Galileo Galilei made discoveries that reshaped humanity’s understanding of the sky, the solar system, and how objects move on Earth. Working in the late 1500s and early 1600s, he used a combination of homemade telescopes and carefully designed experiments to overturn ideas that had stood unchallenged for nearly two thousand years. His contributions spanned astronomy, physics, and the very method of doing science.
Jupiter’s Moons
On January 7, 1610, Galileo pointed a 20-power telescope he had built himself at Jupiter and noticed three points of light near the planet. He initially assumed they were distant stars. Over the following nights, he spotted a fourth point and watched all four shift position relative to Jupiter. By January 15, he had reached a striking conclusion: these were not stars but moons orbiting another planet.
This was a direct challenge to the prevailing view that everything in the heavens revolved around Earth. If Jupiter had its own satellites, Earth was not the only center of motion in the universe. The four moons, now called the Galilean satellites, were later named Io, Europa, Ganymede, and Callisto, after the German astronomer Johannes Kepler suggested using mythological figures associated with Jupiter. Galileo himself simply called them I, II, III, and IV based on their distance from the planet.
Mountains and Craters on the Moon
Through his telescope, Galileo saw features on the Moon that no one had documented before. He observed dark lines whose width changed depending on the angle of sunlight hitting the surface. He watched bright spots appear in the unlit portion of the Moon, then gradually merge with the illuminated side as it grew. From these patterns, he concluded that the changing dark lines were shadows cast by mountains and valleys. The Moon was rough, uneven, and looked a lot like Earth’s terrain.
This was a problem for the dominant worldview inherited from Aristotle, which held that everything above the Moon belonged to a realm of perfection. Celestial bodies were supposed to be flawless spheres. Galileo’s observations made the Moon look ordinary, more like a rocky landscape than an unblemished heavenly object. Some defenders of the old view tried to argue that the Moon was smooth but simply had patches of different density, creating the illusion of terrain. Most astronomers who looked through telescopes, however, agreed with Galileo.
The Phases of Venus
Galileo observed that Venus goes through a full cycle of phases, similar to the Moon, waxing from a thin crescent to a full disk and back again. This was one of the most important pieces of evidence against the old Earth-centered model of the solar system. In Ptolemy’s model, where Venus orbited Earth, Venus could never appear fully lit from our perspective. But in the Sun-centered model proposed by Copernicus, Venus would show a complete set of phases as it orbited the Sun, sometimes on the far side (appearing small and full) and sometimes between us and the Sun (appearing large and crescent-shaped). Galileo saw exactly what the Copernican model predicted.
The Milky Way Is Made of Stars
Before Galileo, the Milky Way was thought to be a band of wispy, cloud-like material stretching across the sky. When Galileo turned his telescope toward it, the cloudy wisps broke apart into countless individual stars, so densely packed that their light blended together to the naked eye. This was the first evidence that what appeared to be a hazy streak was actually a vast collection of stars, a discovery that hinted at the true scale of the universe long before anyone could measure it.
Sunspots and the Sun’s Rotation
Galileo observed dark spots on the surface of the Sun and tracked their movement over days. He noticed that a spot took about 14 days to cross from one side of the Sun to the other, but its speed was not uniform. Spots near the Sun’s edge appeared to move slowly, while spots near the center appeared to move quickly. Galileo recognized this as foreshortening: a spot on the curved surface of a rotating sphere would look like it was barely moving when it was near the edge (coming toward or away from you) and moving quickly when crossing the middle of the disk (traveling sideways relative to your line of sight).
This pattern could only occur if the spots were on or very near the Sun’s surface. If they had been small planets orbiting in front of the Sun, as a rival astronomer named Christoph Scheiner initially proposed, their apparent speed across the disk would have been roughly constant. Galileo also noted that sunspots changed shape and sometimes appeared or disappeared entirely on the Sun’s face, further proving they were features of the Sun itself. This meant the Sun was not a perfect, unchanging body, just as the Moon was not a perfect sphere.
Saturn’s Strange Appearance
Galileo also turned his telescope to Saturn, and what he saw puzzled him. He described the planet as a “composite of three” bodies arranged in a line, with the middle one about three times larger than the two on its sides. He sketched it as “oOo.” He could not figure out what the lateral bodies were. They did not move like Jupiter’s moons, and they were enormous relative to the planet. Then, in 1612, the lateral bodies vanished entirely, leaving Saturn looking perfectly round.
Galileo never solved the mystery. His telescope was not powerful enough to resolve Saturn’s rings clearly, and he died without knowing what he had been looking at. It took another 45 years before the Dutch astronomer Christiaan Huygens, using better optics, identified the structures as a flat ring system surrounding the planet. The disappearance Galileo witnessed was the ring plane aligning edge-on with Earth, making the thin rings temporarily invisible.
How Objects Fall and Accelerate
Galileo’s contributions to physics were just as groundbreaking as his astronomical work. He used inclined planes to study how objects accelerate under gravity, rolling balls down sloped channels and carefully timing their descent. After repeating the experiment a hundred times at various angles, he found a consistent mathematical relationship: the distance an object travels is proportional to the square of the time it has been moving. An object rolling for two seconds covers four times the distance it covers in one second, not twice the distance. This was the first precise, experimentally verified law of motion.
He also discovered that a pendulum’s swing has a constant period regardless of how wide the arc is, as long as the pendulum’s length stays the same. A pendulum pulled far to one side and released takes the same time to complete a swing as one given a small push. This property, called isochronism, later became the basis for accurate timekeeping in pendulum clocks.
A New Way of Doing Science
Perhaps Galileo’s most lasting contribution was not any single discovery but the method behind all of them. Before Galileo, natural philosophy relied heavily on logical reasoning from accepted principles, many inherited from Aristotle. If a respected authority said heavier objects fall faster, that was generally accepted without testing it. Galileo insisted on measurement and experiment. He built instruments, designed repeatable tests, recorded quantitative data, and described nature in mathematical terms. His approach, summarized in the phrase “science is measure,” became the foundation of what we now call the scientific method. The philosopher Bertrand Russell credited Galileo, along with a handful of contemporaries, with launching the scientific revolution that separates the modern world from everything that came before it.