Diagram showing the relationship between the Zenith, the Nadir, and different types of Horizon. Note how the Nadir is opposite the Zenith.
In broad terms, the zenith is the direction pointing directly above a particular location (perpendicular, orthogonal). Since the concept of being above is itself somewhat vague, scientists define the zenith in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the zenith at a given point is the local vertical direction pointing away from direction of the force of gravity at that location.
For reference, the vertical direction at the given location and pointing in the same sense as the gravitational force is called the nadir.
Zenith is also used for the highest point reached by a celestial body during its apparent orbit around a given point of observation.  Often used in this sense about the Sun, it only corresponds to the first concept of zenith for one latitude at a time, and never at all for latitudes outside the tropics.
The word zenith derives from the inaccurate reading of the Arabic word samt ('path'), pronounced sent, by scribes in the Middle Ages (during the 14th century), in the expression samt arrâs ('path above the head').
Relevance and use
The zenith is used in the following scientific contexts:
* It serves as the direction of reference for measuring the zenith angle, which is the angular distance between a direction of interest (e.g., a star) and the local zenith, relative to the point for which the zenith is defined.
* It defines one of the axes of the horizontal coordinate system in astronomy.
* The zenith directions corresponding to two different locations (not on the same vertical direction) are divergent.
* Strictly speaking, the zenith is only approximatively contained in the local meridian plane because the latter is defined in terms of the rotational characteristics of the celestial body, not in terms of its gravitational field. The two coincide only for a perfectly rotationally symmetric body. On Earth, the axis of rotation is not fixed with respect to the planet (for example due to constant displacements of its fluid components) so that the local vertical direction, as defined by the gravity field, is itself changing direction in time (for instance due to lunar and solar tides).
* Zenith distance
* Huschke, Ralph E. (1959) Glossary of Meteorology, American Meteorological Society, Boston, Second printing-1970.
* McIntosh, D. H. (1972) Meteorological Glossary, Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.
* Picoche, J. (1992) Dictionnaire Etymologique du Français, Le Robert, Paris, ISBN 2-85036-458-4.
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