The nadir (, ; ar, نظير, naẓīr, counterpart) is the direction pointing directly ''below'' a particular location; that is, it is one of two

vertical direction In astronomy, geography, and related sciences and contexts, a '' direction'' or '' plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. Conversely, a direction or plane is said to be h ...

s at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the

zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...

Definitions

Space science

Since the concept of ''being below'' is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

geophysics Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

and related sciences (e.g.,

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

), the nadir at a given point is the local vertical direction pointing in the direction of the force of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

at that location. The term can also be used to represent the lowest point that a

celestial object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...

reaches along its apparent daily path around a given point of observation (i.e. the object's ''lower culmination''). This can be used to describe the

position of the Sun The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the ...

, but it is only technically accurate for one

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

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 is 90° below the horizon. ''Nadir'' also refers to the downward-facing viewing geometry of an orbiting

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...

, such as is employed during

remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Eart ...

of the atmosphere, as well as when an astronaut faces the Earth while performing a spacewalk. A ''nadir image'' is a

satellite image Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell ima ...

or aerial photo of the Earth taken vertically. A satellite

ground track A ground track or ground trace is the path on the surface of a planet directly below an aircraft's or satellite's trajectory. In the case of satellites, it is also known as a suborbital track, and is the vertical projection of the satellite's ...

represents its orbit projected to nadir on to Earth's surface.

Medicine

Generally in medicine, ''nadir'' is used to indicate the progression to the lowest point of a clinical symptom (e.g. fever patterns) or a laboratory count. In

oncology Oncology is a branch of medicine that deals with the study, treatment, diagnosis and prevention of cancer. A medical professional who practices oncology is an ''oncologist''. The name's etymological origin is the Greek word ὄγκος (''� ...

, the term nadir is used to represent the lowest level of a blood cell count while a patient is undergoing

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs ( chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemothe ...

. A diagnosis of neutropenic nadir after chemotherapy typically lasts 7–10 days.

Figurative usage

The word is also used figuratively to mean a low point, such as with a person's

spirits Spirit or spirits may refer to: Liquor and other volatile liquids * Spirits, a.k.a. liquor, distilled alcoholic drinks * Spirit or tincture, an extract of plant or animal material dissolved in ethanol * Volatile (especially flammable) liquids, ...

, the quality of an activity or profession, or the

nadir of American race relations The nadir of American race relations was the period in African American history and the history of the United States from the end of Reconstruction in 1877 through the early 20th century when racism in the country, especially racism against ...

References

{{Portal bar, Science, Astronomy, Stars, Spaceflight, Outer space, Solar System Astronomical coordinate systems Technical factors of astrology