A

Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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aberration

1. In astronomy, the apparent angular displacement of the position of a celestial body in the direction of motion of the observer, caused by the combination of the velocity of the observer and the velocity of light. See constant of aberration, planetary aberration. Compare parallax.

2. In optics, a specific deviation from perfect imagery, as, for example: spherical aberration, coma, astigmatism, curvature of field, and distortion.

aberration constant = constant of aberration

ablate

To carry away; specifically, to carry away heat generated by aerodynamic heating, from a vital part, by arranging for its absorption in a nonvital part, which may melt or vaporize, then fall away taking the heat with it. See heat shield, ablation.

ablating material

A material, especially a coating material, designed to provide thermal protection to a body in a fluid stream through loss of mass.

Ablating materials are used on the surfaces of some reentry vehicles to absorb heat by removal of mass, thus blocking the transfer of heat to the rest of the vehicle and maintaining temperatures within design limits. Ablating materials absorb heat by increasing in temperature and changing in chemical or physical state. The heat is carried away from the surface by a loss of mass (liquid or vapor). The departing mass also blocks part of the convective heat transfer to the remaining material in the same manner as transpiration cooling.

ablating nose cone

A nose cone designed to reduce heat transfer to the internal structure by the use of an ablating material.

ablation

The removal of surface material from a body by vaporization, melting, chipping, or other erosive process; specifically, the intentional removal of material from a nose cone or spacecraft during high-speed movement through a planetary atmosphere to provide thermal protection to the underlying structure. See ablating material.

ablatively

By a process of ablation, as in ablatively cooled .

ablative material = ablating material.

ablator

A material designed to provide thermal protection through ablation.

abort

1. To cut short or break off an action, operation, or procedure with an aircraft, space vehicle, or the like, especially because of equipment failure, as to abort a mission, the launching was aborted.

2. An aircraft, space vehicle, or the like that aborts.

3. An act or instance of aborting.

Abridged Nautical Almanac

See Nautical Almanac.

absolute

1. Pertaining to a measurement relative to a universal constant or natural datum, as absolute coordinate system, absolute altitude, absolute temperature.

2. Complete, as in absolute vacuum.

absolute altimeter

An instrument intended to give acceptably accurate, direct indications of absolute altitude.

absolute altitude

Altitude above the actual surface, either land or water, of a planet or natural satellite. Compare true altitude.

absolute coordinate system

An inertial coordinate system which is fixed with respect to the stars.

In theory, no absolute coordinate system can be established because the reference stars are themselves in motion. In practice, such a system can be established to meet the demands of the problem concerned by the selection of appropriate reference stars.

absolute delay

1. The time interval between the transmission of sequential signals. Also called delay .

2. Specifically, in loran, the time interval between transmission of a signal from the A-station and transmission of the next signal from the B-station.

absolute humidity

The amount of water vapor actually present in unit quantity of a gas, generally expressed as mass of water vapor per unit volume of gas + water vapor, e.g., as grains per cubic foot.

absolute index of refraction = index of refraction (sense 1).

absolute instrument

An instrument whose calibration can be determined by means of physical measurements on the instrument. Compare secondary instrument.

absolute magnitude (symbol M)

1. A measure of the brightness of a star equal to the magnitude the star would have at a distance of 10 parsecs from the observer.

where m is apparent magnitude, and p is the parallax of the star (in seconds of arc). Absolute magnitudes may be visual, photographic, etc., according to the way in which the apparent magnitude was measured.

2. The stellar magnitude any meteor would have if placed in the observer's zenith at a height of 100 kilometers.

absolute manometer

1. A gas manometer whose calibration, which is the same for all ideal gases, can be calculated from the measurable physical constants of the instrument.

2. A manometer that measures absolute pressure.

absolute motion

Motion relative to a fixed point. See absolute coordinate system, note.

absolute pressure

In engineering literature, a term used to indicate pressure above the absolute zero value of pressure that theoretically obtains in empty space or at the absolute zero of temperature as distinguished from gage pressure.

In high-vacuum technology, pressure is understood to correspond to absolute pressure, not gage pressure, and therefore the term absolute pressure is rarely used.

absolute refractive index = index of refractory (sense 1)

absolute system of units

1. A system of units in which a small number of units are chosen as fundamental, and all other units are derived from them.

2. Specifically, a system of electrical units put into effect by international agreement on 1 January 1948.

Prior to 1 January 1948 the international system was in effect; the two systems can be converted by the following relationships:

1 mean international ohm = 1.00049 absolute ohm

1 mean international volt = 1.00034 absolute volt.

"Electric units, called "international," for current and resistance had been introduced by the International Electrical Congress held in Chicago in 1893, and the definitions of the "international" ampere and the "international" ohm were confirmed by the International Conference of London in 1908.

Although it was already obvious on the occasion of the 8th CGPM (1933) that there was a unanimous desire to replace those "international" units by so-called "absolute" units, the official decision to abolish them was only taken by the 9th CGPM (1948), which adopted for the unit of electric current, the ampere," which see.

The previous is an excerpt from WWW version of the National Institute of Standards and Technology: Physics Laboratory's International System of Units (SI).

absolute temperature

Temperature value relative to absolute zero.

absolute temperature scale

A temperature scale based upon the value zero as the lowest possible value. Thus, all obtainable temperatures are positive. The Kelvin and Rankine scales are absolute scales.

absolute vacuum

A void completely empty of matter. Also called perfect vacuum.
An absolute vacuum is not obtainable.

absolute vorticity

1. The vorticity of a fluid particle expressed with respect to an absolute coordinate system. 2. The vertical component of the absolute vorticity (as defined above).

absolute zero

The theoretical temperature at which molecular motion vanishes and a body would have no heat energy; the zero point of the Kelvin and Rankine temperature scales.

Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold source which would render a Carnot cycle 100 percent efficient. The value of absolute zero is now estimated to be - 273.15° Celsius, -459.67° Fahrenheit, 0° Kelvin, and 0° Rankine.

absorptance, absorbtance(symbol A, a, or )

The ratio of the radiant flux absorbed by a body to that incident upon it. Also called absorption factor. Compare absorptivity.
Total absorptance refers to absorptance measured over all wavelengths.
Spectral absorptance refers to absorptance measured at a specified wavelength.

absorption

1. The process by which radiant energy is absorbed and converted into other forms of energy. See attenuation.
Absorption takes place only after the radiant flux enters a medium and thus acts only on the entering flux not on the incident flux, some of which may be reflected at the surface of the medium. A substance which absorbs energy may also be a medium of refraction, diffraction, or scattering; these processes, however, involve no energy retention or transformation and are to be clearly differentiated from absorption.

2. In general, the taking up or assimilation of one substance by another. See sorption, adsorption.

3. In vacuum technology, gas entering into the interior of a solid.

absorption band

A range of wavelengths (or frequencies) in the electromagnetic spectrum within which radiant energy is absorbed by a substance. See absorption spectrum.

When the absorbing substance is a polyatomic gas, an absorption band actually is composed of a group of discrete absorption lines which appear to overlap. Each line is associated with a particular mode of vibration or rotation induced in a gas molecule by the incident radiation.
The absorption bands of oxygen and ozone are often referred to in the literature of atmospheric physics.
The important bands for oxygen are: (a) the Hopfield bands, very strong, between about 670 and 1000 angstroms in the ultraviolet; (b) a diffuse system between 1019 and 1300 angstroms; (c) the Schumann-Runge continuum, very strong, between 1350 and 1760 angstroms; (d) the Schumann-Runge bands between 1760 and 1926 angstroms; (e) the Herzberg bands between 2400 and 2600 angstroms; (f) the atmospheric bands between 5380 and 7710 angstroms in the visible spectrum; and (g) a system in the infrared at about 1 micron.
The important bands for ozone are: (a) the Hartley bands between 2000 and 3000 angstroms in the ultraviolet, with a very intense maximum absorption at 2550 angstroms; (b) the Huggins bands, weak absorption between 3200 and 3600 angstroms; (c) the Chappius bands, a weak diffuse system between 4500 and 6500 angstroms in the visible spectrum; and (d) the infrared bands centered at 4.7, 9.6 and 14.1 microns, the latter being the most intense.

absorption coefficient (symbol )

1. A measure of the amount of normally incident radiant energy absorbed through a unit distance or by a unit mass of absorbing medium. Compare transmission coefficient.

The absorption coefficient is frequently identified as follows: where I_Lx is the flux density of radiation of wavelength L, initially of flux density I_L0, after traversing a distance x in some absorbing medium. (Substitute L for lambda.)

2. In acoustics, the ratio of the sound energy absorbed by a surface of a medium (or material) exposed to a sound field or sound radiation to the sound energy incident on the surface. The stated values of this ratio are to hold for an infinite area of the surface. The conditions under which measurements of absorption coefficients are made are to be stated explicitly.

Three types of absorption coefficients associated with three methods of measurement are: chamber absorption coefficient, obtained in a certain reverberation chamber; free-wave absorption coefficient, obtained when a plane, progressive, sound wave is incident on the surface of the medium; sabine absorption coefficient, obtained when the sound is incident from all directions on the sample.

absorption cross section

In radar, the ratio of the amount of power removed from a beam by absorption of radio energy by a target to the power in the beam incident upon the target. Compare scattering cross section. See cross section.

absorption-emission pyrometer

A thermometer for determining gas temperature from measurement of the radiation emitted by a calibrated reference source before and after this radiation has passed through and been partially absorbed by the gas. Both measurements are made over the same wavelength interval.

absorption factor = absorptance.

absorption line

A minute range of wavelength (or frequency) in the electromagnetic spectrum within which radiant energy is absorbed by the medium through which it is passing. Each line is associated with a particular mode of electronic excitation induced in the absorbing atoms by the incident radiation. See absorption spectrum, spectral line, telluric lines, Fraunhofer lines, absorption band.

absorption spectrum

The array of absorption lines and absorption bands which results from the passage of radiant energy from a continuous source through a selectively absorbing medium cooler than the source. See electromagnetic spectrum.

The absorption spectrum is a characteristic of the absorbing medium, just as an emission spectrum is a characteristic of a radiator.
An absorption spectrum formed by a monatomic gas exhibits discrete dark lines, whereas that formed by a polyatomic gas exhibits ordered arrays (bands) of dark lines, which appear to overlap. This type of absorption is often referred to as line absorption. The spectrum formed by a selectively absorbing liquid or solid is typically continuous in nature (continuous absorption).

absorptive index

The imaginary part of the complex index of refraction of a medium. It represents the energy loss by absorption and has a nonzero value for all media which are not dielectrics. Also called index of absorption . Compare absorption coefficient.

absorptive power

The total flux of radiant energy absorbed in a unit area of absorbing substance; expressed, for example, in ergs per square centimeter per second or in watts per square centimeter.

absorptivity (symbol )

The capacity of a material to absorb incident radiant energy, measured as the absorptance of a specimen of the material thick enough to be completely opaque, and having an optically smooth surface.

absorptivity-emissivity ratio

In space applications, the ratio of absorptivity for solar radiation of a material to its infrared emissivity. Also called A/E ratio.

acceleration

1. The rate of change of velocity.

2. The act or process of accelerating, or the state of being accelerated. Negative acceleration is called deceleration .

acceleration of gravity (symbol g)

By the International Gravity Formula, $g\; =\; 978.0495\; [1\; +\; 0.0052892\; sin2(p)\; -\; 0.0000073\; sin2(2p)]$ centimeters per second squared at sea level at latitude p. See gravity.

