cm in diameter on the floor of the church used to calculate the summer solstice. In fact, the sun's rays only manage to pass through the hole in the bronze plate for a few minutes from the end of May to the end of July, thus creating the luminous circle on the cathedral floor.


The moment in which the sun strikes the marble circle in Florence Cathedral
(photo FAN)


1700's drawing of the gnomon

There has been a gnomon in the church of Santa Maria del Fiore since it was built. This valuable astronomical instrument was used for around 300 years to calculate the position of the sun in the sky. At 90 metres, this one is the tallest of its kind. According to historical research published in 1757 by the Jesuit Leonardo Ximenez, Paolo del Pozzo Toscanelli would appear to be the creator of the gnomon whereas 1468 is the date in which the instrument was probably put into use.

Such a large gnomon, besides being an instrument which measures the exact moment of the solstice and therefore an indicator of sowing and harvest times, was also probably used to work out whether the sun's yearly path through the stars is constant. With the invention of telescopes the gnomon lost its importance but Ximenez continued to use it and study from it until his death in 1786.

On the occasion of the 700th anniversary of the cathedral, Arcetri Observatory in Florence has begun using the gnomon again to allow people to learn about a valuable instrument which our ancestors used up until the invention of the telescope.

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