The Galilean Moons

According to Stephen Hawking, Galileo Galilei was, more than any other person, "responsible for the birth of modern science". He was also the first person to study the sky using a telescope, and in the following 1610 letter - a draft of the one he eventually sent to Leonardo Donato, Doge of Venice - described the instrument and then for the first time illustrated Jupiter's four largest moons, all of which he had just discovered.

Translated Transcript & Diagram

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 ("Occhiale") 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.