Everything about Zenith totally explained
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 (for example,
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.
(External Link
) 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.
Origin
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 (for example, 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.
Discussion
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 isn't 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).
Further Information
Get more info on 'Zenith'.
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