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Nadir
From Wikipedia, the free encyclopedia
The nadir (from Arabic nazir, "opposite") is the astronomical term for the point in the sky directly below the observer, or more precisely, the point in the sky with an inclination of −90°. In simple terms, if you are standing on the earth, it is the direction "down" toward your feet.
Geometrically, it is the point on the celestial sphere intersected by a line drawn from the observer's location on the Earth's surface through the center of the Earth. The point opposite the nadir is the zenith. Nadir also refers to a downward-facing viewing angle of an orbiting satellite, such as is employed during remote sensing of the atmosphere.
By association, nadir is also used to mean the lowest point in a number of fields:
- In biology and the natural sciences, nadir refers to a temporal low point in the population of a species, or the lowest point of some other parameter.
- In hydrology nadir may refer to the low point in a lake level varying by season or for a long term natural cycle.
- In economics, nadir means a low in the price of products.
- In medicine, nadir can also be used to refer to the lowest blood count for a given patient in a given period of time (i.e., a patient's ANC Nadir or absolute neutrophil count). For example, patients undergoing chemotherapy will exhibit an ANC Nadir a week after starting therapy due to bone marrow suppression.
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