Sicily - Sundials - 2

Dear Friends
a strange sundial I met in the Cathedral of Palermo: http://en.wikipedia.org/wiki/Palermo_Cathedral .
Built by Giuseppe Piazzi: http://de.wikipedia.org/wiki/Giuseppe_Piazzi , director of the Palermo Astronomical Observatory and discoverer of the dwarf planet Ceres: http://en.wikipedia.org/wiki/Ceres_(dwarf_planet) , has the purpose of indicating the passage of the Sun on the meridian of Palermo, i.e the noon, and to measure the duration of the true solar day, i.e. the time interval between two consecutive passages of the sun on said meridian. Any further information from your side is welcome.
Salutations
Giancarlo