Commissioned by Archbishop Philip Lopez y Royo during the renovation of the Cathedral during the last decades of the 18th century, the sundial was designed in 1794/95 by Giuseppe Piazzi, a Theatine priest, professor of Astronomy at the University of Palermo and director of the Astronomical Observatory of the Royal Palace. Located in the area in front of the Sanctuary, it was completed and inaugurated in 1801, as suggested by a Latin inscription on the white marble plaque of the gnomon. The latter was made by drilling a hole, covered with a metal plate, into a limestone on the dome in front of the Chapel of St Francis of Paola. Consisting of a prismatic brass bar set into the floor, the sundial crosses the space in front of the aisle sanctuary obliquely and is also characterised by the representation of the signs of the zodiac, decorated with polychrome marble.