Galileo became convinced early in life of the truth of the Copernican theory (i.e., that the planets revolve about the Sun) but was deterred from avowing his opinions--as shown in his letter of April 4, 1597, to Kepler--because of fear of ridicule. While in Venice in the spring of 1609, Galileo learned of the recent invention of the telescope. After returning to Padua he built a telescope of threefold magnifying power and quickly improved it to a power of 32. Because of the method Galileo devised for checking the curvature of the lenses, his telescopes were the first that could be used for astronomical observation and soon were in demand in all parts of Europe.

As the first person to apply the telescope to a study of the skies, Galileo in late 1609 and early 1610 announced a series of astronomical discoveries. He found that the surface of the Moon was irregular and not smooth, as had been supposed; he observed that the Milky Way system was composed of a collection of stars; he discovered the satellites of Jupiter and named them Sidera Medicea (Medicean Stars) in honour of his former pupil and future employer, Cosimo II, grand duke of Tuscany. He also observed Saturn, spots on the Sun, and the phases of Venus . His first decisive astronomical observations were published in 1610 in Sidereus Nuncius ("The Starry Messenger").

Although the Venetian senate had granted Galileo a lifetime appointment as professor at Padua because of his findings with the telescope, he left in the summer of 1610 to become "first philosopher and mathematician" to the grand duke of Tuscany, an appointment that enabled him to devote more time to research.

Objective lens of Galileo - Lens

Glass, ivory, gilded brass, ebony. Diameter of lens 30 mm; frame 410 mm x 300 mm.

This is the original objective lens with which Galileo discovered the moons of Jupiter which he christened the "Medicean Planets". The frame is made of ivory mounted on ebony. Galileo made numerous celestial observations between 1609 and 1610. The four satellites of Jupiter were discovered in 1610. This great discovery was announced in the Sidereus Nuncius (Venice, 1610). The telescope lens with which Galileo made this discovery was given as a gift to Grand Duke Ferdinand II de' Medici by Galileo himself. It was accidentally broken at some later date. After Galileo's death in 1642 it remained the property of the Medici family and passed to the hands of Prince, later Cardinal, Leopoldo de' Medici. On the death of Leopoldo in 1675 the lens became part of the family collection of art and rare objects, kept for the most part in the Galleria degli Uffizi. In 1677 the Medici commissioned Vittorio Croster, an ivory engraver who was probably German, to make the frame which contains the lens today. The lens, thus mounted, remained in the Galleria after the foundation of the Imperiale e Regio Museo di Fisica e Storia Naturale in 1775, whereas the other scientific instruments and materials that had been kept in the Galleria were transferred to the new museum. The lens was eventually transferred to the Museo di Fisica in 1793.

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