Ptolemy, in his influential work *Almagest*, explained the changing constellations throughout the year using a geocentric model of the universe, where Earth is at the center. He proposed the following:

* Earth is stationary: He believed that Earth was fixed and did not move.

* Celestial Sphere: The stars were embedded on a giant sphere, the celestial sphere, which rotated around the Earth.

* Earth's Tilt: Ptolemy understood that the Earth's axis is tilted at an angle. This tilt, combined with the Earth's annual rotation around the Sun, causes the celestial sphere to appear to shift its position throughout the year.

Explanation of the Changing Constellations:

1. The Earth's Tilt: Due to the Earth's tilt, different parts of the Earth face the Sun at different times of the year. This means different constellations are visible in the night sky during different seasons.

2. Celestial Sphere's Rotation: As the celestial sphere rotates around the Earth, the constellations that are visible at night shift throughout the year.

3. The Sun's Apparent Path: The Sun's apparent path through the constellations (the Zodiac) is also affected by the Earth's tilt. As the Earth orbits the Sun, the Sun appears to move against the backdrop of the constellations, causing the visible constellations to change with the seasons.

Note: This explanation, while ingenious for its time, is incorrect. We now know that the Sun is the center of our solar system, and the Earth orbits the Sun. However, Ptolemy's model was widely accepted for over 1400 years and paved the way for future astronomical discoveries.