The horizon or sensible horizon is, first of all, the visible place where earth, or sea, and sky appear to meet. At a hundred feet above sea level, the horizon is approximately fourteen miles away. The word has other technical meanings. A true, astronomical, or celestial horizon in astronomy is the circle on the celestial sphere whose plane is at right angles to the visible horizon. In geology, the horizon is the plane of a stratified surface containing a particular series of fossils. In soil science, it is a layer of soil in a cross section of land. In literature it is, of course, often a symbol for opportunity, especially if it is out of reach. Here is Zora Neale Hurston in Their Eyes Were Watching God: “Ships at a distance have every man’s wish on board. For some they come in with the tide. For others they sail forever on the horizon, never out of sight, never landing until the Watcher turns his eyes away in resignation.” In the plural horizons can be sequences of days, as in this seascape from Wallace Stevens’s “The Idea of Order at Key West”: “Theatrical distances, bronze shadows heaped/On high horizons, mountainous atmospheres/Of sea and sky.”

Robert Hass