2.3  The Geocentric Model

The geocentric model was predominant in ancient Greek philosophy as early as the 6th century BC. This model puts the Earth at the center of the Universe, while the Sun, Moon, planets, and stars, all circled around it in uniform motion. This is the apparent view from the perspective of local observers who see the Earth stationary, and the Sun revolving around it once per day, and also the Moon and other planets and stars, although some of these objects also have their own other motions. Furthermore, if the Earth is moving, we should observe the shifting of the positions of stars due to stellar parallax, which means that the shapes of the various constellations should change considerably from time to time across the year.

In reality, however, we do not notice the relative motion of stars because they are much farther away than Greek astronomers had ever thought. This stellar parallax was not detected until the 19th century after the inversion of modern telescopes. However, it might be appropriate to mention here briefly, as we shall discuss it in chapter IV with more details, that Ibn al-Arabi declared clearly that the stars are not fixed at all, and he correctly explained why we don’t observe their relative motion. He also clearly affirmed that the Earth is moving and rotating around its center, and explained why people don’t realize its motion, as we quoted at the top of this chapter. In general his view of the universe is heliocentric, similar to what Copernicus suggested many centuries afterwards as it will be described in section 10.

Plato described the Earth as a stationary sphere at the center of the universe, and the stars and planets were arranged in circular orbits, starting with the Moon and up to the celestial sphere of fixed stars. His student Aristotle added that heavenly bodies are attached to concentric crystalline spheres composed of an incorruptible substance called aether. Unfortunately, this concept of aether required it to have ideal physical properties that could not be conceived in terms of modern physics, especially after Michelson and Morley did not detect the aether drift as it was predicted. As we shall see at the beginning of chapter III, these developments led eventually to Einstein’s theory of relativity which marked the death of aether theories.

The geocentricism was well established by the time of Aristotle, but the detailed model became standard only in the 2nd century AD, after it was developed by the Hellenistic astronomer Claudius Ptolemaeus, or Ptolemy (100-170 AD) who wrote many important books on astronomy and astrology, including: the Almagest, the Planetary Hypotheses, and the Tetrabiblos.

In his most influential work, Almagest, he explained how to predict the behavior of the planets, by introducing a new mathematical tool called the equant. He gave a comprehensive treatment of astronomical models and observations from many previous mathematicians, and he placed the planets in geocentric order that would remain standard in medieval astronomy until the 16th century when it was displaced by the modern heliocentric system. The main reason why Ptolemy’s model was successful is because he was able to explain the observed retrograde motion of planets.

As they wander in the sky, some planets slow down until they stop, and then they start to move backward in retrograde motion, and then again they reverse to resume their normal, or prograde, motion. In order to explain this, Ptolemy suggested that each planet moved in epicycle at the same time as it rotated around the Earth, so the planet moves around the epicycle, which in turn moves along the main path in its original revolution, which is called the deferent.

This geometrical combination of epicycles and deferents explained the observations mathematically, but it was actually not real. Nevertheless, it convinced most astronomers for the following fourteen centuries until the time of Copernicus.

Many other Muslim astronomers, however, doubted the geocentric model and questioned the Earth’s apparent immobility and centrality within the universe, and some have clearly stated: “the Earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the Earth and not the stars.” Young et al. (2006). Alpetragius also declared the Ptolemaic system as an imaginary model that was successful at predicting planetary positions but not real or physical. His alternative system spread through most of Europe during the 13th century.

Other Muslim astronomers from Maragha observatory which was established in 1259, together with astronomers from the Damascus mosque and Samarkand observatory, attempted to solve the equant problem and produce alternative configurations that were more accurate than the Ptolemaic model in numerically predicting planetary positions, and were in better agreement with empirical observations. The arguments and evidence they used resemble those used by Copernicus to support the Earth’s motion.

As we mentioned in the first chapter of the Single Monad Model, Ibn al-Arabi explained the retrograde motion of Saturn in terms of two opposite causes or forces that are acting at the same time, and he gave an example that: “if an animal standing on an object and it moved to a certain direction on this object, and then the object moved to a different direction, then the animal would be moving in the direction of the motion of the object at the same time when it is moving in the opposite direction. So the motion of the animal is a combination between two opposite motions at the same time. ... These planets move, as seen by our eyes, in the orbit from west to east while the greater circumferential orbit (of the Sun) moves with them from east to west. So the planet is moving from east to west at the same time when it is moving from west to east.” [II.443.24, see also: III.203.21].

Among other examples, that will be elucidated further in chapter IV, Ibn al-Arabi also explained that the Aristotelean notion of “fixed stars” is not correct, because these stars are actually moving with relative proper velocities, but the human age is too short to notice it. Moreover, he estimated the proper motion of some visible stars as one arc degree per 100 years, or 0.6 arc-seconds per year, which is quite consistent with the measurements taken only few decades ago, although there are other invisible faint stars, such as Barnard’s Star, being the fastest because of its close proximity to the Earth, which moves at 10.3 arc-seconds per year, while the second is Kapteyn’s Star with 8 arc-seconds per year.

In Western astronomy, more than two thirds of star names are Arabic, with a few others from Greek and unknown origin, but only bright stars have names because there were no telescopes to observe faint stars. Most of these Arab names of stars originated before Islam. In his Almagest, Ptolemy listed the celestial position and brightness of 1,025 stars. Muslim astronomers adopted the method Ptolemy used in identifying stars according to their position within constellations, and added names from traditional Arabic star lore, which they recorded in various Zij treatises, such as the Book of Fixed Stars written by Abd al-Rahman al-Sufi (903-986 AD), known as Azophi in the West, who thoroughly illustrated all the stars known to him along with their observations, descriptions, positions, magnitudes, brightness, and color Kunitzsch (2017).