During his years in Padua, starting in 1592, Galileo had devoted himself - in addition to his teaching duties and his own studies - to fabricating scientific instruments. He thus acquired a thorough understanding of the link between these instruments and scientific investigation, as well as extraordinary skill in designing and building them. In 1597 he devised a new type of geometric and military compass; starting in 1600 he devoted himself to strengthening magnets, and from 1602 to 1609 he designed various devices to determine the physical laws governing falling bodies.

Between 1608 and 1609 Galileo received news from various correspondents of the spyglasses now circulating through Europe, and may have had the chance to example one that had arrived in Venice. Immediately fascinated by the new device, he dedicated himself to making it more powerful, with such success that in late 1609 and early 1610 he managed to increase its power of magnification from the original two or three times to twenty or thirty times. Thanks to the skill he acquired in fabricating lenses and combining them appropriately, the telescope (from the Greek tele = "at a distance" and skopeo = "I observe") lost its original Dutch connotation and became, to all effects, a "Galilean" instrument.