Defaulting to perfection

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A 9th-century diagram of the Ptolemaic system with Earth at the centre, then the Moon (Luna), Sun (Sol), Mercury, Venus, Mars, Jupiter and Saturn, St. Gallen, Stiftsbibliothek, Cod. Sang. 248

The myth that medieval people thought the Earth was flat has long been debunked. Nevertheless, it persists in the imagination of many today. Periodically, popular historians need to remind everyone that for the educated man and woman of the western Middle Ages, the earth was spherical. It had been conceived as a sphere since the classical period. It was flat in representation, but that’s because the parchment surface is flat.

On the other hand, the model of the universe prevalent in the Middle Ages was geocentric. This was another legacy of the classical period, the so-called Ptolemaic model, named after the Roman-Egyptian astronomer Claudius Ptolemy (2nd century AD): the Earth in the centre, and then the Moon, Mercury, Venus, Sun, and the other known planets, excluding Neptune, Uranus and Pluto (too small to discern without optical aid), mounted on circular orbits of increasingly larger sizes around the Earth. Beyond these concentric circles drawn by the orbits of the celestial bodies were the fixed stars and then metaphysical space, which various philosophers and thinkers filled in various speculative ways, not unlike the theoretical models today describing the universe at a cosmological, non-empirical level.

Three fundamental qualities made the Ptolemaic model different from our own post-Copernican understanding of the universe:

  • anchorage: the whole system had to be centred on a fundamental substance/quality/body, on which everything else depended. The conclusion that the universe revolves around the Earth was not an empirical observation, a scientific fact, but nor was it contradicted by observation or mathematical calculation. It was a default position, supported by philosophy and metaphysics. Copernicus’ heliocentrism was not a fundamental departure from the Ptolemaic model, as it anchored the universe in the Sun, rather than Earth. The Copernican principle, however, articulated much later than Copernicus’ own works on cosmology, exploded the need for centrism in mapping the universe. There are no privileged observers in the universe, no point of anchorage.
  • perfection: this was another ‘default position’ into which astronomical observation fell. One of the main differences between the two cosmological models is in regards to the shape of the orbits. Both models postulate the existence of planetary orbits, since it is the only way to explain the movement of the planets (and the Sun, understood as being a planet in the Ptolemaic model). The difference however, is that ancient and medieval observers conceived of these orbits as perfect circles. Since Johannes Kepler (1571-1630), we know that bodies in orbit never draw circles, but ellipses. The pre-modern conclusion was based on astronomical observation, mathematical calculation and metaphysical speculation. The third one provided the ‘default position’, when the other two, insufficiently developed (relative to Kepler’s time), put the matter on the fence. In the absence of compelling evidence, the premodern mind preferred circles, as they represent perfection and are a better fit for articulating cosmological truth than an ellipse or any other kind of geometry.
  • metaphysical: Ptolemy’s geocentric model of concentric circles endured not because it didn’t have its critics. The Greek astronomer Aristarchus of Samos (3rd century BC), for example, had challenged an early form of what was to become the Ptolemaic model about 400 years before Ptolemy, becoming the first proponent of a heliocentric model of the universe. His ideas didn’t catch on not because scholars didn’t ‘believe’ in science, but because people have always shown preference and sympathy to ideas which fit the existing system of thought. This was true of the ancients, it is also true of us, and it explains why there have been relatively few revolutions in the history of humanity. Aristarchus’ idea of the Sun as the ‘central fire’ coordinating not just the solar system but the universe was poorly suited for making sense of the correspondences between the human and the natural which ancient philosophy, especially under Plato and Aristotle, had postulated. A geo- or human-centric model of the universe provided a better key for understanding everything which mattered in life, from the natural environment to ethics, politics and the afterlife. It was only when such a paradigm became weak and the observational tools supported by mathematics became stronger than a Copernicus could turn up and change the world.


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