The Geocentric Model of the Solar System

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An Introduction to Ptolemaic Astronomy

Geocentrism is an astronomical theory, central to the astronomy of Ancient Greece that posits that the Earth lies at the centre of the universe.

The Geocentric model of the Solar System entered Greek astronomy sometime in the 6th Century BC in the writings of the astronomer Anaximander. Anaximander proposed a cosmological model of the universe, in which the Earth was shaped like a like a section of a pillar and held aloft at the centre of the universe. He is also believed to have been the first person to propose that the sun and the planets orbit the Earth. Anaximander also believed that the sun, moon, stars and planets were actually invisible wheels with holes in them, that revealed a concealed fire.

The Geocentric Model in Greek Astronomy

In the 4th Century BC, Plato, and his student Aristotle, wrote a number of important treatises about the Geocentric Model of the Solar System. According to Plato, the Earth was a stationary sphere at the centre of the universe. He also believed that the sun, moon and planets orbited the Earth on individual spheres, which he believed explained the circular motion of the stars and planets.

Adherence to the Geocentric Model of the Solar System was the result of a number of important observations. The first was that if the Earth was moving, it should be possible to observe the phenomenon known as stellar parallax, an optical trick that seems to shift the position of the sun, moon and planets. This was believed to be the case because the shapes of the constellations do not change over the course of the year. The second important observation was that the brightness of Venus does not vary. To the Greeks, this implied that the distance between the Earth and the planets was constant. The only way this could be explained was if the Earth did not move. However, one of the major flaws of Geocentrism that the Greeks could not explain was the variation in brightness of the other planets, over the course of the year.

Sometime around 150 AD, the ideas of Anaximander and other Geocentric astronomers were collected and codified by the Roman astronomer Claudius Ptolemy. For this reason, the Geocentric model is also sometimes called the Ptolemaic model.

Not all Greek astronomers supported the Geocentric view of the universe, however. The followers of Pythagoras believed that numbers had divine meaning. As a result, the followers of Pythagoras worshipped a symbol they called the Monad, a single dot within a circle. They believed that the dot represented represented the gods at the centre of the universe and that the circle represented the orbits of the planets. The Greek astronomer Heraclidies Ponticus also wrote a treatise, in which he claimed that the planets orbited the sun and that the sun orbited the Earth.

Around the same time, another Greek astronomer, Aristarchus of Samos, wrote a treatise arguing for a heliocentric model of the Solar System. However, his theories were not widely accepted and no copies of his writing have survived. Geocentrism would not be seriously challenged until the 16th Century.

The Geocentric Model and the Coperican Revolution

In 1543, the Polish astronomer Nicolaus Copernicus published a paper entitled, On the Revolution of Heavenly Spheres. He set out to mathematically demonstrate the viability of a heliocentric solar system, where the planets orbit the sun, instead of the other way around. Because the Heliocentric, or Copernican model of the solar system could not offer better predictions for eclipses and planetary alignments, Copernicus’s ideas were slow to gain acceptance. Following the invention of the telescope, in 1609, observations made by the Italian astronomer Galileo called some of the basic tenets of Geocentrism into question. Starting in December, 1610, Galileo became the first person to observe the phases of Venus. The Geocentric model of the Solar System states that if Venus is between the Earth and the sun, then Venus should be in continuous darkness. The discovery of the phases of Venus represented the first serious challenge to Geocentrism.

The observations of Galileo and Copernicus eventually led to the discovery of the Three Laws of Planetary Motion by Johannes Kepler between 1609 and 1619. Based on the concept of a heliocentric universe, Kepler was the first person to successfully predict the transit of Venus across the face of the sun.

In 1687, Sir Isaac Newton devised his law of universal gravity using theories developed by Kepler, Tycho Brahe and Galileo.

In 1838, German astronomer Fredich Wilhelm Bessel measured the stellar parallax of the star 61 Cygni, disproving the Geocentric ascertation that such a phenomenon did not exist.

Today, a geocentric framework is still useful in some applications, such as navigation at sea and in planetariums. However a geocentric understanding of the universe is generally not useful for the computation of planetary orbits or spacecraft trajectories. Today astronomers think of the Solar System as being in motion relative to the motion of the Milky Way Galaxy.

Sources

Crowe, Michael J. (1990).Theories of the World from Antiquity to the Copernican Revolution. Mineola, NY: Dover Publications, Inc.

Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998.

Walker, Christopher, ed. Astronomy before the telescope. London: British Museum Press, 1996

Michael Hoskin, The Cambridge Concise History of Astronomy, Cambridge University Press, 1999

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