horizon

horizon in astronomy, roughly circular line bounding an observer's view of the surface of the earth where the sky and earth seem to meet. This is the visible horizon. At sea the visible horizon is a perfect circle with the observer at its center, but on land it is irregular due to topographic features. The distance to the horizon varies as the square root of the observer's elevation for small elevations; at four times the height the distance to the horizon is twice as great. The celestial horizon , the principal axis in the altazimuth coordinate system , lies halfway between the observer's zenith and nadir . In geology horizon refers to sedimentary deposits of a certain period, usually marked by characteristic fossils.

ho·ri·zon / həˈrīzən/ • n. 1. [usu. in sing.] the line at which the earth's surface and the sky appear to meet: the sun rose above the horizon. ∎  (also apparent or visible horizon) the circular boundary of the part of the earth's surface visible from a particular point, ignoring irregularities and obstructions. ∎  (also celestial horizon) Astron. a great circle of the celestial sphere, the plane of which passes through the center of the earth and is parallel to that of the apparent horizon of a place. 2. (often horizons) the limit of a person's mental perception, experience, or interest: she wanted to leave home and broaden her horizons. 3. Geol. a layer of soil or rock, or a set of strata, with particular characteristics. ∎  Archaeol. a level of an excavated site representing a particular period. PHRASES: on the horizon just imminent or becoming apparent: trouble could be on the horizon.

horizon The great circle formed by the intersection of the plane perpendicular to the observer's zenith with the celestial sphere; also called the astronomical horizon.