How did Kepler's laws improve on Copernicus's model of planetary motion?

Johannes Kepler’s laws of planetary motion significantly improved upon the heliocentric theory proposed by Nicolaus Copernicus by replacing circular orbits with a dynamic, physically grounded model.12

While Copernicus correctly placed the Sun at the center of the solar system, he maintained the traditional assumption that planets moved in perfect circular paths. Because this did not align with precise observations—particularly those of Mars—Copernicus’s model remained inaccurate.13

Kepler’s improvements included:

• Elliptical Orbits: Kepler discovered that planets travel in ellipses with the Sun at one focus, rather than in circles. This accounted for the varying distances between planets and the Sun.12
• Variable Orbital Speed: Through his second law, Kepler demonstrated that a planet moves at different speeds depending on its distance from the Sun, sweeping out equal areas in equal times. This replaced the need for complex, artificial mechanisms like the equant.123
• Mathematical Harmony: His third law established a precise mathematical relationship between a planet's distance from the Sun and its orbital period, transforming the solar system into a coherent, dynamic structure.12

Ultimately, Kepler’s work turned the Copernican system into a physical astronomy, providing the foundation for Isaac Newton to later explain these motions through the law of gravitation.12