Museo Galileo
Virtual Museum

To use Jupiter's moons as a clock for determining longitude at sea, observing them through a telescope on the deck of a continuously moving ship, Galileo (1564-1642) designed a device that he called celatone (from celata, a type of helmet called a "sallet" in English). It consisted of a metal helmet with a visor carrying a small telescope. The visor was hinged to the side of the helmet and could be adjusted to align the axis of the telescope with the eye of the observer. The wearer could thus continuously adjust the aim to the ship's pitch and roll, and the planet would always remain within the telescope's field of view. Galileo later came up with a different solution. He imagined a hemispheric vessel in which the sailor assigned to the observation would be seated. The vessel floated on oil in a tub that was also shaped like a hemisphere. Its diameter was only slightly larger, so as to minimize the quantity of oil required. Like gimbals, the oil bath would have neutralized the ship's oscillations, keeping the observer in a stable position.

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