Final answer:
The difference between Copernicus' idea of the solar system and Ptolemy's is the position of the Sun and the explanation of planetary motion.
Step-by-step explanation:
Copernicus' idea of the solar system differed from Ptolemy's in terms of the position of the Sun and the explanation of planetary motion. Ptolemy's model, known as the Ptolemaic model, placed Earth at the center of the universe with the Moon, planets, Sun, and stars revolving around it in complex circular paths. It was purely descriptive and lacked an understanding of the causes of these motions. On the other hand, Copernicus proposed a heliocentric model where the Sun was at the center of the solar system. This model was simpler and more accurate, explaining planetary motion using a small number of laws of physics, including Newton's universal law of gravitation.