An imaginary sphere of infinite radius, centered at the observer. The variation of refraction , or refractive index, with the wavelength or color of light. If the ray curvature within a thermal inversion is stronger than the curvature of the Earth's surface, rays can be continuously guided along the surface of the Earth without ever escaping to space. This region in which rays are trapped is called a duct. An observer within the duct sees a superior mirage of distant objects within the duct.
Extinction is the sum of actual absorption, and scattering .
Sometimes the geodetic horizon is taken to be the line of tangency of this cone with the geoid. See the simulation of a sunset in the Standard Atmosphere. The green rim is so narrow that it can only be seen telescopically. This catch-all category is no longer useful, and should be avoided. To an astronomer, one of these horizons is a collection of directions in space, and in fact a circle on the celestial sphere .