Galileo, having heard of Dutch spyglass designs, built and refined his own telescope and presented it to the Venetian Senate, offering it as a tool of military and maritime value. Within months he turned the instrument skyward, discovering Jupiter's moons, lunar craters, and the phases of Venus—observations that provided powerful evidence for the Copernican model and set him on a collision course with the Catholic Church.
Galileo did not invent the telescope, but he was the first to systematically improve it and turn it toward serious scientific use. In August 1609, he demonstrated a device with about 8x to 9x magnification to Venetian senators from the top of a tower, emphasizing its usefulness for spotting ships and enemy fleets far out at sea—a practical pitch that secured him funding and prestige.
Within the year, Galileo had built telescopes with 20x magnification and turned them to the heavens rather than the harbor. His subsequent observations—mountains and craters on the Moon, four moons circling Jupiter, and countless stars invisible to the naked eye—were published in 1610's Sidereus Nuncius and stunned European intellectuals.
The political and scientific consequences unfolded slowly but irreversibly. Galileo's evidence for a Sun-centered cosmos eventually led to his trial by the Roman Inquisition in 1633, but the genie was out of the bottle: the telescope had permanently changed the scale of the observable universe and the methods by which humans studied it.