What theory did Copernicus propose about the structure of the universe?

Copernicus proposed the heliocentric theory, which asserts that the Sun is at the center of the universe, with the Earth and other planets revolving around it. This was a revolutionary shift from the geocentric model, which placed the Earth at the center. Copernicus' theory was significant because it laid the groundwork for modern astronomy and challenged long-held beliefs based on the works of earlier astronomers like Ptolemy, who argued for an Earth-centered universe.

The heliocentric theory provided a more accurate explanation of celestial movements and offered simpler calculations for predicting the positions of the planets. By suggesting that the Earth is just one of many bodies orbiting the Sun, Copernicus helped to promote a new understanding of our place in the cosmos, which ultimately led to the scientific revolution.