Copernicus, originally of Torun in Poland, although not an observer himself, promoted the heliocentric system, that is, motion of the Earth and planets in orbit around the fixed Sun.
Copernicus recognized that if the planets orbit around the Sun, rather than around the Earth, he could easily explain the observed retrograde motion of planets like Mars.
Among the points he proposed in his "De Revolutionibus Orbium Coelestium" are:
The Sun is the center of the Solar System; the Earth and planets orbit the Sun in circular orbits.
Day and night are the result of the rotation of the Earth on its axis.
Mercury and Venus are closer to the Sun than the Earth is.
Only 3 motions of the Earth are needed:
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