The OED gives the etymology as a Greek word meaning the bounding circle and defines it in the first place as: “The boundry-line of that part of the earth’s surface visible from a given point of view; the line at which the earth and sky appear to meet. In strict use, the circle bounding that part of the earth’s surface which would be visible if no irregularities or obstructions were present.” The horizon with irregularities and obstructions it refers to as “apparent, natural, sensible, physical or visible horizon” opposed to the “astronomical, celestial, mathematical, rational, real or true horizon”, except that “on the open sea or a great plain these coincide.”
Yesterday, walking up the road after getting the mail, I thought about horizons while looking as far into the woods as I could, through the trees. As a sailor, the “true” horizon might be most important, but only when calculating the results of an astronomical “sight”. Otherwise, like other mathematical precision, the true horizon never exists. The earth is not round, more pear shaped, and neither the land nor the sea is level, even the OED’s “the open sea or a great plain”. Waves, swell and ripples impinge on the horizon as much as hills, mountains and trees.
Looking into the woods through the trees one horizon is between the land and trees, especially evergreens, pine and hemlock, that shield the upper edge of the land. The rise and fall of land in the forest defines the water shed, suggests the hill rising to the East, out of sight behind the trees where the sky is visible almost overhead through the tree tops. I look through the trees for fox, deer, grouse, red squirrel, moving legs, flash of feathers, seeing tree trunks, ferns, laurel, saplings, the lay of the land — rarely animal life.
There are horizons for every point of view and every horizon a point of view. We have recently moved from sea shore to wooded hills. The shore view is half at sea and half the land view, rising horizon with plants and houses. In the wooded hills the trees seem to be the horizon until sunset when red and orange glow attracts the eye low into the woods revealing far ridges closer to the natural horizon.