What specific groundbreaking observations did Galileo Galilei achieve using his early refracting telescopes around 1609?

Observations of the Moon's surface, the phases of Venus, and the moons orbiting Jupiter

Galileo Galilei took early telescope designs and dramatically improved them, turning the instrument into a powerful tool for scientific inquiry focused on the heavens, particularly around the year 1609. His documented observations fundamentally altered the understanding of the solar system's structure. He was the first to systematically observe and document the imperfect, cratered surface of the Moon, contradicting the idea of perfect celestial spheres. Crucially, he observed the full cycle of phases for Venus, proving it orbited the Sun, and discovered the four largest moons orbiting Jupiter, providing tangible evidence that not everything revolved around the Earth.