| horizon [hǝ`rīz·ǝn] ASTRONOMY 1. The apparent boundary line between the sky and the earth or sea. Also known as apparent horizon. 2. The distance a light-ray could have traveled since the big-bang explosion at any given epoch in the evolution of the universe. GEOLOGY 1. The surface separating two beds. 2. One of the layers, each of which is a few inches to a foot thick, that make up a soil. |