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Geocentric model says the Sun orbits around the Earth.

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The geocentric model of the universe, in astronomy, is the theory that the Earth is at the center of the universe and the Sun and other objects go around it. Belief in the geocentric model was common in ancient Greece. The geocentric model was embraced by both Aristotle and Ptolemy, and most Greek philosophers assumed that the Sun, Moon, stars, and naked eye planets circle the Earth. Similar geocentric models were held in ancient China. Aristarchus of Samos proposed a heliocentric model of the Solar System.

Geocentric model.
Early printed rendition of a geocentric cosmological model.

The geocentric model was usually combined with a spherical Earth by ancient Greek and medieval philosophers. It is not the same as the older flat Earth model implied in some mythology. The ancient Greeks also believed that the motions of the planets were circular and not elliptical, a view that was not challenged in western culture before the 17th century.

The geocentric model held sway into the early modern age; from the late 16th century onward it was gradually replaced by the heliocentric model of Copernicus, Galileo and Kepler. Today, geocentric cosmology survives in the work of some creationist fundamentalist Protestant elements of Christianity, as well as literary treatments within alternate history science fiction.

The geocentric model: Classical Greece.


In the 4th century BC, two influential Greek philosophers wrote works based on the geocentric model. These were Plato and his student Aristotle. According to Plato, the Earth was a sphere, at rest at the center of the universe. The stars and planets were carried around the Earth on spheres or circles, arranged in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, fixed stars. In the "Myth of Er," a section of the Republic, Plato describes the cosmos as the Spindle of Necessity, attended by the Sirens and turned by the three Fates. Eudoxus of Cnidus, who worked with Plato, developed a less mythical, more mathematical explanation of the planets' motion based on Plato's dictum stating that all phenomena in the heavens can be explained with uniform circular motion. Aristotle elaborated on Eudoxus' system. In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. All heavenly bodies are attached to 56 concentric spheres which rotate around the Earth. (The number is very high because several transparent spheres are needed for each planet.) The Moon is on the innermost sphere. Thus it touches the realm of Earth, which contaminates it, causing the dark spots (macula) and the ability to go through Lunar phases. It is not perfect like the other heavenly bodies, which shine by their own light.

Adherence to the geocentric model stemmed largely from several important observations. First of all, if the Earth did move, then one ought to be able to observe the shifting of the fixed stars due to parallax. In short, the shapes of the Constellations should change considerably over the course of a year. In reality, the stars are so much further away than the Sun and the planets that this motion (which does exist) was not detected until the 19th century. The lack of any observable parallax was considered the death of any non-geocentric theory.

Another influencing observation was that Venus stays about the same brightness most of the time "and therefore is always about the same distance from Earth". In reality that is because the loss of light caused by its phases compensates for the increase in apparent size caused by its varying distance from Earth. Other objections included the idea, put forward by Aristotle, that the natural state of heavy objects like the Earth was at rest, and that some force was required to move them. It was also believed by some that the Earth's rotation on its axis would cause the air and objects in it (such as birds or clouds) to be left behind.


Claudius Ptolemy and the geocentric model.

Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard. This honor was reserved for the Ptolemaic system, espoused by the Claudius Ptolemaeus of Alexandria, Egypt in the 2nd century AD. His main astronomical book, the Almagest, was the culmination of centuries of work by Greek astronomers; it was accepted for over a millennium as the correct cosmological model by European and Islamic astronomers. Because of its influence, the Ptolemaic system is sometimes considered identical with the geocentric model.

Ptolemaic system.


  1. Moon.
  2. Mercury.
  3. Venus.
  4. Sun.
  5. Mars.
  6. Jupiter.
  7. Saturn.
  8. Fixed Stars.


geocentric model.

Geocentric model and the rival systems.

