the circle which bounds that part of the earth's surface visible to a spectator from a given point; the apparent junction of the earth and sky.-a plane passing through the eye of the spectator and at right angles to the vertical at a given place; a plane tangent to the earth's surface at that place; called distinctively the sensible horizon.-a plane parallel to the sensible horizon of a place, and passing through the earth's center; -- called also rational / celestial horizon.-the unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible.-the epoch or time during which a deposit was made.-the chief horizontal line in a picture of any sort, which determines in the picture the height of the eye of the spectator; in an extended landscape, the representation of the natural horizon corresponds with this line.