Nicholas Copernicus 1473-1543 was a Polish priest and astronomer who created an alternative model of the planets which put the Sun at the centre, instead of the Earth as generally accepted at that time.
He knew it would get him into trouble with the church, and his book 'De Revolutionibus' was not published until the year he died. The theory with the Sun at the centre was similar to the old Ptolemaic system with its collection of circles and epicycles allowing for the changes in distance and the changes in planets' speeds, and their departure from the ecliptic.
In the late 1500s Tycho Brahe made new more accurate observations of the planets from Denmark, and these were used by Johannes Kepler to produce a new model published in 1609.
Kepler's model is the one we use today, and it has the Sun at the centre, like the Copernican model, but all the other details of Copernicus's theory were rejected in favour of elliptical orbits.
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Kepler's assumptions about orbits differed from Copernicus in that Kepler proposed that planetary orbits were elliptical rather than circular. Copernicus had assumed circular orbits in his heliocentric model of the universe. Kepler's discovery of elliptical orbits led to his three laws of planetary motion.
Kepler disagreed with Copernicus on the shape of planetary orbits. Copernicus believed in circular orbits, while Kepler's observations led him to propose elliptical orbits. Kepler's laws of planetary motion refined and corrected some of the assumptions in Copernicus' heliocentric model.
Copernicus did not reject the idea that planetary orbits around our Sun were circular.
Kepler's law that describes how fast planets travel at different points in their orbits is called the Law of Equal Areas. This law states that a planet will travel faster when it is closer to the Sun and slower when it is farther away, so that the area it sweeps out in a given time is the same regardless of its distance from the Sun.
It is believed that Nicolaus Copernicus first proposed the heliocentric theory.
Nicolaus Copernicus was the scientist who proposed that the planets move around the sun in circular orbits, known as heliocentrism. His theory laid the foundation for modern astronomy and challenged the geocentric model.