The standard value of gravity, or normal gravity, g, is defined as g$$_o=980.665 centimeters per second squared, or 32.1741 feet per second squared. This value corresponds closely to the International Gravity Formula value of g at 45° latitude at sea level.

accelerator

Short for particle accelerator .

accelerometer

A transducer which measures acceleration or gravitational forces capable of imparting acceleration.

An accelerometer usually uses a concentrated mass (seismic mass) which resists movement because of its inertia. The displacement of the seismic mass relative to its supporting frame or container is used as a measure of acceleration.

acceptor

In transistors, the P-type semiconductor, the electrode containing trivalent impurities (boron, gallium, or indium) to increase the number of holes which can accept electrons. Contrast with donor.

accidental error

In experimental observations, an error which does not always recur when an observation is repeated under the same conditions. Contrast systematic error.

acclimatization

The adjustments of a human body or other organism to a new environment; the bodily changes which tend to increase efficiency and reduce energy loss. Compare adaptation, accustomization.

accommodation coefficient (symbol )

The ratio of the average energy actually transferred between a surface and impinging gas molecules which are scattered by the surface to the average energy which would theoretically by transferred if the impinging molecules reached complete thermal equilibrium with the surface before leaving the surface or $a\; =\; (E$_r-E _i)/E_s-E_i) where a is the accommodation coefficient, Er is the energy carried away from unit surface area per second by the scattered or re-evaporated molecules, Ei is the energy per unit surface area per second carried toward the surface by the impinging molecules, and Es is the energy per unit surface area per second which would be carried away by the molecules if the molecules reached complete thermal equilibrium with the surface before leaving.

accumulator

1. A device or apparatus that accumulates or stores up, as: (a) a contrivance in a hydraulic system that stores fluid under pressure; (b) a device sometimes incorporated in the fuel system of a gas-turbine engine to store up and release fuel under pressure as an aid in starting; (c) an electrical storage battery (British usage).

2. In computer technology, a device which stores a number and upon receipt of another number adds it to the number already stored and stores the sum. See counter.

accustomization

The process of learning the techniques of living with a minimum of discomfort in an extreme or new environment. Compare acclimatization, adaptation.

aclinic line

The line through those points on the earth's surface at which magnetic dip is zero. The aclinic line is a particular case of an isoclinic line. Also call dip equator, magnetic equator. Compare agonic line, geomagnetic equator.

acoustic, acoustical

Containing, producing, arising from, actuated by, related to, or associated with sound.

Acoustic is used to modify terms that designate an object, or physical characteristics, associated with sound waves; acoustical is used when the term being qualified does not designate explicitly something that has such properties, dimensions, or physical characteristics.
The following terms are examples of those modified by acoustic; impedance, intertance, load (radiation field), output (sound power), energy, wave, medium, signal, conduit, absorptivity, transducer.
The following examples do not have the requisite physical characteristics and therefore take acoustical; society, method, engineer, school, glossary, symbol, problem, measurement, point of view, device.
As illustrated, the generic term is usually modified by acoustical, whereas the specific technical term calls for acoustic.

acoustic delay line

A device used in a communications link or a computer memory in which the signal is delayed by the propagation of a sound wave. Also call sonic delay line .

acoustic description

The change of speed of sound with frequency.

acoustic excitation

The process of inducing vibration in a structure by exposure to sound waves.

acoustic generator

A transducer which converts electric, mechanical, or other forms of energy into sound.

acoustic Mach meter

A device which obtains data on sound propagation for the calculation of Mach number.

Some acoustics Mach meters measure transit time or velocity of a sound pulse; others measure an angle, as the angle of the Mach cone.

acoustic radiation pressure

A unidirectional, steady-state pressure exerted upon a surface exposed to a sound wave.

acoustic refraction

The process by which the direction of sound propagation is changed due to spatial variation in the speed of sound in the medium.

acoustics

1. The study of sound, including its production, transmission, and effects.

2. Those qualities of an enclosure that together determine its character with respect to distinct hearing.

acoustic streaming

Unidirectional flow currents in a fluid that are due to the presence of sound waves.

acoustic velocity (symbol ) =speed of sound

acoustic vibration

With respect to operational environments, vibrations transmitted through a gas. These vibrations may be subsonic, sonic, and ultrasonic.

acoustic wave = sound wave.

acquisition

1. The process of locating the orbit of a satellite or trajectory of a space probe so that tracking or telemetry data can be gathered.

2. The process of pointing an antenna or telescope so that it is properly oriented to allow gathering of tracking or telemetry data from a satellite or space probe.

acquisition and tracking radar

A radar set that locks onto a strong signal and track the object reflecting the signal.

actinic

Pertaining to electromagnetic radiation capable of initiating photochemical reactions, as in photography or the fading of pigments.

Because of the particularly strong action of ultra violet radiation on photochemical processes, the term has come to be almost synonymous with ultraviolet, as in actinic rays.

actinic balance =bolometer.

actinogram

The record of a recording actinometer.

actinograph

A recording actinometer.

actinometer

The general name for any instrument used to measure the intensity of radiant energy, particularly that of the sun. See actinometry. See also bolometer, dosimeter, photometer, radiometer.

