Opera magazine
2017-03-29
History and mysteries of the Baptistery's gnomon
A decoration that is part of a complex astronomical machine, the "gnomon", and a history that carries with it obscurities, legends and some certainties: the gnomon of the Baptistery.
Upon entering the Baptistery, the most attentive visitor will notice that one of the beautiful inlaid marble panels on the floor—the one facing the Gates of Paradise—features the design of a wheel with the twelve signs of the zodiac, centered on a sun surrounded by a palindrome (identical in both directions) celebrating the sun as the center and driving force of the universe.
In fact, chroniclers and historians of the past (Villani, 14th century) report that before the Baptistery was surmounted by the current lantern and the marble floor was rearranged, resulting in the relocation of the zodiac (13th century), a ray of sunlight penetrated through a hole in the ceiling and fell precisely on the image of Cancer, a segment of the zodiac, every June 21st, ushering in summer on its solstice.
We do not know for certain whether this decoration was part of this complex astronomical machine whose technical name is “gnomon”, but almost certainly already in this remote era notions of astronomy were circulating in Florence, found in ancient classical texts, such as those of Ptolemy, and perhaps known through the studies of Arab astronomers.
A close-up of the Baptistery's marble zodiac.
In the past, it was also believed (Ximenes, 18th century) that the gnomon was designed between the 10th and 11th centuries by the Florentine Strozzo Strozzi, a man reputed to be a brilliant and multifaceted architect, sculptor, and astronomer. This, however, is a legendary figure, as Follini discovered in the 19th century: the floor inlay dates back to the early 13th century, as demonstrated by the fact that in the floor of the Florentine basilica of San Miniato al Monte, dated 1207, there is a second, almost identical, albeit somewhat simplified, one: perhaps a slightly later copy, perhaps executed by the same craftsmen.
However, suggesting there is some truth to the legend surrounding the Baptistery's gnomon, its "twin" has been discovered to actually calculate the summer solstice: on June 21st, just after noon (a small miscalculation by the anonymous astronomer who created it), a ray of sunlight penetrates the cathedral and still illuminates the sign of Cancer, which rules the sky that month.
In 1467, Paolo Dal Pozzo Toscanelli refined these older astronomical experiments by drilling a gnomonic hole in the dome of Santa Maria del Fiore in Florence, 86 meters above the ground, creating the tallest gnomon in Europe still in use today.
