Zenith

For other uses, see Zenith (disambiguation).

Diagram showing the relationship between the Zenith, the Nadir, and different types of Horizon. Note that the Zenith is opposite the Nadir.

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 the gravitational force pulls, is toward the nadir.

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. 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.

To an astronomer the meridian plane is defined by the zenith and the pole, but a geographic meridian is intended to be defined by the Earth's rotational axis, not by its gravitational field. (The two meridians would coincide only for a perfectly rotationally symmetric body.) On Earth, the axis of rotation is not perfectly fixed in 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).

Origin

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.

Relevance and use

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 zenith is used in the following scientific contexts:

• It 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.
• It defines one of the axes of the horizontal coordinate system in astronomy.

See also

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

References

• 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.