<P> 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 calendars, 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 . </P> <P> The oldest accurately dated star chart was the result of ancient Egyptian astronomy in 1534 BC . The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period (ca . 1531--1155 BC). </P> <P> The first star catalogue in Greek astronomy was created by Aristillus in approximately 300 BC, with the help of Timocharis . The star catalog of Hipparchus (2nd century BC) included 1020 stars, and was used to assemble Ptolemy's star catalogue . Hipparchus is known for the discovery of the first recorded nova (new star). Many of the constellations and star names in use today derive from Greek astronomy . </P> <P> In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear . In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185 . The brightest stellar event in recorded history was the SN 1006 supernova, which was observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers . The SN 1054 supernova, which gave birth to the Crab Nebula, was also observed by Chinese and Islamic astronomers . </P>

A group of stars that form an image