Actinometers may be classified, according to the quantities which they measure, in the following manner: (a) pyrheliometer, which measures the intensity of direct solar radiation; (b) pyranometer, which measure global radiation (the combined intensity of direct solar radiation and diffuse sky radiation); and (c) pyrgeometer, which measures the effective terrestrial radiation.

actinometry

The science of measurement of radiant energy, particularly that of the sun, in its thermal, chemical, and luminous aspects. Compare photometry. See actinometer.

active

1. Transmitting a signal, as active satellite . Antonym of passive.

2. = radioactive, as active sample .

3. = fissionable, as active material .

4. Receiving energy from some source other than a signal, as active element .

active element

In a computer, a circuit or device which receives energy from some source other than the signal input.

active homing

The homing of an aerodynamic or space vehicle in which energy waves (as radar) are transmitted from the vehicle to the target and reflected back to the vehicle to direct the vehicle toward the target. Compare passive homing.

active homing guidance

See homing guidance.

active leg

An electrical element within a transducer which changes its electrical characteristics as a function of the application of a stimulus.

active satellite

A satellite which transmits a signal, in contrast to passive satellite .

active tracking system

A system which requires addition of a transponder, or transmitter on board the vehicle to repeat, transmit, or retransmit information to the tracking equipment, e.g. Dovap, Secor, Azusa, Miran, Minitrack.

active transducer

A transducer whose output is dependent upon sources of power, apart from that supplied by any of the actuating signals, which power is controlled by one or more of these signals.

actuating system

A mechanical system that supplies and transmits energy for the operation of other mechanisms or systems.

adaptation

The adjustment, alteration, or modification of an organism to fit it more perfectly for existence in its environment. Compare acclimatization, accustomization.

Adaptation is applied particularly to evolutionary change.

adaptation brightness= adaptation luminance.

adaptation illuminance = adaptation luminance.

adaptation level = adaptation luminance.

adaptation luminance

The average luminance (or brightness) of those objects and surfaces in the immediate vicinity of an observer. Also called adaptation brightness, adaptation level, adaptation illuminance.

The adaptation luminance has a marked influence on an observer's estimate of the visual range because, along with the visual angle of the object under observation, it determines the observer's threshold contrast. High adaptation luminance tends to produce a high threshold contrast, thus reducing the estimated visual range. This effect of the adaptation luminance is to be distinguished from the influence of background luminance.

adapter

1. Any device or contrivance used or designed primarily to fit or adjust one thing to another, as: (a) a buckle or clip on a parachute harness, used in adjusting the harness to the wearer; (b) a joint attaching an afterburner to a turbine casing on a jet engine; (c) a fitting for connecting pipes, valves, etc., that have different types of threads.

2. Any device, appliance or the like used to alter something so as to make it suitable for a use for which it was not originally designed.

adapter skirt

A flange or extension of a space vehicle stage or section that provides a ready means for fitting some object to the stage or section.

adaptive control system

A control system which continuously monitors the dynamic response of the controlled system and automatically adjusts critical system parameters to satisfy preassigned response criteria, thus producing the same response over a wide range of environmental conditions.

ADC (abbr) = analog to digital converter.

Adcock antenna

A pair of vertical antennas separated by a distance of one-half wavelength or less, and connected in phase opposition to produce a radiation pattern having the shape of a figure eight.

adder

In a computer, a device which can form the sum of two or more numbers or quantities.

additional apparent mass = apparent additional mass.

additive

Any material or substance added to something else. Specifically, a substance added to a propellant to achieve some purpose, such as a more even rate of combustion, or a substance added to fuels or lubricants to improve them or give them some desired quality, such as tetraethyl lead added to a fuel as an antidetonation agent, or graphite, talc, or other substances added to certain oils and greases to improve lubrication qualities.

address

1. Of a computer, a location where information is stored.

2. An expression, usually numerical, identifying an address (sense 1).

ADF (abbr) = automatic direction finder.

adiabat

A line on a thermodynamic diagram representing a constant potential temperature. See adiabatic process.

adiabatic

Without gain or loss of heat.

adiabatic atmosphere

A model atmosphere in which the pressure decreases with height according to: $p\; =\; p$₀[1 - (-gz/c_p,dT₀)] C_p,dR_d where $p$₀ and $T$₀ are the pressure and temperature (° K) at sea level or other datum; z is the geometric height; $R$_d is the gas constant for dry gas; $c$_p,d is the specific heat for dry gas at constant pressure; and g is the acceleration of gravity. Also called dry-adiabatic atmosphere, convective atmosphere, homogeneous atmosphere . See homogeneous atmosphere, barotropy.

adiabatic compression

See adiabatic process.

adiabatic efficiency

The efficiency with which work is done with respect to heat gains or losses. See adiabatic process.

adiabatic equivalent temperature

See equivalent temperature, sense 2.

adiabatic process

A thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, compression always results in warming, expansion in cooling. See diabatic process.

adiabatic recovery temperature

1. The temperature reached by a moving fluid when brought to rest through an adiabatic process. Also called recovery temperature, stagnation temperature .

2. = adiabatic wall temperature.

3. The final and initial temperature in an adiabatic, Carnot cycle.

adiabatic wall temperature

The temperature assumed by a wall in a moving fluid stream when there is no heat transfer between the wall and the stream.

A-display

In radar, a display in which targets appear as vertical deflections from a line representing a time base. Also called A-scan or A-scope .
Target distance is indicated by the horizontal position of the deflection from one end of the time base. The amplitude of the vertical deflection is a function of the signal intensity.

