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Zenith refers to an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir. The zenith is the "highest" point on the celestial sphere (meaning it is the farthest up from the gravitational force). Zenith is sometimes used as a given name, most commonly for males. Its meaning, "highest point," evokes success and power.
The root of the word "zenith" in Proto-Indo-European was reconstructed as '*mei-' ("to change"). In Latin we find 'meare' ("to pass"). With the prefix 'sē-' ("aside") it became 'sēmeare' ("to branch off"). Then the noun 'sēmita' ("side-way") was formed. When the Romans occupied Syria, shortly before the time of Christ, the resident Arameans adopted the word 'sēmita' as 'simta' ("side-way"). The Romans left. As the Arabians conquered the land in the seventh century they took the word 'simta' from the Arameans as 'samt' ("side-path") and also coined astronomical expressions. The word "zenith" derives from the inaccurate reading of the Arabic expression سمت الرأس (samt ar-ra's), meaning "direction of the head" or "path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), possibly through Old Spanish. It was reduced to 'samt' ("direction") and miswritten as 'senit'/'cenit', as the "m" was misread as an "ni". Through the Old French 'cenith', 'zenith' first appeared in the 17th century.
The term zenith is sometimes used to refer to the highest point, way or level reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is often used to describe the location of the Sun ("The sun reached its zenith..."), but to an astronomer the sun does not have its own zenith, and is at the zenith only if it is directly overhead.
In a scientific context, the zenith is the direction of reference for measuring the zenith angle, the angle between a direction of interest (e.g., a star) and the local zenith.
In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles.