Galileo Galilei — the First to Turn a Telescope on the Heavens
Mountains on the Moon, moons around Jupiter, the phases of Venus — with what he saw through his telescope, Galileo shook the age-old belief that "the Earth is the center of the universe." This is the story of the man who opened the door to modern science.
Turning the telescope toward the sky
In 1609, the Italian scholar Galileo Galilei heard that a curious instrument for seeing distant things had been invented in the Netherlands. He ground his own lenses to build a greatly improved telescope, and while others used such devices to watch for ships or enemies, he alone turned his toward the night sky. That simple decision changed forever the way humankind sees the universe.
An imperfect heaven
Through the telescope the Moon was no smooth celestial pearl but a rugged world, covered like the Earth in mountains, valleys, and craters. The Sun bore dark spots and so was not perfect, and the Milky Way, which had looked like a hazy blur, proved to be a gathering of countless stars. For the first time, the belief held since Aristotle that "the heavens are eternal and perfect" began to crack.
Jupiter's four moons and the phases of Venus
In January 1610, Galileo noticed four little stars lined up beside Jupiter, shifting their positions night after night. They were moons orbiting Jupiter — today called the "Galilean moons": Io, Europa, Ganymede, and Callisto. Contrary to the belief that everything circled the Earth, here were clearly worlds orbiting a body other than our own.
The decisive evidence was Venus. Galileo observed that Venus goes through "phases," waxing and waning like the Moon from a thin crescent to a full disk — a phenomenon that can only be explained if Venus orbits the Sun rather than the Earth. It was visible proof supporting the heliocentric model of Copernicus.
The trial, and "yet it moves"
Galileo spread word of the heliocentric model by publishing his findings in plain Italian that anyone could read. But this collided head-on with the teachings of the Church of the day, and in 1633 he was brought before the Inquisition and forced to recant his claims. Legend has it that as he left the courtroom he muttered under his breath, "And yet it moves."
Though he spent his final years under house arrest, his method of proving nature through observation and experiment became the cornerstone of modern science. Today we call him the "father of observational astronomy," and Jupiter's four moons can still be seen by anyone with a small telescope, just as Galileo saw them that day.