ADP (abbr) = automatic data processing.

adsorbate

In the process of adsorption, the adsorbed substance.

adsorbent

A material which takes up gas by adsorption.

adsorption

The adhesion of a thin film of liquid or gas to the surface of a solid substance. The solid does not combine chemically with the adsorbed substance. See sorption, absorption, chemisorption.

advection

The process of transport of an atmospheric property solely by the mass motion of the atmosphere; also, the rate of change of the value of the advected property at a given point.
Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere whereas convection describes the predominantly vertical, locally induced motions.

advective

Pertaining to advection .

aeon

$109$ years. This term was suggested by Harold Urey in 1957.

A/E ratio= absorptivity-emissivity ratio.

aerial

1. = antenna.

2. Of or pertaining to the air, atmosphere, or aviation.

aeroastromedicine= aerospace medicine.

aeroballistics

The study of the interaction of projectiles or high speed vehicles with the atmosphere. See ballistics.
The problem of the effect of reentry on the trajectory of a vehicle is a problem in aeroballistics.

aerobiology

The study of the distribution of living organisms freely suspended in the atmosphere.

aerodontalgia

A toothache brought on by a change in ambient pressure.

aeroduct

A ramjet type of engine designed to scoop up ions and electrons freely available in the outer reaches of the atmosphere or in the atmospheres of other spatial bodies, and by a metachemical process within the duct of this engine, expel particles derived from the ions and electrons as a propulsive jetstream.

aerodynamic

Of or pertaining to aerodynamics.

aerodynamic coefficient

Any nondimensional coefficient relating to aerodynamic forces or moments, such as a coefficient of drag, a coefficient of lift, etc.

aerodynamic force

The force exerted by a moving gaseous fluid upon a body completely immersed in it.
The aerodynamic force is proportional to the expression
$$p u ² L ² R ⁿ
where p is the fluid density; u is the velocity of the undisturbed stream relative to the body; L is a characteristic linear dimension of the body; and Rⁿ is the Reynolds number raised to the power of n, a constant usually determined experimentally. This form for the aerodynamic force is sometimes called Rayleigh formula. The component of the aerodynamic force parallel to the direction of flow is called the drag.

aerodynamic heating

The heating of a body produced by passage of air or other gases over the body; caused by friction and by compression processes and significant chiefly at high speeds. See radiative heating.

aerodynamics

1. The science that deals with the motion of air and other gaseous fluids, and of the forces acting on bodies when the bodies move through such fluids, or when such fluids move against or around the bodies, as, his research in aerodynamics .

2. (a) The actions and forces resulting from the movement or flow of gaseous fluids against or around bodies, as, the aerodynamics of a wing in supersonic flight . (b) The properties of a body or bodies with respect to these actions or forces, as, the aerodynamics of a turret or of a configuration .

3. The application of the principles of gaseous fluid flows and of their actions against and around bodies to the design and construction of bodies intended to move through such fluids, as a design used in aerodynamics.

aerodynamic trail

A condensation trail formed by adiabatic cooling to saturation (or slightly supersaturation) of air passing over the surfaces of high-speed aircraft.
Aerodynamic trails form off the tips of wings and propellers and other points of maximum pressure decrease. They are relatively rare and of short duration compared to exhaust trails.

aerodynamic vehicle

A device, such as an airplane, glider, etc., capable of flight only within a sensible atmosphere and relying on aerodynamic forces to maintain flight.
The term is used when the context calls for discrimination from space vehicle.

aeroelasticity

Any phenomenon which includes the mutual interaction between aerodynamic loads and structural deformation.

aeroembolism

1. The formation or liberation of gases in the blood vessels of the body, as brought on by a too-rapid change from a high, or relatively high, atmospheric pressure to a lower one.

2. The disease or condition caused by the formation of gas bubbles (mostly nitrogen) in the body fluids. The disease is characterized principally by neuralgic pains, cramps, and swelling, and sometimes results in death. Also call decompression sickness .

aeroemphysema

A swelling condition caused by the formation of gas in the tissues of the body.

aerolite

A meteorite composed principally of stony material.

aerology

1. As officially used in the U.S. Navy until early 1957, same as meteorology; this usage was more administrative than scientific.

2. As a subdivision of meteorology, the study of the free atmosphere through its vertical extend, as distinguished from studies confined to the layer of the atmosphere adjacent to the earth's surface.

aeronomy

The study of the upper regions of the atmosphere where ionization, dissociation, and chemical reactions take place.

aero-otitis media

An inflammatory reaction of the middle ear resulting from a difference in pressure between the gas in the middle ear and the surrounding atmosphere. Also called otitic barotrauma . c

aeropause

A region of indeterminate limits in the upper atmosphere, considered as a boundary or transition region between the denser portion of the atmosphere and space.
From a functional point of view, it is considered to be that region in which the atmosphere is so tenuous as to have a negligible, or almost negligible, effect on men and aircraft, and in which the physiological requirements of man become increasingly important in the design of aircraft and auxiliary equipment.

aerophare =radio beacon .

aeropulse engine = pulsejet engine.

aerosinusitis

An inflammatory reaction of one or more of the accessory nasal sinuses resulting from a difference in pressure between the gas in the sinus and the surrounding atmosphere. Also called sinus barotrauma .

aerosonator = resojet engine.

aerospace

(From aeronautics and space).

