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Jupiter’s System and the Search for Longitude

Careful observation allowed Galileo to determine the orbital radii and the periods of the four Medicean stars. This success was due to three tools he had invented: the "micrometer" (a graduated ruler projecting from the telescopic tube and sliding along it), the "jovilabe" (a polar map of Jupiter's system), and a polar map representing the heliocentric orbits of Earth and Jupiter.

Since the Medicean stars revolved uniformly around Jupiter, Galileo thought they could be seen as the hands of an extremely precise celestial clock. This would make them indispensable for establishing the difference in longitude between two locations. In 1612, Galileo told the Spanish Court of his method, which determined longitude using the Medicean stars. No clear favourable or unfavourable response was received, and therefore, in 1636, Galileo felt free to present a renewed proposal to the Merchant Company and the General Estates of Holland. These had decided to reward a prize of 30,000 golden escudos to the inventor of a valid method of measurement.

Like the Spanish Court, the General Estates of Holland were sceptical of Galileo's proposal. In practice, despite the ingenuity of Galileo's idea, the search for a method of measuring longitude had to wait for the mariner's clocks invented by John Harrison (1693-1776).

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