Draft of a letter to Leonardo Donato, Doge of Venice, August, 1609, and Notes on the Moons of Jupiter, January 1610.
This single-leaf manuscript is one of the great treasures of the University of Michigan Library. It reflects a pivotal moment in Galileo's life that helped to change our understanding of the universe.
In the summer of 1609 Galileo received a description of a telescope which had been developed the year before in the Dutch town of Middelburg by an optician, one Jan Lippershey. Applying his knowledge of optical science, Galileo built such a glass or telescope for himself, and in the draft letter shown below offers his new "occhiale" to the Doge of Venice, pointing out its potential use in warfare. The final letter, revised from this draft, was sent on August 24, 1609. It is in the State Archives in Venice.
The lower part of this sheet shows the use to which Galileo put this optical device a few months later. As he viewed the skies on successive evenings in January, 1610, he had noticed several bright objects around Jupiter that changed position from night to night. On this page, he plotted their positions over the course of one week and, when he drew the diagram in the lower left imagining how these movements would look if they were viewed from above Jupiter, he realized that the objects were moons of that planet. This was the first observational data that showed objects orbiting a body other than the earth.
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."
[Below the text are diagrams and notes documenting the position of Jupiter's moons on several nights in January 1610.]
Bequest of Tracy W. McGregor, 1938.