Galileo and the Heliocentric Model of the Solar System

Astronomy until the 1400s was dominated by the idea that Earth stood at the center of the universe; this was called the Ptolemaic system, after Claudius Ptolemy, a Roman astronomer who worked circa 150 CE. He is considered the last of the great ancient astronomers, and he took the astronomical systems developed by the ancient Greeks, such as Pythagoras and Aristotle. These Greeks argued that the planets (which at this time were known only as far as Saturn), the moon, the sun, and the stars circled the Earth. There were, however, problems with the notion that the planets orbited the Earth as did the sun and moon; their behavior through the heavens is not one of constant motion but of jerky motion, sometimes in one direction, sometimes in another. This phenomenon is known as retrograde motion, and occurs only among the planets with orbits exterior to Earth’s — Mars, Jupiter, Saturn, and the other outer planets discovered since the 1700s. Mercury and Venus, however, behaved in a completely different manner; at times they appeared before the sun rose, at other times after the sun set, and they were in fact considered different stars. Consider the mythical references to the Morning Star and the Evening Star; both were Venus but had different mythical properties. The question confronting astronomers (who were really nothing more than philosophers) during those ancient times was this: supposed, as appears likely, that the planets encircle the Earth, how can their odd behavior be explained?

The solution, as Ptolemy described it, was that the planets did orbit the Earth, but that they also orbited something else. This is called the flywheel system; each planet was affixed to a flywheel, so sometimes it was moving in the reverse direction (clockwise) of its orbit (counterclockwise), but its overall motion was always in the counterclockwise direction. With a few refinements, this system remained the system of choice, and the Roman church found explanations for the system within the Bible, that the system showed God’s grand design for the universe. This is actually an extension of the ancient Greek philosophy, particularly that of Pythagoras, as he believed that the heavens consisted of ordered spheres. It is from this that the phrase “the Music of the Spheres” emerges.

Astronomy during the middle ages was largely unknown. It did occur, but it was mostly concerned with the observation of the heavens, particularly the search for comets, which were believed to be omens heralding bad tidings. This obsession with comets can be seen in artwork dating to the middle ages, particularly in the Bayeux Tapestry (showing the passage of Halley’s Comet in 1066, presaging William the Conqueror’s invasion of England and his overthrow of Harold II) and Giotto’s painting “Adoration of the Magi” which shows Halley’s Comet (again) as the Star of Bethlehem. The fourteenth and fifteenth century ushered in a rediscovery of ancient Greek and Roman texts and a new sense of humanism, something that we tend to call now the Renaissance, and from this a rediscovery and appreciation of the ancient Greek astronomers and their cosmological systems.

The man that we credit today with the heliocentric system (with the Sun at the center of the solar system) is Nicolas Copernicus, a Polish canon of the late 15th and early 16th centuries. Copernicus should actually be considered the last of the Aristotelian or Ptolemaic astronomers; he didn’t so much advocate that the sun was at the center of the solar system so much as advocate that the Earth actually moved among the heavens. Ironically, the heliocentric system had already been fashioned in the 300s BCE by Aristarchus, sometimes known as the last of the Pythagorean astronomers. While we cannot today reconstruct Aristarchus’ argument which has been lost to antiquity, quite possibly in the Great Fire of Alexandria, we do know that Aristarchus argued for the heliocentric model through the testimony of Archemedes and Plutarch. I think it is quite likely that the Aristarchus system was profoundly simpler than the Ptolemaic system which followed several centuries later, though this is pure speculation on my part, and I offer this information for your own knowledge, as Aristarchus is not credited in modern times as having fashioned the heliocentric system.

