Geocentric model and planets were holes in invisible wheels surrounding Earth; through the holes, humans could see concealed fire. About the same time, Pythagoras thought that the Earth was a sphere (in accordance with observations of eclipses), but not at the center; they believed that it was in motion around an unseen fire. Later these views were combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe. In the 4th century BC, two influential Greek philosophers, Plato and his student Aristotle, wrote works based on the geocentric model.
Central place theory Central place theory Central place theory is a geographical theory that seeks to explain the number, size and location of human settlements in a residential system. The theory was created by the German geographer Walter Christaller, who asserted that settlements simply functioned as 'central places' providing services to surrounding areas. To develop the theory, Christaller made the following simplifying assumptions: All areas have: Therefore, the trade areas of these central places who provide a particular good or service must all be of equal size The theory then relied on two concepts: "threshold" and "range". The result of these consumer preferences