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The nadir (UK: //, US: //; from Arabic: نظير / ALA-LC: naẓīr, meaning "counterpart")[a] is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite the nadir is the zenith.
In medicine, the term nadir is used to represent the lowest level of something being measured; for example, of a blood cell count while a patient is undergoing chemotherapy, or of a blood glucose level after a baby is born. A diagnosis of neutropenic nadir after chemotherapy typically lasts 7–10 days, and a newborn's blood sugar normally reaches its nadir around 2 hours after birth.
The word also refers to the downward-facing viewing geometry of an orbiting satellite, such as is employed during remote sensing of the atmosphere, as well as when an astronaut faces the Earth while performing a spacewalk.
A nadir line is a line traced on the ground directly beneath an aircraft while taking aerial photographs of the ground from above. This line connects the image centers of the successive vertical photographs.
The term nadir can also be used to represent the lowest point reached by a celestial body during its apparent orbit around a given point of observation. This can be 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. The sun is said to be at the nadir at a location when it is at the zenith at the location's antipode and the sun is 90 degrees below the horizon. This only occurs between the Tropic of Cancer and Tropic of Capricorn at solar midnight. The tropic circles experience this once a year on their winter solstices and all other tropical regions experience this twice.
Virtual Reality Video Capture (360º Video Footage)
The term nadir also applies to video footage produced for use in Virtual Reality aka 360º video. Cameras capable of capturing spherical footage are generally mounted on a tripod or extension boom that sits directly below and perpendicular to the horizontal plane of the camera lenses (when using two, 180º FOV lenses). In the case of camera rigging that utilizes more than 2 lenses to capture 360º footage, the nadir location follows the same logic as the astronomical definition in that it is located opposite of the zenith, or in the same direction as the gravitational pull in relation to the camera rigging.
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