The nadir (/ - /,, also UK: //; Arabic: نظير, romanized: naẓīr, lit. 'counterpart') 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.
The direction opposite of the nadir is the zenith.
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). 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 faces the Earth while performing a spacewalk. A nadir image is a satellite image or aerial photo of the Earth taken vertically. A satellite ground track represents its orbit projected to nadir on to Earth's surface.
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.
- McLaughlin, Richard J.; Warr, William H. (2001). "The Common Berthing Mechanism (CBM) for International Space Station" (PDF). Society of Automotive Engineers. Retrieved March 23, 2012.
- "Bone marrow suppression". Chemotherapy Principles: An In-depth Discussion. American Cancer Society. Archived from the original on July 21, 2010. Retrieved March 5, 2013.
- Le, Tao; Bhushan, Vikas; Skapik, Julia (2007). First Aid For The USMLE Step 2 CK (6th ed.). New York: McGraw-Hill Medical. p. 479. ISBN 9780071487955.
- Chisholm, Hugh, ed. (1911). Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. p. 149. .
- Turner, Janice (December 1, 2007). "The lowest point in British journalism". The Times. Archived from the original on September 5, 2008. Retrieved March 23, 2012.
- Logan, Rayford Whittingham (1997) . The betrayal of the Negro, from Rutherford B. Hayes to Woodrow Wilson (Reprint ed.). Da Capo Press. ISBN 9780306807589. OCLC 35777358.