This astronomy concept diagram of the celestial sphere depicts an arbitrarily large radius sphere concentric with Earth. All objects in the observer's sky can be imagined as being projected onto the celestial sphere's inside surface as if it were the underside of a dome. A celestial sphere is a useful tool in spherical astronomy because it allows observers to plot the positions of objects when their distances are unknown or unimportant. Because astronomical objects are so far away, casual observation of the sky provides no information on their actual distances. All things appear to be equally distant as if they are fixed to the inside of a sphere of unspecified radius that rotates from east to west overhead while underfoot.