Published in Nuremberg in 1543, The Book of the Revelations of the Heavenly Spheres recounted Copernicus’ theory which was actually an reworking of the Ptolemaic theory, but with the sun as the center of the solar system and the earth moving around it. This is not counter-intuitive for two reasons: one, it makes no difference whether the Earth orbits the sun or the sun orbits the Earth because relative to each other the motion is identical, and two, there had already been a revision of the Ptolemaic system placing Mercury and Venus in orbit around the Sun as an explanation for their Morning Star/Evening Star behavior once they were recognized as the same objects (this theory having been formulated by Herakleides and later rediscovered and advocated by the last of the great naked-eye astronomers, Tycho Brahe). What Copernicus did with his theory was place all objects — Earth, sun, moon, the planets — in orbit, not of the sun or the earth, but around an unseen point in space. This created an incredibly messy system, for Ptolemy’s system required forty flycycles to account for all observed motions, while the Copernican system required forty-eight flycycles since the Earth now orbited the point on its own system of flycycles.

Galileo didn’t so much refashion the Copernican system into the current heliocentric model so much as he misinterpreted it. If anyone deserves the true credit for the heliocentric system, it would Johannes Kepler and not Galileo; Kepler fashioned the three laws of planetary motion, based on the assumption that the sun sat at the center of the solar system and the planets carved out ellipses around the Sun. His laws would not have worked under the 48 flycycle system of Copernicus, though Copernicus largely gets the credit for formulating the heliocentric system because at the very beginning of The Revelations of the Heavenly Spheres Copernicus specifically says that the sun occupies the central point of the solar system, but then backs away from that analysis in his attempt to reconcile his theory with the Ptolemaic system.

Galileo’s role in these events is his use of the telescope, until this time used only for land observations, in the study of the heavens; he made the discovery of the Galilean satellites of Jupiter (Io, Europa, Ganymede, and Callisto). It was for the discovery that he was placed on trial, as these bodies, which clearly did not orbit the earth, altered the unchanging nature of the heavens. Now, the discovery of the Galilean satellites is seen as confirming the Copernican system, though at the time it was seen as a great heresy, and Galileo was forced to recant his discovery. If Galileo did anything counter-intuitive, if was his use of the telescope to observe the heavens, not creating the heliocentric model. While he did argue for it, he also tried to make the heliocentric model fit under the Christian doctrinal beliefs regarding the heavens. This, more than anything, led to his trial, because Galileo cast doubt on the authenticity of the Bible, angering the Church fathers. His trial, then, pitted the nascent science of telescopic astronomy against the doctrinal beliefs of the Church and Galileo’s conviction and recantation stand as one of the greatest crimes committed in religious history and mark a low point in the Counter-Reformation for the Church clearly placed itself against the advance of knowledge.

Was Galileo acting in a counter-intuitive manner? I would argue that he was not. Galileo was first and foremost a scientist and a thinker who sought to know more about man and his place in the universe. For that he should be and will be remembered; to argue that he acted counter-intuitively by acting against the interests of the Church is to profoundly misunderstand the history of astronomy.


Cosmos, by Carl Sagan, which provides a good, entry-level history of astronomy.

The Sleepwalkers, by Arthur Koestler, which provides a solid history of the development of astronomy from Pythagoras to Kepler and Galileo.

The Copernican Revolution, by Thomas Kuhn, which discusses the standard interpretation of Copernicus and the scientific and technological revolutions which followed from the heliocentric model.

Allyn Gibson
University of Richmond
13 April 1999

2 Replies to “Galileo and the Heliocentric Model of the Solar System”

  1. great info very well said and also really truethful and inspiring to look into the solar system further. :)

  2. I enjoyed your article and some of the insights about Galileo and Keppler.  I have an interest in the debate between science and religion and its struggles to arrive at truth together.  Do you think that at the present time science is too heavily weighted with their own views and not being fair to religion For example: To deem Intelligent Design as a pseudo-science stems from bias and not from a true science which might be more in line with: Since we do not know for sure the origin of the Universe, it would be good science to neither state that there is a creator or that there is not a creator.  Alternative hypotheses can continue invesitigation which can assist in identifying errors for either outcome.  Why would science want to pressure believers to be more flexible in accepting scientific study and then turn-a-round and impose negative pressure on believers, i.e. religion is a product of superstition; religion is for the ignorant or the poor, etc.

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