The nadir 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 (geometry, geography), direction'' or ''plane (geometry), plane'' passing by a given point is said to be vertical if it contains the local gravity direction at that point. ...

s at a specified location,

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

to a horizontal flat surface. The direction opposite of the nadir is the

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

Etymology

Although it entered English via other European languages, the word “nadir” is ultimately an

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

loanword A loanword (also a loan word, loan-word) is a word at least partly assimilated from one language (the donor language) into another language (the recipient or target language), through the process of borrowing. Borrowing is a metaphorical term t ...

. It comes from the Arabic word “nazir”, meaning “opposite to”. More specifically, it originated from the Arabic phrase “nazir as-samt”, meaning “ heopposite direction”. Hebrew (whether

ancient Ancient history is a time period from the beginning of writing and recorded human history through late antiquity. The span of recorded history is roughly 5,000 years, beginning with the development of Sumerian cuneiform script. Ancient h ...

modern Modern may refer to: History *Modern history ** Early Modern period ** Late Modern period *** 18th century *** 19th century *** 20th century ** Contemporary history * Moderns, a faction of Freemasonry that existed in the 18th century Philosophy ...

) is a related language to Arabic, as they are both

Semitic languages The Semitic languages are a branch of the Afroasiatic languages, Afroasiatic language family. They include Arabic, Amharic, Tigrinya language, Tigrinya, Aramaic, Hebrew language, Hebrew, Maltese language, Maltese, Modern South Arabian language ...

. Hebrew also has a word “nadir” (נדיר), but with a somewhat different meaning: it is an adjective meaning “rare”. However, the same word also has a specialized usage to match its meaning in other languages like English.

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 the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

geophysics Geophysics () is a subject of natural science concerned with the physical processes and Physical property, properties of Earth and its surrounding space environment, and the use of quantitative methods for their analysis. Geophysicists conduct i ...

and related sciences (e.g.,

meteorology Meteorology is the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

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

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

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 within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

reaches along its apparent daily path around a given point of observation (i.e. the object's ''lower

culmination In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian. These events are also known as meridian tran ...

''). 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 coordinate system, geographic location of observation on Earth's surface. As Earth's orbit, Earth orbits the Sun over the course of a year, the Sun appears to mo ...

, but it is only technically accurate for one

latitude In geography, latitude is a geographic coordinate system, geographic 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 t ...

at a time and only possible at the

low latitudes The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's axi ...

. 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 an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

, such as is employed during

remote sensing Remote sensing is the acquisition of information about an physical object, 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 inform ...

of the atmosphere, as well as when an

astronaut An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...

faces the Earth while performing a

spacewalk Extravehicular activity (EVA) is any activity done by an astronaut in outer space outside a spacecraft. In the absence of a breathable atmosphere of Earth, Earthlike atmosphere, the astronaut is completely reliant on a space suit for environme ...

. 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 i ...

aerial photo Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other airborne platforms. When taking motion pictures, it is also known as aerial videography. Platforms for aerial photography include fixed-wing airc ...

of the Earth taken vertically. A satellite

ground track A satellite ground track or satellite ground trace is the path on the surface of a planet directly below a satellite's trajectory. It is also known as a suborbital track or subsatellite 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 Etymology, etymological origin is the Greek word ὄγ ...

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

chemotherapy Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) in a standard chemotherapy re ...

. A diagnosis of

neutropenic Neutropenia is an abnormally low concentration of neutrophils (a type of white blood cell) in the blood. Neutrophils make up the majority of circulating white blood cells and serve as the primary defense against infections by destroying bacteria ...

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(s) commonly refers to: * Liquor, a distilled alcoholic drink * Spirit (animating force), the non-corporeal essence of living things * Spirit (supernatural entity), an incorporeal or immaterial being Spirit(s) may also refer to: 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, and particularly anti-bl ...

Notes

References

{{Portal bar, Science, Astronomy, Stars, Spaceflight, Outer space, Solar System Astronomical coordinate systems Horizontal coordinate system Technical factors of astrology Orientation (geometry)