In general terms, the zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers to the highest point 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, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.
Strictly speaking, the zenith is only approximately 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).
The word zenith derives from the inaccurate reading of the Arabic expression سمت الرأس (samt ar-ra's) meaning “direction of the head”/"path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), probably through Old Spanish. It was incorrectly reduced to 'samt' ("direction") and imprecisely written as 'senit'/'cenit' by those scribes. Through Old French 'cenith', Middle English 'senith' and finally 'zenith' first appears in the 17th century.
The Arabic word for zenith is zawâl, meaning "decline", that is, when the sun ceases to rise and starts to decline.
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.
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