The Cathedral sundial is inserted into the floor parallel to the counterfaçade, it was made in 1786 by astronomers of the Astronomical Observatory of Brera Giovanni Angelo de Cesaris and Guido Francesco Reggio.
It is a long gilded brass strip broken up by the twelve zodiac signs, and it runs up the left wall.
Being a watch and a solar calendar perfectly working, it had required over centuries of several checks and restores. One of these was made in 1827 and it was necessary because of the lowering of the floor plan.
A second test was made in 1929 by astronomer Louis Gabba on behalf of the Royal Astronomical Observatory of Brera.
The last check was made in 1976 as the excavations of the first subway line and the lowering of the phreatic aquifer caused a further lowering of the Cathedral floor. The gnomic hole, site at the first vault of the first bay of south aisle, was also widened.
Sundial is currently working.