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.