 Historically , stars have been important to civilizations throughout the world . They have been part of religious practices and used for celestial navigation and orientation . Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable . By convention , astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun . The motion of the Sun against the background stars ( and the horizon ) was used to create <unk> , which could be used to regulate agricultural practices . The Gregorian calendar , currently used nearly everywhere in the world , is a solar calendar based on the angle of the Earth 's rotational axis relative to its local star , the Sun . 
