High on the white and green marble facade designed by Leon Battista Alberti sit two astronomical instruments installed in 1572 by Dominican astronomer Ignazio Danti:
The Marble Armillary Sphere. Mounted on the left facade pillar to determine the exact moment of the equinoxes.
The Astronomical Quadrant. Mounted on the right pillar to measure solar elevation angles.
The Pinhole Gnomon Aperture. Danti pierced the rose window with a small gnomonic aperture and began tracing a meridian across the nave floor — a line he abandoned when Bologna called him to San Petronio, and which restorers have traced again in recent decades.
Each carries its full hermetic decipherment and an observational protocol — where to stand, when, and what to watch for — in the Florence field codex.