Time measuring, time keeping





In the 19th century, when longitude could be calculated mainly by comparing local time with the time shown on a clock at a known meridian, time was really precious. And so were accurate clocks. In 1872, the newly founded Istituto Idrografico was in charge, among others, of time measuring and time keeping. Time measuring was carried out through astronomical observation, using the transit instruments installed in the geodetic-astronomical observatory. Time keeping was carried out through extremely accurate fundamental astronomical pendulum clocks, which were housed in a suitable room designed and equipped by admiral Magnaghi in 1883. The Istituto Idrografico was in charge of time signalling as well, and a cannon electrically controlled by one of the automatic signal clocks, shot every noon at Forte Castellaccio.



The same clock controlled a powerful light on top of the San Benigno signal station, which could be seen from a distance. The signal lit up at given times and remained on for five minutes sharp, so that the ships in the port and the people in the city could adjust their clocks and watches.



In 1915 the clock at IIM controlled also the seconds hand of an electric clock placed by the Municipality of Genoa in Via XX Settembre, in the city centre. Officers from Navy vessels and merchant ships in the port of Genoa used to come to the Istituto Idrografico to check their clocks against the pendulum clocks. Any discrepancies were recorded in the ship’s clock register. When at sea, the difference between the time indicated by the ship’s clock and the time when the sun reached its highest point at the ship’s meridian made it possible for the mariners to calculate the longitude.