The projection of space onto the night sky, an imaginary hollow sphere of infinite radius surrounding the Earth but centred on the observer. (First postulated by Ptolemy.) It is the basis of sky charts, and the celestial co-ordinate system. The coordinate system most commonly used is right ascension and declination. The sphere itself is split up into arbitrary areas known as constellations. 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.