Final answer:
The apparent motion of stars and constellations over a year is due to Earth's revolution around the Sun. This causes a change in our vantage point, resulting in the stars appearing to shift their position in the sky.
Step-by-step explanation:
The apparent motion of stars and constellations over a period of a year is due to the revolution of the Earth around the Sun. As the Earth orbits the Sun, it causes a change in our vantage point of the stars. This results in the stars appearing to shift their position in the sky over time.
For example, as Earth moves from one side of its orbit to the other, the stars that were visible during the night in summer will be visible during the day in winter. This is because our position in space changes relative to the stars due to Earth's revolution.
The observation of the apparent motion of stars and constellations over a year can be explained in terms of stellar parallax, which is the apparent shift in the direction of a star due to Earth's orbital motion. The Greeks tried to observe stellar parallax but were unable to detect it because the stars were too far away.