What major change did Copernicus make in the ptolemaic system why was this a significant change?

Copernicus introduced the heliocentric model, proposing that the Sun, rather than the Earth, is at the center of the universe. This was a significant change because it challenged the long-standing Ptolemaic geocentric system, which placed Earth at the center, and altered humanity's understanding of its place in the cosmos. The shift to a heliocentric perspective laid the groundwork for modern astronomy and revolutionized scientific thought, leading to further discoveries by figures like Galileo and Kepler.