The geocentric theory, also called the geocentric model or geocentrism, is an astronomical theory that places the Earth at the center of the universe , while the rest of the stars revolve around it.
This worldview was predominant in most ancient civilizations, such as the Babylonian. Today we'll discuss what it was like and how it has changed from theory to the present day.
geocentric theory
In reality there was not a single geocentric theory but throughout the history of philosophy it has varied. Of course, the theory began in ancient Greece.
For the pre-Socratics
Anaximander, in the 6th century BC, believed in an arrangement of the stars in which the Earth, with a cylindrical shape (like a piece of tube), was located at the center, floating . He explained the other celestial bodies by visualizing invisible wheels that surrounded the Earth; these wheels had small holes that allowed the inhabitants of the world to see a hidden fire (the sun).
The Pythagoreans believed the earth was round after observing eclipses and said it was moving due to an invisible fire.
Both versions eventually merged, resulting in the view that spread among all Greeks who could pursue higher education. The Earth was a sphere located at the center of the universe. Plato and Aristotle dedicated works to this theocentric theory of the world.
Plato
Plato said that the Earth was a sphere resting at the center of the universe. The stars and other planets revolved around it in celestial circles, each having its own circle, and they went from closest to Earth to farthest away in the following order: Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, and the fixed stars. In his book The Republic, he described how the Sirens and the Fates guarded the cosmos.
Aristotle
Following his teacher's system, he continued to place the Earth at the center of everything, arranging the planets and stars around it. This time, all celestial bodies were connected to invisible spheres (between 47 and 55) that revolved around the Earth, concentric with it. He called these spheres "crystalline spheres," and theorized that their movement differed. He established that celestial bodies were composed of ether. The Moon remained the closest object to Earth, and when it came into contact with it, it caused maculae (dark spots).
He continued writing, explaining the terrestrial elements such as earth, water, fire, air, and also ether. The different weights of each of these elements were what had positioned them on Earth. Earth was the heaviest; water was attracted by the weight of the earth and thus formed a layer around it. Air and fire tended further outward, away from the center, with fire being the lightest. Beyond all of the above lay ether.
Ptolemy
Following the initial Greek theories, this theory rested on several points. First, if the Earth were moving, this should be noticeable in the fixed stars (which were thought to be much closer than we now know them to be). The brightness of Venus was another element supporting the theory; the planet always had a consistent brightness, indicating that it was always at the same distance from Earth.
However, the changes in the apparent size of the planets, attributed to changes in distance, could not be explained. Ptolemy, in his work *The Almagest* , established the solution to this problem. Each planet moved on two or more spheres, one of which anchored it around the Earth. The other, much smaller sphere caused the planet to rotate and thus move closer to or farther from the Earth, following the path of the smaller sphere that moved around the Earth. He maintained that the system was in uniform, circular motion. This theory was accepted by the West and became the geocentric theory, which persisted for centuries.
Although the theory was not accurate, it did predict some celestial movements such as retrograde motion.
The discovery that the Earth revolves around the sun
Not all Greek models followed this theocentric system. Some Pythagoreans argued that the Earth was one of the planets that revolved around the sun.
Aristarchus of Samos in the 2nd century BC was the most radical of all the Greeks in defending this theory, which was contrary to the general Greek belief. He wrote a book on heliocentrism. But that theory did not become popular. Only Seleucus of Seleucia continued to promote that idea.
Muslim astronomers initially accepted the theory that was completed by Ptolemy, but around the 10th century, several writings questioned the supposed immobility of the Earth, just as they questioned its being the center of everything . Some established that the Earth rotated on its own axis and that the movement seen in the sky was due to the Earth's movement, not the movement of the sky itself.
Copernicus
In 1543, Copernicus entered the scene, asserting that the Earth and the other planets revolved around the Sun. However, the geocentric theory remained the most widely accepted. This shift in theory overturned everything that had been believed up to that point, at all levels, both philosophical and religious.
His theory held that the Earth rotated on its axis once a day and revolved around the Sun once a year. He also stated that the Earth's rotation was around its own axis. All of this represented a significant advance over the geocentric theory. It is true, however, that Copernicus retained some elements of the older theory, such as the idea of Aristotle's crystalline spheres and an outer sphere where all the motionless stars were located.
Newton and Kepler
Kepler formulated his three laws between 1609 and 1619, supporting the heliocentric model, in which the planets move in elliptical orbits. Specifically, he documented the orbit of Venus in 1631.
For his part, Newton and his law of gravity would put the final nail in the coffin of the entire heliocentric theory. This was the force that kept the planets in orbit. This allowed for the establishment of a model for the solar system that resolved the doubts that had arisen up to that point.
