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.