The uses of the telescope, 1609-10

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Commentary

Only a few months after receiving a description of the telescope and building one for himself, Galileo penning this draft of a letter to the Doge of Venice in August 1609, describing the practical utility of the instrument to the great maritime republic and promising to keep it a secret.

Most Serene Prince.

Galileo Galilei most humbly prostrates himself before Your Highness, watching carefully, and with all spirit of willingness, not only to satisfy what concerns the reading of mathematics in the study of Padua, but to write of having decided to present to Your Highness a telescope that will be a great help in maritime and land enterprises. I assure you I shall keep this new invention a great secret and show it only to Your Highness. The telescope was made for the most accurate study of distances. This telescope has the advantage of discovering the ships of the enemy two hours before they can be seen with the natural vision and to distinguish the number and quality of the ships and to judge their strength and be ready to chase them, to fight them, or to flee from them; or, in the open country to see all details and to distinguish every movement and preparation.

Pointing the instrument at the heavens a few months later, Galileo made his sensational series of telescopc observations, the most significant of which is recorded for the first time on the lower part of this same sheet of paper. On successive evenings in January 1610, he observed several bright objects close to Jupiter which changed their relative position while never straying far from the planet. After plotting their relative positions over the course of one week, he drew the diagram indicating that these movements could only be explained by regarding these lights as moons orbiting Jupiter on the same plane as it orbits the sun. This observation of moons orbiting a moving planet contradicted fundamental premises of Aristotelian-Ptolemaic cosmology while providing empirical evidence of the reality of the heliocentric cosmology postulated by Copernicus.

The final revised draft of the letter, sent on 24 August 1609, is preserved in the Venetian state archives. This single-leaf manuscript is one of the great treasures of the University of Michigan Library.

The most extensive contemporary record of Galileo's first telescopic observations of Jupiter and its satellites is contained in his autograph diary in the Biblioteca Nazionale Centrale (Florence), Ms. Gal. 49.