The geocentric model of the solar system (and indeed of the universe) asserts that the earth sits, unmoving, at the centre of all existence. Every other object in the sky revolves around the earth, following paths dictated by a variety of mathematical rules – some of them quite complex!
From our humanist point of view, earthbound and vastly separate from the heavens, the model instinctively makes sense. After all, we can clearly see the sun, moon, planets and stars revolving across the sky, and if the earth were moving we would surely feel it. Additionally, many religions have their creator place the earth at the centre of the universe – and mankind doesn’t like to argue with scripture.
The accuracy of the geocentric model, as calculated by astronomers and astrologers over thousands of years, is impressive. This is all the more incredible given that we now know that its axioms are completely incorrect. Over the last 400 years, scientists have discredited the geocentric theory in favour of more elegant cosmic mechanics. These theories have not only relegated the Earth to a position of inferiority in the solar system, but don’t even allow our sun, solar system or galaxy to take any special position in the universe. Indeed, the very concept of ‘the centre of the universe’ cannot be defined in modern cosmology.
The geocentric model was strong enough to survive from its formal description, in mathematical language, by the ancient Greeks around 550BC, until late into the 17th century. By this time, scientific observations and knowledge had advanced too far for the geocentric model to explain them. The work of scientists such as Copernicus, Galileo and Newton posed too many questions which couldn’t be answered and, despite fervent opposition from the church, a theory of heliocentrism superceded the old science and the sun was placed firmly at the centre of creation.
Geocentrism survived because it was accurate in determining planetary movements and was believed to explain some astronomical phenomenon better than a heliocentric model. Aristotle rejected heliocentrism for two main reasons. First, he observed that Venus changed little in brightness over the course of the year. This made sense under geocentrism as Venus was always the same distance away. It didn’t make sense to him under heliocentrism as it ought to be dimmer when on the other side of the sun. Then, he observed that if the Earth were moving, then the positions of the stars should change relative to one another over the course of the year – a parallax effect.
Of course, we can understand Aristotle’s mistakes in hindsight. He was unaware that Venus goes through phases, like the moon, and that the increase in illumination from its phases balances out the reduction in illumination from its distance. The other solution to Aristotle’s parallax problem was that the stars could be so far away that the parallax isn’t observable. The concept of a universe of that vast size would have been unthinkable to early astronomers. Stellar parallax was not measured until 1838.
After hundreds of years of work by the ancient Greeks, a successful and reasonably accurate set of rules was drawn up by Ptolemy in the 2nd century AD. All the objects in space orbited in a small circle called an epicycle. This epicycle then orbited the Earth on another circular path called a deferent… except it didn’t orbit exactly around the Earth, but around a point half way between the Earth and another point called the equant. Simple!
In this system, the sun, moon, planets and ‘sphere of the stars’ were all perfect bodies, moving in perfectly circular orbits, as befits a perfect creation. It was scientifically and theologically sound. It predicted the movements of heavenly bodies with reasonable accuracy, but it didn’t explain why or how any of these movements occurred. The beginning of the end for the geocentric model came with the work of Copernicus.
In the early 16th century, Copernicus began to study the recorded observations of earlier astronomers. He decided that the figures could be more simply explained by a heliocentric model than by the prevailing geocentric model. He published his calculation demonstrating this – all the planets moving in circular orbits around the sun. However, the predictions he could make with his new system were no more accurate than the old system, and there was no evidence that the Earth was moving around the sun, and so there was little impetus for a change in the status quo.
In the early 17th century, Galileo’s observations using the newly invented telescope began to cause problems for the geocentric model. He was the first person to observe the moons of Jupiter, and therefore the first person to discover an object which was provably not orbiting the Earth. He also observed the phases of Venus which had deluded Aristotle so many centuries earlier. It took the work of Johannes Kepler, also early in the 17th century, to really turn the tide of cosmological thinking. He abandoned the perfection of circles and eventually examined ellipses. He realised that the combination of a heliocentric solar system with elliptical planetary orbits, provided a new system which was both simpler and more accurate than the geocentric model.
Then, in 1687, Isaac Newton published his work on gravity. This explained why the solar system behaved like it did, and the mathematics perfectly complimented the law of planetary motion that Kepler had devised from experimental observation. Geocentrism couldn’t compete with the explanatory powers of these new theories and faded into scientific history.
The geocentric model is still believed by many people today – nearly 20 percent of the population of the UK according to a 1999 poll. A significant proportion of these views might be down to substandard science education, but among strong proponents, this is generally associated with a strict adherence to particular theological point of view rather than convincing evidence in favour.