zenith

Zenith is a term used in astronomy to describe the point on the celestial sphere that is directly above an observer. At the zenith, a straight line from the observer to the sky would pass through the center of the celestial sphere. The opposite point is the nadir, located directly below the observer.

Because the zenith depends on the observer’s location, it changes as the observer moves. In practical terms, the position of celestial objects is described using the zenith distance (the angular separation from the zenith) and the altitude (the angle above the horizon). These quantities are related by zenith distance = 90 degrees − altitude; an object at the zenith has an altitude of 90 degrees and a zenith distance of 0 degrees. Atmospheric effects, such as extinction, are also influenced by the zenith distance because light passes through more air at larger zenith distances.

Etymology and usage: the word zenith comes from Old French zénith, borrowed from Arabic samt, meaning direction or bearing. Beyond its astronomical sense, zenith is used metaphorically to denote the highest point or culmination of something, such as the zenith of a career.

Other uses: the term appears as a proper noun in brands and products, notably Zenith Electronics, an American electronics manufacturer known for its televisions, among other names that evoke the idea of peak quality or a pinnacle location in the sky.