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

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of ''perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

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

Definitions

Space science

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 term can also be used to represent the lowest point that a celestial object 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 were also known as meridian transits ...

''). This can be used to describe the position 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 is 90° below the horizon. ''Nadir'' 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 An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...

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 environmen ...

. A ''nadir image'' is a satellite image or

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 aircra ...

of the Earth taken vertically. A satellite ground track 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, the term nadir is used to represent the lowest level of a blood cell count while a patient is undergoing chemotherapy. 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, the quality of an activity or profession, or the nadir of American race relations.

References

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