Galileo's Sidereus Nuncius (translated as either The Sidereal Messenger or The Starry Messenger) was published in Venice in March 1610. The first scientific study conducted with a telescope, it began with Galileo's observations of the Moon. But he then revealed a major discovery—that the planet Jupiter itself had four orbiting moons. This observation struck an important blow for Copernicanism. The full implications of the discovery would not be demonstrated by Galileo until two decades later, at which time he found himself at dangerous odds with the Roman Inquisition.

Galileo recorded in this engraving the roughness of the moon's surface and the position of spots and prominences on the light and dark sides. Photo ID: RB-GG1610a-3

Galileo's success with the telescope won him election to the Accademia dei Lincei in Rome, the first academy of science. In an engraved portrait first published in his book on sunspots (History and Demonstration of Sunspots, 1613), the title “Linceo” is appended to his name. His two most important inventions, the mathematical compass and the telescope, are pictured above in the hands of cherubs, the latter depicted in a somewhat fanciful trumpet-like shape.

Portrait of Galileo in 1613, at the height of his reputation. Photo ID: RB-GG1613a-2

In his Dialogue of the Two Worlds Systems, published in Florence in 1632, Galileo presented a comparison of the ancient cosmology of Aristotle and Ptolemy with that of the modern Copernicus, cast in the form of a series of dialogues. In it he included a diagram of the Copernican system that showed the ordering of the planets, moving out from the Sun. The diagram is similar to one that appeared in Copernicus' Concerning the Revolutions of the Heavenly Bodies ninety years previously, except that Galileo adds the four moons of Jupiter which he discovered in early 1610.

Galileo's diagram of the Copernican universe, 1632. Photo ID: RB-GG1632-3.

The full gamut of original editions of Galileo's works published during his lifetime, with the inclusion of three manuscript copies, were purchased for Caltech in 1955 as part of the library of Count Giampaolo Rocco of Bologna. The Rocco library was the gift of trustee Harry Bauer.