Aristotle and Plato

Aristotle's Cosmic Theory

Aristotle (384-322 BC) recognised the essential qualities of hot & cold and wet & dry. From combinations of these qualities he imagined everything to be made from:

  • cold + dry = Earth
  • cold + wet= Water
  • hot + wet = Air
  • hot + dry = Fire

Using these basic 'elements' he constructed the 'cosmos' shown here.

The Earth (earth) is 'obviously' static and at the centre. This is surrounded by water (sea) and air (atmosphere). Beyond that is a layer of fire and beyond that is the rotating celestial sphere carrying the fixed stars.

 

Plato's Cosmic Theory

Plato (~427 - 348BC) pointed out that the stars move round the Earth in a regular pattern but the 'planets', (Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn were all considered 'planets' at that time) seem to wander about in an irregular manner.

Plato challenged his contemporaries to explain these movements.

One approach was to insert additional spheres between the 'fire' and the sphere containing the 'fixed stars'. Each sphere would carry a planet and would rotate at an appropriate speed. This left much room for debate as to the physical nature of the spheres. However, the Greeks were more interested in finding the geometry rather than explaining the physical properties.

Of course, the idea of planets rotating on a simple sphere did not fit very well with observations. In particular several of the planets sometimes reverse direction in relationship to the background "fixed" stars. This "retrograde motion" required further explanation.

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Tony Evans 2004-2011

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