How did Copernicus challenge the Ptolemaic system?

Nicolaus Copernicus challenged the Ptolemaic system—a geocentric model that placed Earth at the center of the universe—by proposing a heliocentric theory in his 1543 work, De revolutionibus orbium coelestium libri VI.123

Copernicus’s primary challenges to the Ptolemaic model included:

• Sun-Centered Universe: He placed the Sun at the center of the solar system, with Earth and other planets orbiting it.13
• Planetary Order: By assuming Earth was in motion, he established a logical, orderly relationship for the planets based on their orbital periods, resolving long-standing disagreements regarding the positions of Mercury and Venus.24
• Scientific Adequacy: Copernicus criticized the Ptolemaic system for its lack of unity and harmony, describing it as a "disorderly monster." He argued that a valid astronomical theory must describe the real, physical system of the world rather than serving merely as a mathematical tool.24

While Copernicus retained some of Ptolemy’s complex apparatus, such as epicycles, his work provided a new criterion for scientific adequacy and set the stage for the Scientific Revolution.123