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12 August Aug 2016 1433 2 months ago

The Duomo’s Meridian: science and history in the Cathedral

A surprising tale among the monument’s naves

Friday, 16 June 1786
Having read the letter from the Regio Imperiale Supremo Consiglio di Governo, which requires this chapter
to cooperate with the Brera Astronomical Observatory in creating a meridian in the Duomo, so that the chapter may dedicate itself to this necessary endeavour, (…) it has been requested of the rector to provide the relevant provisions for the execution.
(Annals of the Veneranda Fabbrica del Duomo di Milano, Vol. VI)

In Milan’s Duomo, stretching North to South along the length of the five naves that make up the Christian Prayer area and the Visitor’s Area, parallel to the façade, runs the meridian as we know it today after multiple renovations, the last of which overseen by the Veneranda Fabbrica in 1976 in order to fully restore its functionality.

The meridian is “a brass rod encased in white marble; it is half an arm wide and approximately three once thick, set into a stable foundation”. It is the projection of the intersection of the plane, or rather of the Duomo’s vertical line, with the axis of the Earth’s rotation, and it represents an important chapter in the nation’s history: a sign of significant economic growth, crowned by multiple reforms, during the reign of the Austro-Hungarian empire in Lombardy beginning in 1713, in particular under Maria Teresa and Giuseppe II.

In fact, the so-called “Time Reform“ ordered that from 1 December 1786 all clocks in Lombardy conform to “Transalpine time”, or rather “French time”, which dictated that the day begin at “true noon” and no longer at the first sound of the bells for the Ave Maria at vespers, or approximately half an hour after sunset. This meant a more constant and less variable length of day, given that the time of sunset varied greatly with the changing of the seasons.

With the 12 May 1786 injunction by the Regio Imperiale Supremo Consiglio di Governo, astronomers Brera Giovanni Angelo De Cesaris and Guido Francesco Reggio were commissioned to construct a meridian line within the Duomo di Milano in order to “precisely regulate the hour according to the point of physical noon and with greater precision”.

In fact, contrary to sundials the purpose of the meridian is not to indicate each hour of the day, but only the moment in which the sun is exactly at the zenith, or rather at “true or physical noon”, in this specific case the point which marks the start of the day.

In order to position the brass bar within the monument, what was considered an innovative system at the time was used. That is, upon determining the precise instant of physical noon, the signal was transmitted to the Brera Astronomical Observatory, where a clock with a precision to a ¼ of a second was located. Subsequently the position of the centre of the solar disk was traced from the gnomon hole onto the Cathedral floor. The meridian line was created by repeating this operation over and over again and then connecting the points obtained. Taking into account the time required for the manual transmission of the signal and the determination of the solar circle, in 1 or 2 seconds De Cesaris estimated the precision of the meridian’s time indications.

In 1787 a manual entitled “Daily register of the French and Italian clock” was published which included the difference between the French and Italian time for each day of the year. “Convenient for anyone, especially for whom, for reasons of their own, cannot make use of the meridian”.

Even relatively recently the Duomo di Milano’s meridian was found to be one of the most precise and functioning of its kind currently in existence worldwide: another valuable part of this enormous treasure chest of religious faith and art which the Veneranda Fabbrica has safeguarded for almost seven centuries with enormous responsibility.