Machina Mundi >The Geocentric Heritage

The Geocentric Heritage

Historically, the lack of evidence concerning the mobility of the observer, his perception of a heavenly sphere surrounding him, and the periodicity of astronomical phenomena, backed up such basic ideas as the immobility and centrality of the Earth, and the uniform circularity of celestial motion. However, despite the Sun and the Moon moving almost uniformly along the Zodiac, the other planets display motions that hardly can be seen as uniform. To explain such anomalies the astronomers conceived two types of planetary models, based on the possibility that a non-uniform motion could be a mixture of many uniform circular motions.

Using a group of three or four concentric spheres for each planet, Eudoxus of Cnydus (4th century B.C.) was able to explain the motion of Mercury, Jupiter and Saturn along the Zodiac. Callippus of Cyzicus (4th century B.C.) introduced some additional spheres for the Moon, the Sun, Venus and Mars. The resulting models were used by Aristotle (384-322 B.C.) to develop his physical conception of the Universe: the immovable Earth was surrounded by eight groups of crystalline spheres, carrying the Moon, the Sun, the planets and the fixed stars. This structure was attractive to the medieval mentality. The eight groups of spheres were housed within a ninth sphere, called the "first movable", and thereafter into a tenth immovable sphere, the "empyrean sky", or Dome of God. Finally, this empyrean sky was divided into nine metaphysical spheres which matched the nine degrees of the angelic hierarchy.

Apollonius of Perge (3rd century B.C.) conceived the second construct of planetary models using a composition of two circles. Such models were developed by Hipparchus of Nicea (2nd century B.C.) and, in the form described by Claudius Ptolemy (2nd century A.D.) in his Almagest, they became the basis for computing planetary ephemererides. They solved some inconsistencies of Eudoxus' models, but produced other problems. For instance, some of the circles that Ptolemy had introduced appeared as mere artefacts, and contradicted the uniform circularity of celestial motion.


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