Which astronomer improved Copernicus's heliocentric model by introducing elliptical orbits?

Johannes Kepler improved Copernicus's heliocentric model by showing that planets travel in elliptical orbits rather than perfect circles. This was a crucial step in turning a broadly correct Sun-centered arrangement into a far more accurate mathematical description of planetary motion.

Copernicus had placed the Sun at the center, but he retained the ancient belief that heavenly bodies moved in uniform circles. To match observations, his system still required additional circular devices called epicycles. Kepler worked with exceptionally precise observations made by Tycho Brahe, especially measurements of Mars, and found that circular paths could not account for the data.

In Astronomia nova, published in 1609, Kepler introduced his first two laws: planets follow elliptical paths with the Sun at one focus, and they sweep out equal areas in equal times. His third law, linking orbital periods with distances from the Sun, followed in Harmonices Mundi in 1619. Isaac Newton later explained why Kepler's laws work through gravity and laws of motion. Galileo supported heliocentrism but did not adopt Kepler's elliptical orbits in his own major dialogue.