All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere centered on the observer, half of which resembles a hemispherical screen over the observing location.
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.