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Celestial sphere
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<p>In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a> and <a href="/facts/Navigation/sGGAJ0AQ">navigation</a>, the celestial sphere is an <a href="/facts/Abstraction/p7l9eHGd">abstract</a> <a href="/facts/Sphere/4Ivb1cTj">sphere</a> that has an arbitrarily large <a href="/facts/Radius/9Gr9deYD">radius</a> and is <a href="/facts/Concentric_objects/xR1I5N8y">concentric</a> to <a href="/facts/Earth/HLLmDfj0">Earth</a>. All objects in the <a href="/facts/Sky/wwvQ84J1">sky</a> can be conceived as being <a href="/facts/Projective_geometry/rHKSaEbx">projected</a> upon the inner surface of the celestial sphere, which may be <a href="/facts/Geocentric_model/yIc6na3w">centered on Earth</a> or the observer. If centered on the observer, half of the sphere would resemble a <a href="/facts/Sphere/4Ivb1cTj">hemispherical</a> <a href="/facts/Projection_screen/EfJYBwt2">screen</a> over the observing location. </p><p>The celestial sphere is a conceptual tool used in <a href="/facts/Spherical_astronomy/MIs7fBeu">spherical astronomy</a> to <a href="/facts/Celestial_coordinate_system/23HhqvHj">specify</a> the position of an <a href="/facts/Astronomical_object/BCHLa0Q2">object</a> in the sky without consideration of its linear distance from the observer. The <a href="/facts/Celestial_equator/P9Lyq2DE">celestial equator</a> divides the celestial sphere into <a href="/facts/Northern_celestial_hemisphere/ao3Xwp5G">northern</a> and <a href="/facts/Southern_celestial_hemisphere/PP2aeueF">southern</a> hemispheres. </p>