Celestial Sphere

The night sky looks like an upside down bowl set on the horizon, but as it turns around during the night it is easy to think of it as a giant sphere with the stars lying on the inside surface. The apparent canopy of the stars, which is a projection of the earth sphere into space. The co-ordinates of the Earth grid can be "projected" to give Celestial measurements. It is a two-dimensional system, radial measurement ignored, all objects considered to be on its surface. Three- dimensional phenomena in astronomy such as parallax are treated by other means.