A precise knowledge of time is essential for astronomy, and astronomers have always been experts in measuring it. Requests from governments to Astronomical Observatories to keep and distribute the exact time were frequent. The meridian line built by the astronomers of the Brera Astronomical Observatory in 1786 inside the Duomo of Milan is one example.
The Observatory has been active in measuring and giving the exact time until a few decades ago, until the 1960s, when atomic clocks became more accurate than stellar transits and took over the job.
The distribution of the exact time from Brera during the 20th century is well documented in the media: a newsreel made by the Istituto Luce (acronym for “L’Unione Cinematografica Educativa”, i.e. “The Educational Film Union”, which in italian stands for “light”), shows how the exact time was measured and transmitted. The video (Milan, determining the exact time at the Brera Astronomical Observatory) shows the instrument bought to solve the problem of the public clocks in Milan. The electric clocks were synchronized with the pendulum clocks of the Observatory, but the way the information was transmitted (a phone call from the Observatory to the town hall) wasn’t completely reliable, and the newspapers report that the clocks were often set to the wrong time.
To fix the problem, in 1921 senator Senatore Borletti (Senatore was actually his name and he had been elected senator!), owner of a watch factory and of the department store “La Rinascente”, funded the purchase of a high precision pendulum clock, which was installed and operated by the Brera Astronomical Observatory. Every day, exactly at noon, a signal was sent to the department store, which activated a siren whose sound could be heard anywhere in the city.
Transit instruments were used, small telescopes that observe the time of transit of a star through the local meridian, that is the moment when the star passes its point of maximum height above the horizon (pointing the telescope South). When the star passed behind the threads of the micrometer attached to the transit instrument, the observer sent a signal that was recorded by a chronograph on a paper strip; right beside it, the seconds struck by the clock were registered, so that the astronomers could determine the correction to apply to obtain the exact time. A radio signal was transmitted regularly from Brera to the national radio station EIAR, that would then provide the exact time to all through the radio and television network.
The bombs that hit Palazzo Brera during the night between August 7 and 8, 1943 destroyed the set-up. The station was rebuilt with a higher precision equipment in 1957, during the International Geophysical Year. The Observatory kept measuring the exact time with stellar transits until 1969, and distributed the information to SIP (the italian society for telecommunications) and ATM (Azienda Trasporti Milanesi, the local public transportation company).
Today, the exact time is measured with high precision atomic clocks and Astronomical Observatories are no longer in charge of keeping time for all.
Video taken from the archives of the Istituto Luce Cinecittà: Milano, la determinazione dell’ora esatta nell’Osservatorio di Brera (in italian – no subtitles)
Information based on L’Osservatorio Astronomico di Brera nel XX secolo
