Galileo Galilei

Galileo Galilei is often called the "father of observational astronomy" and the "father of modern science." In 1609, upon hearing of a Dutch invention called the spyglass, he quickly built his own improved version and became the first person to systematically turn a telescope toward the sky. What he saw changed everything: mountains and craters on the Moon (proving it was not a perfect sphere), four moons orbiting Jupiter (proving not everything orbited Earth), the phases of Venus (supporting heliocentrism), sunspots, and the resolution of the Milky Way into countless individual stars. His publication Sidereus Nuncius (Starry Messenger, 1610) caused a sensation across Europe. His advocacy of the Copernican system brought him into conflict with the Catholic Church, leading to his famous trial and house arrest in 1633. Despite this, his work laid the foundations for modern physics and astronomy.

First telescopic observations of the Moon's craters and mountains (1609). Discovered the four largest moons of Jupiter (Io, Europa, Ganymede, Callisto) — the "Galilean moons." Observed the phases of Venus, providing direct evidence for the heliocentric model. Resolved the Milky Way into individual stars. Discovered Saturn's rings (though he could not resolve their true shape). Observed sunspots and used them to determine the Sun's rotation period.