1. Of or pertaining to both the earth's atmosphere and space, as in aerospace industries.

2. Earth's envelope of air and space above it; the two considered as a single realm for activity in the flight of air vehicles and in the launching, guidance, and control of ballistic missiles, earth satellites, dirigible space vehicles, and the like.
Aerospace in sense 2 is used primarily by the U.S. Air Force. The term aerospace first appeared in print in the Interim Glossary; Aero-Space Terms (edited by Woodford Agee Heflin) published in February 1958 at the Air University, Maxwell Air Force Base, Alabama.

aerospace medicine

That branch of medicine dealing with the effects of flight through the atmosphere or in space upon the human body and with the prevention or cure of physiological or psychological malfunctions arising from these effects.

aerospace vehicle

A vehicle capable of flight within and outside the sensible atmosphere.

aerothermodynamic border

An altitude at about 100 miles, above which the atmosphere is so rarefied that the skin of an object moving through it at high speeds generates no significant heat.

aerothermodynamic duct

The full term for athodyd .

aerothermodynamics

The study of aerodynamic phenomena at sufficiently high gas velocities that thermodynamic properties of the gas are important.

aerothermoelasticity

The study of the response of elastic structures to the combined effect of aerodynamic heating and loading.

AFC (abbr) = automatic frequency control.

afterbody

1. A companion body that trails a satellite.

2. A section or piece of a rocket or spacecraft that enters the atmosphere unprotected behind the nose cone or other body that is protected for entry.

3. The afterpart of a vehicle.

afterburner

A device for augmenting the thrust of a jet engine by burning additional fuel in the uncombined oxygen in the gases from the turbine.

afterburning

1. Irregular burning of fuel left in the firing chamber or a rocket after fuel cutoff.

2. The function of an afterburner, a device for augmenting the thrust of a jet engine by burning additional fuel in the uncombined oxygen in the gases from the turbine.

aftercooling

1. The cooling of a gas after compression.

2. The necessary cooling of a reactor core after its shutdown by pumping a liquid or gas through it to carry off the excess heat generated by continuing radioactive decay of fission products within the core.

afterglow

1. A broad, high arch of radiance or glow seen occasionally in the western sky above the highest clouds in deepening twilight, caused by the scattering effect of very fine particles of dust suspended in the upper atmosphere.

2. The transient decay of a plasma after the power has been turned off.
The decay time involved is a direct consequence of the charged particle loss mechanisms, such as diffusion and recombination. The magnitude of these quantities is determined by measuring the decay time under controlled conditions.

afterheat

The heat generated in a reactor core after shutdown by continuing radioactive decay of fission products.

AGE (abbr) = aerospace ground equipment. See GSE.

age of the moon

The elapsed time, usually expressed in days, since the last new moon. See phase of the moon.

aging

In a metal or alloy, a change in properties that generally occurs slowly at room temperature and more rapidly at higher temperatures.

Agiwarn

Code name for the Western Hemisphere Regional Center for the IGY World Warning Agency.

agonic line

A line joining points at which the magnetic variation is zero. The agonic line is a particular case of an isogonic line.

agravic

Or pertaining to a condition of no gravitation. See weightlessness.

agravic illusion

An apparent movement of a target in the visual field due to otolith response in zerogravity. Also called oculoagravic illusion .

air

1. The mixture of gases comprising the earth's atmosphere.
The percent by volume of those gases found in relatively constant amount in dry air near sea level in very nearly as follows:

ELEMENT	%
nitrogen (N2)	78.084
oxygen (O2)	20.9476
argon (A)	0.934
carbon dioxide (CO2)	0.0314 (variable)
neon (Ne)	0.001818
helium (He)	0.000524
methane (CH4)	0.0002 (variable)
krypton (Kr)	0.000114
hydrogen (H2)	0.00005
nitruous oxide (N2O)	0.00005
xenon (Xe)	0.0000087

_{This table is from the 1965 edition of the Aerospace Dictionary.}

In addition to the above constituents there are many variable constituents. Chief of these is water vapor, which may vary from zero to volume percentages close to 4 percent. Ozone, sulfur dioxide, ammonia, carbon monoxide, iodine, and other trace gases occur in small and varying amounts.
The above composition of dry air is true to about 90 kilometers. See upper atmosphere.

2. The realm or medium in which aircraft operate.

air breakup

The breakup of a test reentry body after reentry into the atmosphere.
Air breakup is sometimes provided for, as by the firing of a cartridge inside the reentry body, so as to retard the fall of certain pieces and increase the chances of their recovery. See blowoff.

airbreather

An aerodynamic vehicle propelled by fuel oxidized by intake from the atmosphere; an air breathing vehicle.

airbreathing

Of an engine or aerodynamic vehicle, required to take in air for the purpose of combustion.

aircraft

Any structure, machine, or contrivance, especially a vehicle, designed to be supported by the air, being borne up either by the dynamic action of the air upon the surfaces of the structure or object, or by its own buoyancy; such structures, machines, or vehicles collectively, as, fifty aircraft.

Aircraft, in its broadest meaning, includes fixed-wing airplanes, helicopters, gliders, airships, free and captive balloons, ornithopters, flying model aircraft, kites, etc., but since the term carries a strong vehicular suggestion, it is more often applied, or recognized to apply, only to such of these craft as are designed to support or convey a burden in or through the air.

aircraft rocket

A rocket missile designed to be carried by, and launched from, an aircraft.

airflow

A flow or stream of air. An airflow may take place in a wind tunnel, in the induction system of an engine, etc., or a relative airflow can occur, as past the wing or other parts of a moving craft; a rate of flow, measured by mass or volume per unit of time. See flow.

airfoil

A structure, piece, or body, originally likened to a foil or leaf in being wide and thin, designed to obtain a useful reaction on itself in its motion through the air.

airframe

The assembled structural and aerodynamic components of an aircraft or rocket vehicle that support the different systems and subsystems integral to the video.

