Aristarchus and Copernicus

The heliocentric model did not originate with Copernicus. As withso many scientific ideas – including basic theories of evolutionand the atom – Greece was the origin. First proposed by Aristarchusin the second century BC, it was later promoted by Copernicus in thesixteenth century. The model placed the Sun at the centre of thesolar system, and relegated the Earth to be just one of the planetsorbiting the Sun, with all the orbits supposed to be circular.

The apparent motion of Mars as seen from the Earth is shown below,

As Earth and Mars move from positions 1 to 2, 2 to 3 and so on topositions 5 Mars appears to move from A to B then backwards from B toC to D thence forwards again to E.

The model also explained why Mercury and Venus appear close to theSun, but there were small differences which at that time could not beexplained, due to the fact that the orbits of the planets were notcirclesm but ellipses. To account for these differences Copernicusadded epicycles, which Kepler later made unnecessary by modelling theorbits as ellipses.

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