Final answer:
The term 'celestial spheres' refers to the ancient astronomical concept where the stars and other heavenly bodies were thought to be embedded in crystal spheres rotating around the Earth. This model has been superseded by current astronomical knowledge recognizing the Earth's rotation as the cause of the apparent movement of celestial objects in the sky.
Step-by-step explanation:
The heavenly bodies were once thought to move on crystal spheres called celestial spheres, surrounding the Earth. This model, conceptualized by ancient astronomers, depicted the stars as embedded in these crystalline spheres, which rotate around the Earth. However, today we understand that this is not the actual structure of the universe, but rather an illusion caused by the Earth's rotation on its axis. In modern times, we still reference the idea of the celestial sphere for convenience in locating objects in the night sky, though we now know these objects are at various distances from us in space, not fixed to a dome.
Celestial objects appear to rise and set because of the Earth's rotation, not due to the movement of the celestial sphere itself. The concept of the celestial sphere is still useful in astronomy for mapping the apparent positions of stars and planets in the sky. Additionally, planetariums provide a visual representation of the celestial sphere by projecting stars onto a dome, simulating the night sky.