Traced on the paving in front of the ancient Palazzo dei Castellani, seat of the Institute and Museum of the History of Science, this sundial marks true local time throughout the year, while also serving a calendar function. The signs of the zodiac, in water-green glass, flank the meridian line in Tuscan travertine, which extends about 15 m from the sidewalk on the Lungarno, where it marks the summer solstice, to the Museum's entrance, where it indicates the winter solstice. The hour lines, which for reasons of space show the hours only from 9:00 am to 3:00 pm, form an array of slender brass bands between the solstitial curves at the edges of the great dial. The hours are indicated by a monumental gnomon, a double bronze stele 6 meters high that rises at the center of a "pool", also made of water-green glass, encircled by a wind rose.
The materials used clearly allude to the four elements, which, along with the seasons, indicated on the sundial by the course of the Sun, evoke the ancient Platonic doctrine of the correspondence between microcosm and macrocosm, that is, between man and the universe. The "water" effect of the glass recalls winter and the phlegmatic humor, while the wind rose around it evokes spring and the sanguine temperament. The "solid" stele – mirroring autumn and the melancholy humour – captures the "fire" of the Sun's rays in a glass polyhedron – the image of summer and the choleric temperament – whose shadow marks the hours on the stone paving of the square.
Unlike any other sundial, this one still offers a fascinating spectacle after sunset. Lights installed underground transform the signs of the zodiac into luminous traces, and the gnomon into a sort of Moon-Sun that shines even by night. A beam of light imprisoned between the two stelae lights up the polyhedral sphere atop the gnomon, which thus continues to serve as a visual signal in the urban landscape of