"A geometrical and military compass designed by Galileo Galilei, described in Le operazioni del compasso geometrico et militare (1606). It is thought to have been made by Marc'Antonio Mazzoleni circa 1604. This compass is shown with a quadrant, which fixes the two sides of the compass at a 90-degree angle, prohibiting the compass from opening and closing but allowing it to measure angle." Putnam Gallery, Harvard University.

Title page of the first edition of Galileo's Le operazioni del compasso geometrico, the first computer manual, issued in an edition of only 60 copies. Museo Galileo.

# Galileo Produces his "Compasso" & The First "Computer Manual"

1597 to 1606

Drawing of Galileo's Compasso from the Museo Galileo website (September 2020).

Beginning in 1597 Galileo Galilei developed his geometric and military compass into a general-purpose mechanical analog calculator, later known in English as the sector. Galileo produced several examples of his compasso. Images of an example that Galileo may have presented to Cosimo II are available from the Virtual Museum of the Museo Galileo at this link. During the seventeenth century the sector became one of the most widely used mechanical calculators for scientific purposes.

"The Galilean compass—not to be confused with drawing compasses—is a sophisticated and versatile calculating instrument for performing a wide variety of geometrical and arithmetical operations, making use of the proportionality between the corresponding sides of two similar triangles. It comprises three parts:

- the two legs, held together by a round disk (pivot), whose faces (front and back) are engraved with numerous scales;

- the quadrant, graduated with various scales, which is fixed by means of wing nuts to the holes in the compass legs;

- the clamp, a cursor inserted into one of the compass legs; keeps the instrument vertical and can serve as an extension for the leg holding it" (http://catalogue.museogalileo.it/object/GeometricMilitaryCompass_n01.html, accessed 01-23-2014).

As an instruction manual for purchasers of the compass, and to establish his priority for the invention, in 1606 Galileo published from his own house in Padua, printed by Peitro Marinelli, Le Operazioni del Compasso Geometrico et Militare in an edition of only sixty copies. To avoid having the compass pirated, Galileo had no illustrations of the device included in the pamphlet, which may be considered the first "computer manual."

In January 2014 a digital facsimile of the 1606 edition was available from the digital library of the Museo Galileo at this link.  A video describing Galileo's compasso and its functions narrated in English could be downloaded from the same website as a .zip file at this link.