Nicolaus Copernicus reorganized planetary astronomy by placing Earth among the planets moving around the Sun. His 1543 De revolutionibus orbium coelestium by Nicolaus Copernicus treated terrestrial motion as part of the explanation of observed celestial appearances.
This changes the geometry of apparent backward planetary motion. The observer is moving too: when Earth overtakes an outer planet, the changing relative position can reverse the observed direction of its progress against distant stars. Copernicus's system still used combinations of circular motion; the heliocentric model did not already contain Johannes Kepler's elliptic orbits.[ref]
De revolutionibus orbium coelestium by Nicolaus Copernicus. Dedication and Book I: motivation for terrestrial motion and planetary arrangement
Portrait of Nicolaus Copernicus
. Unknown painter, circa 1580 portrait in the Town Hall museum at Toruń; faithful reproduction. Public domain (Commons PD-Art, author life+100; U.S. faithful-reproduction position stated). Source and usage information on the linked Wikimedia file page. Source.
Nicolaus Copernicus (1473–1543) was a Renaissance astronomer and mathematician who is best known for formulating the heliocentric model of the universe, which posited that the Earth and other planets revolve around the Sun. This was a significant departure from the geocentric model that had dominated Western astronomy, which placed the Earth at the center of the universe.
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