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Grouping apparent constellation of stars on the celestial sphere. The use of grouping stars into constellations or asterisms is probably originated among ancient peoples of Mesopotamia. The configurations that with the stars that formed were ideally associated with certain images of deities, heroes, animals, everyday objects, for better or worse is framed within the boundaries of the c. themselves, and gave them the name. The C. current of the northern sky and the part we have seen the southern are due mainly to the Greeks. In 1925 and in 1928 the names and boundaries of the various constellations have been definitely established the International Astronomical Union. In our latitudes there are c. which they are always above the horizon (the two Bears, Cassiopeia, Cepheus etc.) and therefore visible all year, others that are visible or invisible depending on the season; some (including the Southern Cross and other c. S Zodiac) are always invisible. A constellation is each of the 88 districts into which the celestial sphere is conventionally divided in order to map the stars. The groupings so formed are entities exclusively perspective, to which modern astronomy does not recognize any real significance, in fact: in three dimensional space stars forming a same constellation can also be separated by vast distances, as well as different may be the size and brightness, vice versa, two or more stars that appear on the celestial sphere perhaps distant between them, in three-dimensional space can be on the contrary separated by distances smaller than those that separate them from other stars of its constellation, during a hypothetical interstellar travel would not be able to identify any constellation, and each stop near any star there would identify if anything new, visible only from this new perspective. in the course of time different constellations, some have been added have been defined, other have been joined together. Man excels in finding regular patterns and throughout history have grouped stars that appear close in constellations. A constellation “unofficial”, ie an array of stars that form simple geometric figures, called asterism.