21 How did Kepler deviate from the beliefs of Aristotle and Copernicus in his first law?

Answer 1

Johannes Kepler came up with his first law in 1618 which says that every planet moves in an ellipse with the Sun at one focus (an ellipse has two of these).

It differs from earlier theories, which were not bad, just complicated, that the planets move in a system of circles. There would be a circle for the main orbit, then another small circle to allow for the fact that each planet is sometimes closer to the Sun or further away than normal, then another small circle to allow for the inclination of the orbit to the ecliptic, and so on.

Circles were added to make the planets' observed positions agree with the predictions. Each planet moves round each of its small circles once in the time it takes to go right round its main circle. Copernicus's system with the Sun at the centre had a total of 48 circles for the planets out to Saturn.

Answer 2

Kepler was not the first to suggest that planets revolve around the sun.

Copernicus was.

However, Kepler deviated from Copernicus's beliefs because he said the

orbits of the planets are ellipses, not circles.