In January 1610, Galileo Galilei swung his crude telescope skyward, smashing the perfect, crystalline celestial spheres of Aristotle,* and knocking the Earth off its immobile, biblically enshrined pedestal. Galileo's early observations of Venus, Jupiter, and the Moon were nails sealing the coffin of the pre-Copernican worldview.
Studying Venus, Galileo saw what anyone with a small backyard telescope and the patience to watch for a few months can see today: the evening star is approaching and receding from Earth. He realized that Venus is shining by reflected sunlight and, from Earth's perspective, passing alternately in front of and behind the Sun. This only makes sense if Venus and Earth are both traveling around the Sun.
Turning his glass toward Jupiter, Galileo discovered that the giant planet was attended by four tiny companions that tag along on its orbit, rearranging themselves night after night. He had found the moons of Jupiter, the first new worlds. The existence of moons orbiting Jupiter showed that not everything travels around the Earth. This spelled doom for the old Earth-centered cosmos of Aristotle.
The surface of our Moon, viewed through Galileo's telescope, displayed a complex topography of shadows, pits, and mountains. This was not the flawless, smooth sphere required by Aristotle's dichotomy between a perfect, spiritual celestial realm and an imperfect Earth. The Moon's "flaws" suggested to Galileo that it was a world like Earth. Suddenly, it didn't seem at all preposterous that the other planets might be Earth-like. The abstract Copernican universe became real. Galileo concluded that the other planets are worlds, and that "the world"—our Earth—is merely one of many planets circling the Sun.
Galileo caught hell from the Church. In what has become a modern myth of science's collision with biblical authority, he was brought before the Inquisition, forced to recant his Copernican beliefs, and lived out his days under house arrest.
*Aristotle's spheres were made of crystal because they had to be solid to hold up the Sun, Moon, planets, and stars, yet transparent since we can see through them.
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