ZENITH AND NADIR A. Spherical Projection. Blue equatorial and orange meridional planes. B. Dark blue plane describes a circle on the sphere. C. Lines forming cones on the stereographic net. D and E illustrate the derivation of lower hemisphere projection by joining traces of planes on the hemisphere with zenith and the cyclographic traces or great circles on the projection. MERIDIONAL PLANE A plane that contains zenith and nadir both and is orthogonal to the equator in spherical projection is called the meridional plane. The orange plane in A and B parts of the figure is the meridional plane.
