because the world is spinning at a different angle and speed as it its orbiting the sun.
Stars appear to move across the sky at night because Earth spins on its axis and it is moving around the Sun. Because the Earth spins half a turn in a night, a star visible low in the east early in the evening will appear to rise and move in an arc across the southern sky and set in the west before morning. Stars in the center of the southern sky will disappear below the western horizon halfway through the night. Also, other stars will rise in the east all through the night. The stars that can be seen are always in the sky at some time during the day, but some rise while the sun is in the sky and are hidden by the glare of the bright sky.
Hope this helps!
