High Quality Content by WIKIPEDIA articles! In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the observer. All objects in the observer`s sky can be thought of as projected upon the inside surface of the celestial sphere, as if it were the underside of a dome or a hemispherical screen. The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant.