In geometry, a projection of the sphere on the plane of the meridian, orthographically made by straight lines, circles and ellipses, the eye being supposed at an infinite distance, and in the east or west points of the horizon.

An instrument of wood or brass on which this kind of projection is drawn, with a horizon and cursor fitted to it, in which the solstitial colure, and all circles parallel to it, will be concentric circles; all circles oblique to the eye will be ellipses; and all circles whose planes pass through the eye, will be right lines.