Who was Copernicus and what was heliocentric theory?

Answer 1

Nicholas Copernicus 1473-1543 was a Polish priest and astronomer who created an alternative model of the planets, known as the heliocentric theory, 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.
His theory was taken up by Galileo in a famous dispute with the church. At around the same time 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.