In
astronomy and
navigation, the
celestial sphere is an
imaginary sphere of arbitrarily large
radius,
concentric with Earth. 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.