Not all Greeks agreed with the geocentric model. The Pythagorean system has already been mentioned; some Pythagoreans believed the Earth to be one of several planets going around a central fire. Hicetas and Ecphantus, two Pythagoreans of the 5th century BC, and Heraclides Ponticus in the 4th century BC, believed that the Earth rotated on its axis but remained at the center of the universe. Such a system still qualifies as geocentric. It was revived in the Middle Ages by Jean Buridan. Heraclides Ponticus is also sometimes said to have proposed that both Venus and Mercury went around the Sun rather than Earth, but the evidence for this claim is not clear. Martianus Capella definitely put Mercury and Venus on epicycles around the Sun.

Aristarchus of Samos was the most radical. He wrote a work, which has not survived, on heliocentrism, saying that the Sun was at the center of the universe, while the Earth and other planets revolved around it. His theory was not popular, and he had only one known follower, Seleucus of Seleucia.

Copernican system over geocentric model.

In 1543 the geocentric system met its first serious challenge with the publication of Copernicus's De revolutionibus orbium coelestium, which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both natural philosophy and scripture.


Phases of Venus
Phases of Venus.

In December 1610, Galileo Galilei used his telescope to show that Venus went through phases, just like the Moon. This observation was incompatible with the Ptolemaic system.

Ptolemy placed Venus inside the sphere of the Sun (between the Sun and Mercury), but this was arbitrary; he could just as easily swapped Venus and Mercury and put them on the other side of the Sun, or made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun. In this case, if the Sun is the source of all the light, under the Ptolemaic system:



Astronomers of this time period saw the result of this being unsalvageable for a Ptolemaic cosmology, if the results were accepted as true. As a result, later 17th century competition between astronomical cosmologies focused on variations of Tycho Brahe's Tychonic system (in which the Earth was still at the center of the universe, and around it revolved the Sun, but all other planets revolved around the Sun in one massive set of epicycles), or variations on the Copernican system.

Gravitation: Newton and Einstein.

Johannes Kepler,after analysing Tycho Brahe's observations, constructed his three laws in 1609 and 1619, based on a heliocentric view where the planets moves in elliptical paths. Using these laws, he was the first astronomer to successfully predict a transit of Venus (for the year 1631).

In 1687, Isaac Newton devised his Law of universal gravitation, which introduced gravitation as the force that both kept the Earth and planets moving through the heavens and also kept the air from flying away, allowing scientists to quickly construct a plausible heliocentric model for the solar system.

In 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, disproving Ptolemy's argument that parallax motion could never be observed.

A geocentric frame is useful for many everyday activities and most laboratory experiments, but is a less felicitous choice for solar-system mechanics and space travel. While a heliocentric frame is most useful in those cases, galactic and extra-galactic astronomy is easier if the sun is treated as neither stationary nor the center of the universe, but rotating around the center of our galaxy.

Geocentric model today. Modern geocentrism.

Some religious fundamentalists still interpret their scripture as stating that the Earth is the physical center of the universe-known as modern geocentrism. Astrologers, while they may not believe in geocentrism as a principle, still employ the geocentric model in their calculations.


In planetariums.

The geocentric (Ptolemaic) model of the solar system is still of interest to planetarium makers, as, for technical reasons, a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion. The celestial sphere, used for teaching purposes and sometimes for navigation, is also still based on a geocentric system.

Geocentric model in science fiction.


Richard Garfinkle's Celestial Matters (1996) is set in a more elaborated geocentric cosmos, where Earth is divided by two contending factions, the Classical Greece-dominated Delian League and the (Chinese) Middle Kingdom, both of which are capable of flight within an alternate universe based on Ptolemaic astronomy, Aristotle's physics and Taoist thought. Unfortunately, both superpowers have been fighting a thousand-year war since the time of Alexander the Great.

References to the geocentric model.

  • Dreyer, J. L. E.. A History of Astronomy from Thales to Kepler. 2nd edition. New York: Dover Publications, 1953.
  • Heath, Thomas. Aristarchus of Samos. Oxford: Clarendon Press, 1913.

Notes about the geocentric model.

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