A perfect geocentric universe

One of the oldest known model of the universe, after realizing that the earth has to be spherical, has been produced by the ancient Greeks around the 6th century BC.

When you look up in the sky you will notice that every body up there seems to turn around the Earth. So it is easy to think that we are located in the center of this giant "Carousel". As for example the moon or the sun aren't falling down on Earth, it was supposed that they had to be somehow suspended or fixed on something. So the Greeks first imagined giant circular tubes, which later became transparent spheres, called celestial orbs, on which everything was attached. Therefore each known body had to be fixed on one of those spheres, the stars were "fixed" in the background on the last biggest sphere.

That kind of system, where you place the Earth in the center of the universe and assume that everything turns around it, is called a geocentric system.

Geocentric system (self-made diagram - not at all to scale!)

Aristotle (4th century BC) adopted the geocentric model. He supposed that the universe had to be perfect, existing since eternity and lasting forever. He wasn't convinced by evolution as he deduced that every human will make birth to another human, every plant to another plant, and nothing would ever change. He divided the universe in a sublunary and a supralunary world. The sublunary world includes everything between the Earth and the moon, it's where things change, the imperfect world. From the moon on to the stars was the perfect world, where only geometric shapes would exist. He thought that all the stars and planets were illuminated by the sun. He imagined that the planets would have to be perfect spheres, circling in a steady and unchanging movement. This supralunary world would be plunged in a substance called Ether, an unchanging, strong and light substance with the circular motion being it's natural state. This substance would be responsible for the movement of the planets around the Earth. By cons, in the sublunary world elements were heavy, they need to be pushed to get moving and their natural state would be to rest.

Side note : Many of ancient Greece thought that nature was based on geometry, so pure geometric forms were considered to be perfect and somewhat divine. But there were some problems, for example the number Pi can not be exactly calculated as it is not a rational number.

In fact, Aristotle believed in a natural state of things. For example it would be in the nature of water to flow, or of fire to burn. Aristotle wasn't really a good scientist, even though he deduced his ideas on observation, he mostly imagined by himself how the universe could work. He did not test the universe, he had no proof of the Ether or anything he described. And yet, his view was endorsed by many people for a long time.

Of course there were some anomalies in his model, like the comets. As their orbit regularly cross orbits of other planets, they were incompatible with the celestial spheres model. But Aristotle had his own explanation: they were merely atmospheric events in the sublunary world.

Note that even if Aristotle was well known, he got famous in Europe only in the 12th century, when his works have been translated into Latin.

The geocentric system was picked up by Ptolemy in the first century AD to explain the movements of the known stars in the sky. While keeping the earth at the center, he also assumed that the stellar objects were orbiting on specific crystal orbs. Its diagram illustrates this idea : The Earth is placed in the center, on the first orbit comes the Moon, then Mercury, Venus, the Sun, Mars, Jupiter, Saturn, three layers of stars and finally "the empire of heaven, abode of God and of all the elect".

Geocentric system according to Ptolemy

Soon some people started to see flaws in the system, as some planets would not circle steadily around the Earth, and doing sometimes some funny movements instead.