Polaris , the North Star, lies almost directly above the north pole. A constellation is a group of stars that form a pattern in the sky. The planets are heavenly bodies that shift their positions regularly with respect to the stars. In the ptolemaic system , the earth is stationary at the center of the universe. In the copernican system , the earth rotates on its axis and, with the other planets, revolves around the sun.
Observational evidence supports the copernican system.
Kepler's laws are three regularities that planets obey as they move around the sun. The gravitational forces the sun exerts on the planets are what hold them in their orbits. Kepler's laws are explained by the law of gravity . The tides are periodic rises and falls of sea level caused by differences in the gravitational pulls of the moon and sun. Water facing the moon is attracted to it more than the earth itself is, and the earth is attracted to the moon more than water on the far side of the earth.