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Copernican Revolution

Kepler's theories were even more revolutionary than the ideas of Copernicus.

Early ideas about the Solar System were based on everyday experience. We do not feel the Earth move, so it is reasonable to think that the Earth stands still. We can see the Sun and Moon rise and set, so it is reasonable to think that they orbit around the Earth.

Ancient Greek scientists like Ptolemy developed a mathematical system with the Earth at the centre of the Universe. This accounted for the motion of the Sun and the Moon and all of the planets.

Other scientists, like Aristarchus, believed that the Sun was the centre of the Universe, not the Earth, but this idea was ridiculed.

The Polish priest Copernicus revived the idea of a Sun-centred solar system believing that it could explain the motion of the planets more simply.

Copernicus still thought that all heavenly motions must be composed of uniform circular motions.

Kepler's discovery of elliptical orbits was even more revolutionary than Copernicus' theory. For the first time theories were expected to match exactly with observation, no matter how bizarre those theories might be.


Aristotle's cosmology
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Copernican cosmology
magnifyCopernican cosmology


Johannes Kepler
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