A constellation is a group of stars in the celestial vault that suggests an imaginary shape (a particular asterism) on the celestial vault. In astronomy, an asterism is a remarkable figure drawn by particularly bright stars.
In the sky, the stars of a constellation are very far from each other but appear grouped together in figures, so that the constellation does not have its own distance. In theory we could see at a glance about 3800 stars in the sky. The western constellations are grouped into two parts, dividing the sky more or less following the two terrestrial hemispheres, the austral sky for the south and the boreal sky for the north. The boreal constellations are the oldest and correspond to the plane of the sky visible from the regions of the Mediterranean by the astronomers of antiquity. The southern constellations were named by Western astronomers after the 15th century. The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise borders, so that each point in the sky belongs to a constellation. It is the rotation of the Earth that creates the horizon bisecting the celestial sphere.
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