10 How the World Began
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The movements of the planets were not easy to understand, since sometimes they even went in the wrong direction with respect to the other stars. At
the beginning of the second century CE, Claudius Ptolemy, an astronomer of
Alexandria, elaborated a geocentric description of the universe, in which an
articulated machinery explained all of the apparent anomalies in the motions
of the celestial bodies.
This mechanism is explained in Fig. 10.1. The earth lies off-centre from a
circle, known as a deferent. The planet revolves around an epicycle, the centre
of which revolves around the deferent. Another position, known as the equant
lies opposed to the earth on the opposite side of the centre of the deferent.
The significance of the equant is that to a hypothetical observer located there,
the centre of the planet’s epicycle moves at a constant angular speed around
the deferent. The presumption that all planetary orbits are circular (and all
celestial bodies are perfectly spherical) was an article of faith for Aristotle and
his followers, and the equant helped to maintain this illusion.
Fig. 10.1 Example of the Ptolemaic model to describe a planetary orbit, showing
the use of a deferent and epicycle