The nadir [lower-alpha 1] [lower-alpha 2] 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, [2] 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. [3] A diagnosis of neutropenic nadir after chemotherapy typically lasts 7–10 days. [4]
The word is also used figuratively to mean a low point, such as with a person's spirits, [5] the quality of an activity or profession, [6] or the nadir of American race relations. [7]
Geodesy or geodetics is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. Geodesy is an earth science and many consider the study of Earth's shape and gravity to be central to that science. It is also a discipline of applied mathematics.
An azimuth is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system.
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, peaking in number at an altitude around 800 km (500 mi), while the farthest in LEO, before medium Earth orbit (MEO), have an altitude more than about one-third of the radius of Earth, roughly at the beginning of the inner Van Allen radiation belt.
The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane to define two angles of a spherical coordinate system: altitude and azimuth. Therefore, the horizontal coordinate system is sometimes called the az/el system, the alt/az system, or the alt-azimuth system, among others. In an altazimuth mount of a telescope, the instrument's two axes follow altitude and azimuth.
The zenith is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction opposite to the gravity direction at that location (nadir). The zenith is the "highest" point on the celestial sphere.
An astrolabe is an astronomical instrument dating to ancient times. It serves as a star chart and physical model of visible heavenly bodies. Its various functions also make it an elaborate inclinometer and an analog calculation device capable of working out several kinds of problems in astronomy. In its simplest form it is a metal disc with a pattern of wires, cutouts, and perforations that allows a user to calculate astronomical positions precisely. It is able to measure the altitude above the horizon of a celestial body, day or night; it can be used to identify stars or planets, to determine local latitude given local time, to survey, or to triangulate. It was used in classical antiquity, the Islamic Golden Age, the European Middle Ages and the Age of Discovery for all these purposes.
In astronomy, an analemma is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure eight. Globes of the Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun.
An apsis is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides is the line connecting the two extreme values.
The orbital period is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. It may also refer to the time it takes a satellite orbiting a planet or moon to complete one orbit.
In observational astronomy, culmination is the passage of a celestial object across the observer's local meridian. These events are also known as meridian transits, used in timekeeping and navigation, and measured precisely using a transit telescope.
A geocentric orbit, Earth-centered orbit, or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. More than 16,291 objects previously launched have undergone orbital decay and entered Earth's atmosphere.
In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon. Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis. For a location not on this axis, there is a unique meridian plane in this axial-pencil through that location. The intersection of this plane with Earth's surface defines two geographical meridians, and the intersection of the plane with the celestial sphere is the celestial meridian for that location and time.
Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy.
An orbital pole is either point at the ends of the orbital normal, an imaginary line segment that runs through a focus of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator.
Atmospheric sounding or atmospheric profiling is a measurement of vertical distribution of physical properties of the atmospheric column such as pressure, temperature, wind speed and wind direction, liquid water content, ozone concentration, pollution, and other properties. Such measurements are performed in a variety of ways including remote sensing and in situ observations.
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 or subsatellite track, and is the vertical projection of the satellite's orbit onto the surface of the Earth.
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets, as well as natural satellites and minor-planet moons.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
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.
Twice every year, the Sun culminates at the zenith of the Kaaba in Mecca, the holiest site in Islam, at local solar noon, allowing the qibla to be ascertained in other parts of the world by observing the shadows cast by vertical objects. This phenomenon occurs at 12:18 Saudi Arabia Standard Time on 27 or 28 May, and at 12:27 SAST on 15 or 16 July. At these times, the Sun appears in the direction of Mecca, and shadows cast by vertical objects determine the qibla. At two other moments in the year, the Sun passes through the nadir of the Kaaba, casting shadows that point in the opposite direction, and thus also determine the qibla. These occur on 12, 13, or 14 January at 00:30 SAST, and 28 or 29 November at 00:09 SAST.
