One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth, and referred generally to a horizontal direction (i.e., at right angles to the zenith). Considerable contradiction exists between the nomenclatures for the several concepts of horizon. Aside from the distinctly different geological horizons (strata of earth material), it may be said that there are two types of horizons: earth–sky horizons (1, 2, and 3 below) and celestial horizons (4 and 5 below). Meteorology is primarily concerned with the former, astronomy with the latter. Specifically, the following constitute the major variant usages, with suggested nomenclature along with other names that have been applied. 1) Local horizon: the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions. 2) Geographic horizon (also called apparent horizon, local horizon, visible horizon): the distant line along which earth and sky appear to meet. In both popular usage and weather observing, this is the usually conception of horizon. Nearby prominences are said to obscure the horizon and are not considered to be a part of it. The minimum desirable horizon distance should be of the order of three miles. 3) Sea level horizon (also called ideal horizon, sensible horizon, sea horizon, visible horizon, apparent horizon): the apparent junction of the sky and the sea level surface of the earth; the horizon as actually observed at sea. This type of horizon is used as the reference for establishing times of sunrise and sunset. 4) Astronomical horizon (also called sensible horizon, real horizon): the plane that passes through the observer's eye and is perpendicular to the zenith at that point; or, the intersection of that plane with the celestial sphere (i.e., a great circle on the celestial sphere equidistant from the observer's zenith and nadir). It is the projection of a horizontal plane in every direction from the point of orientation. 5) Celestial horizon (also called rational horizon, geometrical horizon, true horizon): the plane, through the center of the earth, that is perpendicular to a radius of the earth that passes through the point of observation on the earth's surface; or, the intersection of that plane with the celestial sphere. See also artificial horizon, fog horizon, haze horizon, smoke horizon.