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The telescope, measurement of time and longitude

6.2 - Galileo's proposal

Tito Lessi, Galileo e Viviani [Galileo and Viviani], oil on wood, 1892 (Istituto e Museo di Storia della Scienza, Florence, Dep. Osservatorio di Arcetri) Engraving of Giovanni Domenico Cassini (G.D. Cassini, La meridiana del Tempio di S. Petronio [The sundial in the Church of St. Petronius], Bologna, 1695, flyleaf)

Within two years of his discovery, Galileo decided that the satellites of Jupiter could be used to determine longitude at sea. They orbited around Jupiter like the hands of a clock visible from every place on Earth.

The configurations of the satellites could be predicted and tabulated. Thanks to the telescope, a navigator could compare the time predicted for a configuration with that of its effective observation. The difference between the time predicted and the time measured indicated the difference in longitude between the place of observation and the reference place in the tables.

Galileo proposed his method to the governments of Spain and The Netherlands, adding to it in 1637 the project for a pendulum clock to measure local time. None of his proposals were however accepted. Moreover, the tables needed to complete Galileo's method were prepared only in 1668 by Cassini.

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