Equatorial Coordinate System Explained: How Astronomers Navigate the Celestial Sphere
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Plane of the Ecliptic Tilted on Celestial Sphere
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The Celestial Sphere
In astronomy and navigation, the celestial sphere is an abstract sphere with an arbitrarily large radius concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location. The celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant.