What Came Before Heliocentrism?
Aristotelian cosmology, Ptolemy's Almagest, epicycles, deferents, and equants: the elaborate geocentric system that heliocentrism eventually replaced.
What Came Before Heliocentrism?
The heliocentric model that Copernicus published in 1543 was not a free-standing invention. It was a response to a long-established, technically sophisticated, and intellectually coherent system of astronomy that had been the working cosmology of European and Islamic civilisation for more than a thousand years. To understand why heliocentrism was so controversial, and what problems it claimed to solve, it is necessary to understand the system it replaced.
This page is an article in the Heliocentrism and the New Astronomy section, under the Major Discoveries. For the Copernican model itself, see the Nicolaus Copernicus, and for the chain of evidence by which heliocentrism was eventually established, see How Was Heliocentrism Proven?. For the broader intellectual setting — the recovery of classical texts, the rise of humanism, the invention of printing — see The Role of the Renaissance in the Origins and Causes.
The Aristotelian Universe
The cosmological framework that heliocentrism displaced was set out in Aristotle’s De Cælo (On the Heavens) of the mid-fourth century BC. Aristotle divided the universe into two qualitatively distinct realms. The terrestrial realm, from the centre of the Earth out to the orbit of the Moon, was made of the four elements — earth, water, air, and fire — and was characterised by change, decay, and rectilinear motion. The celestial realm, beyond the Moon, was made of a special fifth element, the aether (or quinta essentia, “fifth essence”), and was characterised by permanence, perfection, and circular motion.
Within this framework, the Earth was stationary at the centre of the universe because its natural place was at the centre, where the heaviest element (earth) settled. The natural motion of a terrestrial object was toward the centre of the Earth; the natural motion of a celestial object was circular around that centre. The heavens were a set of nested crystalline spheres carrying the Moon, Sun, planets, and fixed stars around the Earth in perfect circles at constant speed.
This picture was not, in its original Aristotelian form, a detailed predictive astronomy. It was a philosophical framework within which any astronomical system had to operate. For the technical machinery that turned the framework into a working predictive science, the medieval and early modern world relied on Ptolemy.
Ptolemy’s Almagest
Claudius Ptolemy, working in Alexandria in the second century AD, produced a thirteen-book treatise originally titled Mathēmatikē Syntaxis (Mathematical Compilation) and known in its Arabic translation as the Almagest (the “Greatest”). The Almagest was, for more than a thousand years, the definitive reference work of mathematical astronomy.
Ptolemy’s central technical problem was to predict the positions of the Sun, Moon, and five known planets (Mercury, Venus, Mars, Jupiter, Saturn) against the background of the fixed stars, using only circular motions. Circular motion was, in the Aristotelian framework, the only natural motion for celestial bodies; any deviation would have implied a physical impossibility.
To reconcile this philosophical commitment with the observed behaviour of the planets, Ptolemy developed a layered set of devices:
- Deferents: a large circle centred on (or, in some cases, near) the Earth, carrying the planet around at constant angular speed.
- Epicycles: a smaller circle whose centre rode on the deferent, and on which the planet itself moved. The combination of the two circular motions could produce the observed retrograde loops of the outer planets.
- Equants: a point offset from the centre of the deferent, with respect to which the centre of the epicycle moved at constant angular speed. The equant allowed a uniform circular motion to be defined relative to a point other than the geometric centre, which was necessary to fit the actual speeds of the planets.
- Eccentrics: a circle whose centre was displaced from the Earth, used to model the unequal lengths of the seasons and similar effects.
By combining these devices in different ways for different planets, and by carefully choosing the radii, periods, and inclinations, Ptolemy was able to match observed planetary positions to within a few minutes of arc — a level of accuracy that was not seriously improved upon for more than a thousand years.
The Arabic Inheritance
When the Western Roman Empire collapsed, much of the technical content of Greek astronomy survived in Arabic translations and in the working observatories of the Islamic world. The Almagest was translated into Arabic in the ninth century, and the Arabic astronomers of the following centuries — al-Farghani, al-Battani, Thabit ibn Qurra, and later al-Tusi, Ibn al-Shatir, and Ulugh Beg — produced improved planetary tables, refined observations, and new theoretical devices.
