In 1786 a sundial was built for the Duomo, or rather a solar clock that indicated noon by projecting the sun's rays onto a bronze line inserted in the marble pavement of the Cathedral, subsequently accompanied by tiles representing the signs of the zodiac.
The sundial was designed by several members of the Brera Astronomical Observatory by creating an aperture at a height of approximately 24 meters on the south facing wall.
The subsequent completion of the external embrasure on the south side resulted in the sundial's aperture being obstructed, thus making it impossible for the sunlight to pass through it during the winter months when the sun tends to be weaker and lower in the sky.
In 1923 "the Board of Directors appointed architect [Adolfo Zacchi] to study together with engineer Luigi Gabba if and how work on the embrasure could solve this problem with the sundial, without altering the artistic integrity" (Annals, 23 March 1923).
The solution adopted, which was not actually put into effect until the second half of the 1940s, was that of creating a clean cut in the embrasure, thus avoiding blockage of the light as it passed through the opening.
As can be seen in the photo shown here, currently preserved in the photo library of the Veneranda Fabbrica del Duomo, from an ornamental point of view a high-relief decoration was selected featuring all the signs of the zodiac, clustered together one on top of the other and with rather realistic features. The new "window" carved into the marble, sculpted by brothers Carlo and Luigi Rigola, permitted the light to pass through the sundial's aperture once again, whatever the season.