On this night in 1609, astronomer Galileo Galilei trained his telescope on the Moon for the first time. What he saw would overturn an ancient model of the universe.
In 1609, most European scientists and philosophers saw the universe almost exactly as the Greek philosopher Aristotle described it in the third century BCE: Earth was the corrupt, ever-changing realm of mortal life, and the celestial bodies were unchanging, smooth, perfect spheres. Obviously, the Moon’s surface isn’t uniformly pale and glowing; darker patches are visible even with the unaided eye. Aristotle blamed Earth and its corruption for contaminating the Moon, our nearest celestial neighbor. Later philosophers suggested that the dark patches came from variations in the density of the material that made up the Moon.
But when Galileo watched the Moon through his telescope, he noticed something. In the patterns of light and shadow along the terminator — the line between the illuminated day side of the Moon and the shadowed night side, which moves just like it does on Earth — smaller patterns of light and shadow stood out in stark relief. (If you look at the Moon through a good set of modern bincoluars, you’ll notice the same thing.) Those smaller shadows could, perhaps, still have been explained away by the “density” argument, but as Galileo watched the distant shadows on the Moon’s surface, he saw them changing, and he knew that he was seeing something important. The width of the shadows changed along with the angle of the sunlight hitting the Moon, just like shadows here on Earth shorten as the Sun rises high in the sky at midday and lengthen as it drops toward the horizon in the late afternoon. That could only mean that physical features on the Moon’s surface were actually casting the shadows.
Galileo wasn’t the first person to propose that the Moon might have terrain similar to Earth. In the first century CE, the philosopher Plutarch had described mountains and valleys on the Moon back in the first century CE, and suggested that, like Earth, the Moon was inhabited. But Aristotle’s view held sway until Galileo’s observations came along to challenge it.
A few months before Galileo’s late November observations, on July 26, 1609, English astronomer Thomas Harriot also studied the Moon through a telescope, but he seems not to have drawn any particular conclusions based on what he saw, and he didn’t publish his drawings or notes. Galileo, on the other hand, published his sketches and conclusions in his 1610 work Sidereus Nuncius. The claim sparked some debate, but the Church quickly accepted the idea of uneven terrain on the Moon.
But another idea Galileo published in Sidereus Nuncius landed Galileo in trouble with the Church. He supported the radical idea that Earth and the other planets revolve around the Sun, challenging the accepted doctrine that God had placed Earth at the center of everything — and he dared to stick to his guns. In the early 1600s, that amounted to heresy, and Galileo’s refusal to back down landed him a sentence of house arrest for life. Today, of course, the radical claims that Galileo first made in Sidereus Nuncius are the foundation of our understanding of how solar systems work: planets orbit stars, and other worlds follow the same laws of physics and geology as Earth.