Eureka! by Andrew Gregory

Author:Andrew Gregory
Language: eng
Format: epub
Publisher: Icon Books Ltd

Figure 20: The epicycle, the basic unit of Ptolemaic astronomy. The planet moves around the small circle, whose centre moves around the large circle, giving a combination of regular circular motion.

With this, allied with two more complex devices based on the epicycle, known as the ‘eccentric’ and the ‘equant’, Ptolemy was able to account for most of the problems that beset the concentric sphere model. Planets could have varying distances from the earth with this system, so variation in apparent size and brightness could be accounted for. It was also easier to explain variations in the apparent velocity of the planets. In addition, Ptolemy’s system could provide several shapes for retrograde motion, and so was able to account for this phenomenon more accurately. This system was mathematically very powerful, and could in principle have explained virtually every astronomical phenomenon. The downside of this power was complexity – greater accuracy demanded more and more epicycles. Ptolemy’s astronomy and cosmology lasted throughout the Roman empire, the dark ages, middle ages and Renaissance, only to be displaced during the scientific revolution, when Copernicus proposed that the earth orbited the sun.

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