In the Ptolemaic system, the apparent shift of stars throughout the year, known as parallax, was explained as a result of the Earth being at the center of the universe and the stars being fixed on a celestial sphere surrounding it. According to this geocentric model, the stars were considered to be at a vast distance from the Earth, which made their parallax effect negligible and imperceptible.
In contrast, the Copernican system, proposed by Nicolaus Copernicus, placed the Sun at the center of the solar system, with the Earth and other planets orbiting around it. In this heliocentric model, the parallax effect of stars could be explained by the Earth's yearly motion around the Sun. As the Earth moves to different positions in its orbit, the apparent positions of stars change relative to each other due to the observer's shifting perspective. This shift in stellar positions provided evidence for the heliocentric model and helped establish its validity over the geocentric Ptolemaic system.