Riding the wave


Gian Domenico Cassini, and the problem of longitude

Meanwhile Gian (Gio) Domenico Cassini (Perinaldo, 1625 - Paris 1712) was working on the chart of France, passed into history by the name "Cassinis chart" because it derived from the work of four generations of the same family, and was eventually completed by Jean Dominique Cassini IV, son of César-François Cassini de Thury - known as Cassini III - in 1790, in 182 sheets.

However, the primary concern of Cassini senior was the problem of determining longitude, which had become the target of constant research in the early eighteenth century. England, with the Longitude Act of 1714, had offered a substantial award for those who would find a solution, as indeed, without success, the King of Spain had already done at the end of the sixteenth century.

Sailors in fact continued to use the method based on lunar distances, developed in the late fifteenth century, and certainly used by Amerigo Vespucci in 1497 and by later explorers, up to James Cook in the second half of the eighteenth century. It was therefore clear that a marine chronometer was needed, set on the time of a meridian of reference, thus enabling the determination of longitude by comparison with the local time obtained through astronomical observations.

In 1735 John Harrison presented his first chronometer in London, which was improved in later years, after several trials at sea. In France, similar studies were conducted by different scientists, until Pierre Le Roy and Ferdinand Berthoud presented their chronometers, successfully tested in 1767.

The following year Captain De Tronjoly was put in command of the frigate " Enjouée " for a trial expedition to Newfoundland and then to Africa, Spain and Portugal, to allow Le Roy and the astronomer Francesco Cesare Cassini to verify the effectiveness of the former's chronometer through astronomical observations.

On his return Francesco Cesare Cassini published a report of the expedition entitled Voyage / fait par ordre du Roi / en 1768, / pour éprouver les montres marines inventées par M. le Roy, / par M. Cassini Fils. / Avec le Mémoire / sur la meilleur manière / de Mesurer le temps en mer, / qui a remporté le prix double en jugement de l'Académie / Royale des Sciences. / Contenant la description de la montre à longitudes, présentée / à Sa Majesté le 5 Août 1766. / Par M. Le Roy l' Aîné, Horloger du Roi / Paris, Rue Dauphine, / Chez Charles-Antoine Jombert, Libraire du Roi pour / l'Artillerie & le Genie, à l'Image Notre-Dame. / M.DCC.LXX. / Avec approbation et privilege du Roi.
(Genoa, Italian Navy Hydrographic Institute)

During the return journey along the Atlantic coast of Africa, the "Enjouée "made a stop in the Moroccan port of Salé, of which Cassini drew a view and described its monuments and customs.

Paola Presciuttini , 2002