zénith

The zénith is the point in the sky that lies directly above an observer on the Earth’s surface. In practical terms, it is the location on the celestial sphere with an altitude of 90 degrees relative to the observer’s horizon. The concept is used in celestial coordinate systems to describe the position of objects such as stars and planets.

The zenith has a fixed relationship with the nadir, the point directly opposite it on the celestial sphere. A related measure is the zenith distance, the angular distance between an object and the zenith. When an object is at the zenith, its zenith distance is zero and its altitude is 90 degrees. For an observer at a given latitude, an object’s maximum altitude occurs when it crosses the local meridian; a star with a declination equal to the observer’s latitude would pass through the zenith.

In observational astronomy, the zenith is important for planning and interpretation. As an object moves away from the zenith toward the horizon, its light passes through more atmosphere, increasing extinction and atmospheric distortion. The concept also underpins the term zenith angle, used in calculations of airmass and sky brightness.

Etymology and usage: zénith is the French term for this concept, derived from historical forms in Old French and ultimately adopted into other languages as zenith. Beyond science, zénith is used metaphorically to denote the peak or culmination of something, and it appears as a proper name in contexts such as large concert venues.