Zenith

Diagram showing the relationship between the zenith, the nadir, and different types of horizon. Note that the zenith is opposite the nadir.

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The zenith is 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.

Origin [edit]

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.^[1][2]

Relevance and use [edit]

Shadows of trees when the sun is directly overhead (at the zenith). This happens at solar noon if the tree's latitude equals the sun's declination at that moment.

The term zenith is sometimes used to refer to the highest point reached by a celestial body during its apparent orbit around a given point of observation.^[3] 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 doesn't 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.

Zenith is also used to determine the time of Dhuhr (midday) prayer by Muslims which is when the sun passes the zenith.

See also [edit]

• Azimuth
• Geodesy
• History of geodesy
• Keyhole problem
• Midheaven
• Subsolar point
• Vertical deflection
• Zenith distance

References [edit]

• Glickman, Todd S. (2000). Glossary of meteorology. American Meteorological Society. ISBN 978-1-878220-34-9. 
• McIntosh, D. H. (1972). Meteorological Glossary (5th ed.). ISBN 978-0-8206-0228-8. 
• Picoche, Jacqueline (2002). Dictionnaire étymologique du français. Paris: Le Robert. ISBN 978-2-85036-458-7.