Rearranging the geocentric system

Last time I explained Aristotle's geocentric system, where the Earth is supposed to be fixed immovable in the center of the universe and the celestial bodies are placed on concentric spheres, all plunged in the Ether which made them rotate.

But the observations didn't match the predictions of the proposed geocentric system. The first signs of the system being non-consistent were the movements of the Sun's nearest planets : Mercury and Venus. From our view they will always be close to the sun, we will never see them in an opposition position (when the Earth is exactly between the planet and the sun). They not only seem to travel next to the sun, but also moving back and forth around it. 

Heraclides(-388 to -310),Greek philosopher and astronomer, proposed that Venus and Mercury revolve around the sun, and so he changed the geocentric system. He would also have been one of the first to support the thesis that the Earth revolves es around an axis of rotation, unlike Aristotle, who assumed that the celestial sphere revolved around the Earth,the Earth itself remaining immutable.

Adapted geocentric system by Heraclides

An other problem is that sometimes planets happen to be in an apparent retrograding motion, which does not fit to their supposedly circular motion. This happens during opposition position. As the Earth is rotating faster than the outer planets, during opposition the Earth is "overtaking" the outer planet (for example Mars), and the planet appears to move backwards in front of our starfield.

Self-made animation to show the apparent trajectory of an outer planet during opposition position

Aristarchus of Samos (-310 / -230), Greek astronomer and mathematician, was the first known man who placed the sun in the center of the universe. He assumed that the Earth rotates on its axis and around the Sun, but his ideas were considered unclean and were therefore rejected.

There is a hint that Heraclides preceded him, through an indirect source : according to Simplicia of Sicilia (490 - 560 AD), Heraclides proposed that the irregular movements of the planets could be explained if the earth moves while the sun stays still.

As I mentioned in the previous post, Ptolemy (90 - 168 AD), a greco-roman mathematician, astronomer, geographer, astrologer and poet of Alexandria, adopted Aristotle's view. He tried to explain the strange behavoir of the planets by assuming that they are moving on epicylces, revolving along a deferent. He observed also that even then the movements of the planets were not regular, they apparently did not turn steadily and circularly around the Earth. So he did a quite complicated model where he placed the Earth away from the center of the system, the center being in the middle between the Earth and an imaginary point named the equant. Note that the Earth and the equant did not move in this system. Now, he supposed that viewed from the equant the planets, respectively the center of their epicycle would appear to move at a steady speed. Which means that they did not move uniformly on the deferent.

Ptolemy's view of the movements of the planets

In order to keep his system coherent with the celestial orbs, Ptolemy considered the orbs to be thick spherical slices rather than a thin sphere. In each slice would be another one in which the planets were located. His geocentric system was more detailed and more accurate than other modes before. His model was almost universally accepted until the 16th century.

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.