Get Normal in Geometry
The input connection is not used by this node (consistent with all Get nodes).
Normal in Geometry is the surface normal, in world space, of the original undisplaced point on the surface being rendered. When sampling an atmosphere shader, Normal in Geometry is usually set to the local "Up" vector with respect to the planet to which the atmosphere shader is attached.
A shader is a program or set of instructions used in 3D computer graphics to determine the final surface properties of an object or image. This can include arbitrarily complex descriptions of light absorption and diffusion, texture mapping, reflection and refraction, shadowing, surface displacement and post-processing effects. In Terragen 2 shaders are used to construct and modify almost every element of a scene.
A vector is a set of three scalars, normally representing X, Y and Z coordinates. It also commonly represents rotation, where the values are pitch, heading and bank.