History
Before the technological advances of the 20th century, people based their assumptions of the motion of the planets and stars on what they could see. If you simply look up in the sky, it appears that stars are stationary and that planets sometimes move a little. For 1,400 years, people believed that the Earth was the center of the motion and that the planets, stars and even the sun revolved around the earth. This notion accounted for the rising and falling of the sun at the time. The cycles were described as circles, which were considered by Greeks to be the perfect shape.
A Shape Discovery
Science later proved that the orbits of the planets were not circular; instead, they were elliptical. An ellipse is a elongated circle, which accounts for the changing distances of certain stars and all of the planets when viewed from the Earth. The misconceptions that existed at the time of this discovery were that the motion of the stars was a result of Earth's rotation. People still believed that the other planets and stars did not spin like the earth.
Newton's Law
One of the most important scientific theories -- if not the most important -- for explaining the planets' movement came from Issac Newton. Newton's first law states that there is a force that controls the motion of the planets and stars. We refer to that force as gravity. He further describes that the larger the mass of the object, the stronger the force of gravity. Distance also plays a factor, as the farther a planet is from the source, the less the force of gravity. For this reason, the sun, which has the largest mass in the solar system, controls the orbits of the planets, as well as their rotation. Similarly, stars in the Milky Way orbit the center of the galaxy.
Stars Move?
Stars may seem like they sit in a stationary position in the sky. However, most notably with constellations, stars will slightly shift locations. This is because of the Earth's rotation. As the Earth rotates around the sun, causing seasons to change, familiar stars will appear to have moved. This is referred to as parallax. Parallax helps to measure distance of objects outside of our solar system, including stars. The stars' actual position does not change much because, unlike the planets, they are extremely far away from the sun.