The word airframe, a carryover from aviation usage, remains appropriate for rocket vehicles since a major function of the airframe is performed during flight within the atmosphere. There is disagreement as to whether the nose cone and combustion chambers are included in the term airframe while they are attached to the vehicle.

airglow

The quasi-steady radiant emission from the upper atmosphere as distinguished from the sporadic emission of the auroras.

Airglow is a chemiluminescence due primarily to the emission of the molecules O2 and N2, the radical OH, and the atoms O and Na. Emissions observed in airglow could arise from three-body atom collisions forming molecules, from two-body reactions between atoms and molecules, or from recombination of ions.
Historically, airglow has referred to visual radiation. Some recent studies use airglow to refer to radiation outside the visual range.

air launch

To launch from an aircraft in the air, as to air launch a guided missile .

air light

Light from sun and sky which is scattered into the eyes of an observer by atmospheric suspensoids (and, to slight extent, by air molecules) lying in the observer's cone of vision. That is, air light reaches the eye in the same manner that diffuse sky radiation reaches the earth's surface.
Air light is not be confused with airglow .

air lock

1. A stoppage or diminution of flow in a fuel system, hydraulic system, or the like, caused by a pocket of air or vapor. 2. A chamber capable of being hermetically sealed that provides for passage between two places of different pressure, as between an altitude chamber and the outside atmosphere.

air position indicator (abbr API)

An airborne computing system which presents a continuous indication of the aircraft position on the basis of aircraft heading, airspeed, and elapsed time.

airscoop

A hood or open end of an air duct or a similar structure, projecting into the airstream about a vehicle in such a way as to utilize the motion of the vehicle in capturing air to be conducted to an engine, a ventilator, etc.

air shower

A grouping of cosmic-ray particles observed in the atmosphere; a cascade shower in the atmosphere. Also called shower .
Primary cosmic rays slowed down in the atmosphere emit bremsstrahlung photons of high energy. Each of these photons produces secondary electrons which generate more photons and the process continues until the available energy is absorbed.

airsickness

Motion sickness occurring in flight.

air sounding

The act of measuring atmospheric phenomena or determining atmospheric conditions at altitude, especially by means of apparatus carried by balloons or rockets. See sounding.

airspace

Specifically, the atmosphere above a particular portion of the earth, usually defined by the boundaries of an area on the surface projected upward.
Airspace is sometimes particularized by altitude, as the airspace above 20,000 feet.

air-space

Of or pertaining to both the atmosphere and space.
Because this adjective is pronounced as the noun airspace is, it is subject to misunderstanding. Aerospace is commonly used instead.

air start

An act or instance of starting an aircraft's engine while in flight, especially a jet engine after flameout. Compare in-flight start, ground start.

airstream = airflow

air vane

A vane that acts in the air, as contrasted to a jet vane which acts within a jetstream. See control vane.

air vehicle = aircraft

Aitken dust counter

An instrument developed by John Aitken for determining the dust content of the atmosphere. In operation, a sample of air is mixed, in an expandable chamber, with a larger volume of dust-free air containing water vapor. Upon a sudden expansion, the chamber cools adiabatically below its dewpoint, and the droplets form with the dust particles as nuclei (Aitken nuclei). A portion of these droplets settle on a ruled plate in the instrument and are counted with the aid of a microscope. Also called Aitken nucleus counter .

Aitken nuclei

The microscopic particles in the atmosphere which serve as condensation nuclei for droplet growth during the rapid adiabatic expansion produced by an Aitken dust counter. These nuclei are both solid and liquid particles whose diameters are of the order of tenths of microns or even smaller.

The Aitken nuclei play an important role in atmospheric electrical processes, for they are the particles which capture (by adsorption or other surface electrical processes) small ions and thereby form large ions. In air containing large numbers of Aitken nuclei, the small ion population is small, the large ion population is large, and the air conductivity is low.

Aitken nucleus counter =Aitken dust counter.

albedo

The ratio of the amount of electromagnetic radiation reflected by a body to the amount incident upon it, often expressed as a percentage, as, the albedo of the earth is 34% . Compare Bond albedo.

The concept defined above is identical with reflectance. However, albedo is more commonly used in astronomy and meteorology and reflectance in physics.
Albedo is sometimes used to mean the flux of the reflected radiation as, the earth albedo is 0.64 calorie per square centimeter. This usage should be discouraged.
The albedo is to be distinguished from the spectral reflectance which refers to one specific wavelength (monochromatic radiation).
Usage varies somewhat with regard to the exact wavelength interval implied in albedo figures; sometimes just the visible portion of the spectrum is considered, sometimes the totality of wavelengths in the solar spectrum.

albedometer

An instrument used for the measurement of the reflecting power, the albedo, of a surface. A pyranometer adapted for the measurement of radiation reflected from the earth's surface is sometimes employed as an albedometer.

Alford loop

A multielement antenna, having approximately equal and in-phase currents uniformly distributed along each of its peripheral elements, producing a substantially circular radiation pattern in the plane of polarization.

Alfvén Mach number

The ratio of the local flow velocity to the local Alfvén speed. See Alfvén wave.

Alfvén speed

The speed at which Alfvén waves are propagated along the magnetic field.

For a perfectly conducting fluid with a mass density of 1 kilogram per cubic meter in a magnetic field of 10,000 gauss, the Alfvén speed is about 1,000 meters per second while the speed of sound in air is about 300 meters per second.

Alfvén wave

A transverse wave in a magnetohydrodynamic field in which the driving force is the tension introduced by the magnetic field along the lines of force. Also called magnetohydrodynamic wave .

