When I was first asked this question, “who discovered the Earth”, I thought it was ridiculous. If you use your eyes and look down beneath your feet, you’d be able to discover the Earth. This was how the first humans would have done it hundreds of thousands of years ago. But maybe a better question is: who discovered that the Earth is a planet?
In ancient times, people thought the Earth was the center of the Universe, and that the Sun, Moon, planets and stars rotated around us. Although some thought the Earth was flat, the ancient Greeks, like Plato, were convinced that the Earth was a sphere. They thought that each of the worlds and stars were in crystal spheres surrounding us.
This idea is natural and intuitive. Anyone who stands outside and looks up can clearly see that the stars and the planets are turning around the Earth. But ancients astronomers who studied the heavens found a few problems. Instead of following a straight path in the sky, some of the planets would appear to stop, move backwards, stop again, and then move forwards. To explain this, the Greek astronomer Ptolemy said that the planets were in tiny spheres and made little circles as they orbited around the Earth.
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It wasn’t until the 16th century that the Polish mathematician and astronomer Nicolaus Copernicus presented the heliocentric model of the Solar System, where the Earth and the other planets orbited around the Sun. His model of the Solar System was backed up by observations by Galileo, who saw that Jupiter had moons of its own, and that Venus went through phases like the Moon.
It took a few years for the ideas to catch on, and for the scientific establishment to agree that yes, the Earth is just another planet, orbiting the Sun, and it’s not the center of the Universe.
We have written many articles about the Earth for Universe Today. Here’s an article about a new telescope that will let you see what Galileo saw.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.
NASA Earth Observatory: Planetary Motion