The nearly 600 years long construction history (1386-1965) of the Milan Cathedral – il Duomo di Milano – is necessarily entangled with the history of the city. The cathedral is a special ‘imprint’ of the different architectural styles and the changes of public taste during that period, and of many trifle and defining moments in the city’s political, economic and social life. One of these stories is the construction of the cathedral’s sundial in the late 18th century.
In 1786 Maria Teresa issued a decree for the Lombardy region (under Austrian control that time) to change the official measurement of days from ‘Italian hours’ – a day lasting from sunset until sunset – to ‘French hours’ – a day beginning 12 hours after local solar noon (the moment when the sun transits the celestial meridian – roughly the time when it’s highest above the horizon on that day). To comply with this regulation a precise instrument had to be constructed for the city of Milan, capitol of Lombardy, to determine the moment of solar noon each day – that was the purpose of creating ‘La Meridiana‘, the sundial of the Milan Cathedral.
The cathedral as site was chosen by the Austrian and local authorities because the building seemed to meet all the technical and economic conditions – darkness, width, convenient access for citizens, relatively low expense of realization. The assignment was carried out by Giovanni Angelo De Cesaris (1749-1832) and Guido Francesco Reggio (1745-1804), astronomers of the Brera Astronomical Observatory, by creating a special sundial adapted to the characteristics of the building.
A sundial for indicating solar noon consist of a gnomon (signifier) and a surface with the local meridian line – if positioned precisely, the gnomon’s shadow (or sun spot, depending on the type of the gnomon) crosses the meridian line exactly at solar noon. The Latin contractions am and pm – ante meridiem and post meridiem – refer to the Sun passing the meridian.
In the Milan Cathedral the gnomon is a 25.2 mm diameter hole created on the vault of the building at 23.82 meters above the floor, and the meridian is a 15 mm thin brass line embedded in the marble floor, crossing the nave of the cathedral from south to north just at the entrance, ending with an about 3 meter long section running on the north wall, to the point that signals the winter solstice. The reason of this unusual extending of the meridian is the fact that the gnomonic hole couldn’t be adjusted lower because of the thickness of the marble coating of the vault’s structure, and the width of the cathedral turned out to be insufficient to contain the whole meridian line on the floor.
The sun’s light projecting through the gnomonic hole remains visible for about half an hour each day, the spot reaches the meridian exactly at solar noon. The position of the crossing point changes constantly along the brass line through the year according to the Sun’s relative height. To indicate this movement there are marble slabs embedded along the meridian marking the period of the year with the corresponding Zodiac signs and the date when the Sun enters them. The sign of Capricornus, marking the winter solstice, is in a special position placed on the north wall, due to the above mentioned circumstances.
The constructing of the sundial began in May 1786 and was completed in October of the same year. The passage of the Sun at the meridian was signaled every day by attendants through the tower of Palazzo della Ragione to the tower of Castello Sforzesco, where a canon was fired announcing midday for the whole city. In the 19th century the sundial was used to adjust the city’s mechanical public clocks.
During the centuries the sundial went through a few modifications, audits and restorations mainly due to alterations of the cathedral’s interior and roof. The last audit was performed in 1976, when the excavations of the metro-line construction caused a lowering of the building’s floor level. These days, thanks to restoration, La Meridiana works precisely again.
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