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1. Timeline Of The Historical Development Of Models Of The Universe Aristotle (384- 322BC) The Greek philosopher Aristotle proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects such as the sun, the moon and the visible planets as well as the stars were attached and which rotated at different velocities, with the Earth at the centre. This type of model with the earth at the centre of the universe is called a geocentric model. Ptolemy (AD 140) In AD 140, Aristotle’s geocentric model of the universe was developed and refined by Ptolemy. Ptolemy contrived an elaborate model of circles within circles that was remarkably successful in predicting the observed motions of the heavens. Copernicus (1473-1542) In a book called On the Revolutions of the Heavenly, Copernicus proposed that the Sun, not the Earth, was the centre of the Solar System. Such a model is called a heliocentric system. The ordering of the planets known to Copernicus in this new system is illustrated in the following figure, which we recognize as the modern ordering of those planets. In this new ordering the Earth is just another planet (the third outward from the Sun), and the Moon is in orbit