Answer: The stars in the galaxy tug on one another gravitationally, which forces them to shift around.
Step-by-step explanation: The monthly positions of the stars change because of the interaction between the rotation of the earth around its axis and the orbit of the earth around the sun. The stars rotate around the north and south celestial poles; hence the stars are always moving relative to a point on the earth's surface.
As our Milky Way rotates, our sun is carried once around the galaxy every 250 million years, slowly drifting up and down through the galaxy's disk, like a horse on a carousel.