Galileo Galilei’s Geometric and military compass

Galileo Galilei’s Geometric and military compass, gilded copper, Padua, 1606  (inv. bronzi 299)

Location: Rocchetta, 2nd floor, Room XXIX

The geometric compass was designed by Galileo Galilei in Padua in 1597 and built by Marcantonio Mazzoleni.
It is composed of two arms, on which numerous “lines” are engraved, converging at the opening pivot, which is equipped with a hole for a plumb line, by a quadrant marked with a variety of scales and by a “forked” clamp and a slider that allows for the leg in which it is threaded to be lengthened and for the compass to be positioned vertically.
This instrument of calculation allowed its users to quickly make more than forty different types of complex geometric and arithmetic calculations and exploits the theory of proportions to solve various civil and military problems. It could be used for instance, as a gunner’s quadrant, to redraw a map to scale or to calculate exchange rates.
In 1606 Galileo published 60 copies of a treatise describing the uses of the instrument, The Uses of the Geometric and Military Compass, which were provided along with the compass itself.
Of the sixty original compasses only 5 examples are known to have survived. As well as the compass exhibited at the Sforza Castle’s Museo di Arti Applicate (Museum of Applied Arts), the others can be seen at the Museum of the History of Science in Florence, the Istituto Geografico Militare - also in Florence, the University of Pisa and in Cambridge, England.