Two of these refinements are particularly relevant to the history of heliocentrism. The first is the Tusi couple, devised by Nasir al-Din al-Tusi in the thirteenth century. The Tusi couple is a mathematical device in which a small circle rolls inside a larger circle of twice the radius; a point on the smaller circle traces a straight line back and forth along a diameter of the larger circle. The device converts uniform circular motion into uniform linear motion, and it was used by later astronomers to replace some of the more awkward combinations of epicycles in Ptolemy’s system.
The second is the work of Ibn al-Shatir, a fourteenth-century Damascene astronomer whose planetary models used epicycles in such a way that, when translated into heliocentric coordinates, they became identical to the models that Copernicus would publish two centuries later. The resemblance is so close that some historians have suggested Copernicus had access, directly or indirectly, to Ibn al-Shatir’s work, although the chain of transmission is not fully established.
The Medieval Christian Synthesis
In the Latin West, the Aristotelian-Ptolemaic system was preserved in a Christian synthesis developed in the thirteenth century, largely through the work of Thomas Aquinas and the Parisian scholastics. Aristotle’s physics tookn over, with adjustments, and integrated with the biblical account of creation. The heavens were populated with intelligences (angels) that moved the spheres. The Earth was at the centre because it was the place of fallen humanity, and the heavens were above because that was the place of God.
This synthesis was not monolithic. Some scholastics were sceptical of particular Ptolemaic devices; the condemnations of 1277 at the University of Paris included propositions denying that God could not have created more than one world, which had implications for the uniqueness of the Earth’s location. There were also astronomers, particularly at the University of Vienna and in the school of George Peurbach and Regiomontanus in the fifteenth century, who were working on technical improvements to the Ptolemaic tables.
By the time Copernicus was a student at Kraków, Bologna, and Padua in the late fifteenth and early sixteenth centuries, the Ptolemaic system was clearly under strain. The tables derived from it — particularly the Alfonsine Tables of the thirteenth century and the Prutenic Tables of the sixteenth — had accumulated errors of a degree or more in some planetary positions. The system workeding, but it workeding imperfectly, and astronomers were looking for ways to improve it.
The Problems Heliocentrism Claimed to Solve
Copernicus’s De revolutionibus (1543) was, in part, a response to these accumulated difficulties. Copernicus argued that several features of the Ptolemaic system that required arbitrary adjustments became natural consequences of heliocentrism. The most striking of these was the retrograde motion of the outer planets, which in the Ptolemaic system required a complex arrangement of epicycles, but which in the Copernican system was simply the optical consequence of the Earth overtaking a slower outer planet on an inner orbit.
The Copernican system also explained why Mercury and Venus were never seen far from the Sun (their orbits lie inside the Earth’s), and it explained the order of the planets by orbital period. The Ptolemaic system could accommodate these observations only by stipulating that the relevant planets had epicycles that kept them tied to the Sun, an arrangement that had always looked ad hoc.
Heliocentrism did not, however, solve all of the problems of the older system on its first publication. Copernicus retained circular orbits and required epicycles of his own. The Tychonic compromise of the late sixteenth century preserved the observational advantages of the new astronomy while keeping the Earth stationary. Kepler’s ellipses, Galileo’s telescope, and Newton’s gravitation would all be required before heliocentrism became unambiguously superior to its rivals.
The Transition
The period from Copernicus to Newton — roughly 1543 to 1687 — is the period in which heliocentrism was made, in the technical sense, scientific. The conceptual foundations were laid in the late sixteenth century; the empirical evidence accumulated in the seventeenth; the theoretical synthesis arrived with Newton. The story is treated in detail in the Heliocentrism and the New Astronomy section, and the biographical context — the careers of Nicolaus Copernicus, Tycho Brahe (if a section on him exists), Johannes Kepler, Galileo Galilei, and Isaac Newton — is covered in the Key Figures. For the cultural and intellectual setting in which this transition took place, see The Scientific Revolution: Origins, Causes, and Timeline.