Southern celestial hemisphere
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The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form constellations, appears to rotate westward around a polar axis due to Earth's rotation.
At any given time, the entire Southern Sky is visible from the geographic South Pole, while less of this hemisphere is visible the further north the observer is located. The northern counterpart is the northern celestial hemisphere.
For the purpose of celestial mapping, the sky is considered by astronomers as the inside of a sphere divided in two halves by the celestial equator.[according to whom?] The Southern Sky or Southern Hemisphere is therefore that half of the celestial sphere that is south of the celestial equator. Even if this one is the ideal projection of the terrestrial equatorial onto the imaginary celestial sphere, the Northern and Southern celestial hemispheres must not be confused with descriptions of the terrestrial hemispheres of Earth itself.[according to whom?]
From the South Pole, in good visibility conditions, the Southern Sky features over 2,000 fixed stars that are easily visible to the naked eye, while about 20,000 to 40,000 with the aided eye.[dubious ] In large cities, about 300 to 500 stars can be seen depending on the extent of light and air pollution. The farther north, the less is visible to the observer.
The brightest stars are all larger than the Sun.[dubious ] Sirius in the constellation of Canis Major has the brightest apparent magnitude of –1.46; it has a radius twice that of the Sun and is 8.6 light-years away. Canopus and the next fixed star Toliman (α Centauri), 4.2 light-years away, are also located in the Southern Sky, having declinations around –60° – too close to the south celestial pole that neither are visible from Central Europe.