The dynamics of such waves are analogous to those in a vibrating string, the phase speed C being given by

where u is the permeability; H is the magnitude of the magnetic field; and p is the fluid density. Dissipative effects due to fluid viscosity and electrical resistance may be also present.

alga (plural, algae)

Any plants of a group of unicellular and multicellular primitive organisms that include the Chlorella, Scenedesmus, and other genera.

The green algae and blue-green algae, for example, provide a possible means of photosynthesis in a closed ecological system, also a source of food.

algorism

The art or system of calculating with any species of notation, as in arithmetic with nine figures and a zero. Also called algorithm.

Different algorisms have been used in the design of computing machines.

algorithm

1. A special mathematical procedure for solving a particular type of problem.

2. = algorism.

alidade

That part of an optical measuring instrument comprising the optical system, indicator, vernier, etc.
In modern practice the term is used principally to refer to a telescope mounted over a compass or compass repeater to facilitate observation of bearings, and to a surveying instrument consisting of a telescope mounted over a compass rose, for measuring directions.

alkali metal

A metal in group IA of the periodic system; namely, lithium, sodium, potassium, rubidium, cesium, and francium. Alkali metals have been considered as coolants (in liquid state) for nuclear reactors for spacecraft. See liquid-metal corrosion.

all burnt

The time at which a rocket consumes its propellants. See burnout, note.

all-inertial guidance

The guidance of a rocket vehicle entirely by use of inertial devices; the equipment used for this.

alloy

A substance having metallic properties and being composed of two or more chemical elements of which at least one is an elemental metal.

alloying element

An element added to a metal to effect changes in properties and which remains within the metal.

almucantar = parallel of altitude.

alpha decay

The radioactive transformation of a nuclide by alpha-particle emission. Also called alpha disintegration.

The decay product is the nuclide having a mass number four units smaller and an atomic number two units smaller than the original nuclide.

alpha disintegration = alpha decay

alphanumeric (alphabet plus numeric)

Including letters and digits.

alpha particle

A positively charged particle emitted from the nuclei of certain atoms during radioactive disintegration. The alpha particle has an atomic weight of 4 and a positive charge equal in magnitude to 2 electronic charges; hence it is essentially a helium nucleus (helium atom stripped of its two planetary electrons). Compare beta particle, gamma ray.

alpha ray

A stream of alpha particles.

altimeter

An instrument for measuring height above a reference datum; specifically, an instrument similar to an aneroid barometer that utilizes the change of atmospheric pressure with altitude to indicate the approximate elevation above a given point or plane used as a reference. See absolute altimeter, pressure altimeter, radio altimeter.

altitude (symbol h)

1. In astronomy, angular displacement above the horizon; the arc of a vertical circle between the horizon and a point on the celestial sphere, measured upward from the horizon.
Angular displacement below the horizon is called negative altitude or dip. See horizon system.

2. Height, especially radial distance as measured above a given datum, as average sea level. See absolute altitude, true altitude.

In space navigation altitude designates distance from the mean surface of the reference body as contrasted to distance, which designates distance from the center of the reference body.

altitude acclimatization

A physiological adaptation to reduced atmospheric and oxygen pressure.

altitude chamber

A chamber within which the air pressure, temperature, etc., can be adjusted to simulate conditions at different altitudes; used for experimentation and testing.

altitude circle = parallel of altitude

altitude difference

In navigation, the difference between computed and observed altitudes, or between precomputed and sextant altitudes. It is labeled T (toward) or A (away) as the observed (or sextant) altitude is greater or smaller than the computed (or precomputed) altitude. Also called altitude intercept, intercept .

altitude intercept = altitude difference

Often shortened to intercept.

altitude sickness

In general, any sickness brought on by exposure to reduced oxygen tension and barometric pressure.
Many writers have proposed specific definitions for this term but the definitions are highly variable.

altitude wind tunnel

A wind tunnel in which the air pressure, temperature, and humidity can be varied to simulate conditions at different altitudes.
In an altitude wind tunnel for testing engines, provision is made for exchanging fresh air for exhaust-laden air during operation.

alveolar air

The respiratory air in the alveoli (air sacs) deep within the lungs.

alveolar oxygen pressure

The oxygen pressure in the alveoli. The value is about 105 millimeters of mercury.

alveoli

The terminal air sacs deep within the lungs. The inhaled oxygen diffuses across the thin alveolar membranes (the walls of the air sacs) into the blood stream, and at the same time carbon dioxide diffuses from the blood into the alveoli and is exhaled through the lungs.

AM (abbr) = amplitude modulation.

ambient (symbol , used as a subscript)

Surrounding; especially, of or pertaining to the environment about a flying aircraft or other body but undisturbed or unaffected by it, as in ambient air, or ambient temperature.

ambient noise

The pervasive noise associated with a given environment, being usually a composite of sounds from sources both near and distant.

ambiguity

In navigation, the condition arising when a given set of observations defines more than one point, direction, line of position, or surface of position.

American Ephemeris and Nautical Almanac

An annual publication of the U.S. Naval Observatory, containing elaborate tables of the predicted positions of various celestial bodies and other data of use to astronomers and navigators.

Beginning with the editions for 1960, The American Ephemeris and Nautical Almanac issued by the Nautical Almanac Office, United States Naval Observatory, and the Astronomical Ephemeris issued by H.M. Nautical Almanac Office, Royal Greenwich Observatory, were unified. With the exception of a few introductory pages, the two publications are identical; they are printed separately in the two countries, from reproducible material prepared partly in the United States of America and partly in the United Kingdom.

American Nautical Almanac

See Nautical Almanac.

ampere (abbr A)

The unit of electric current; the constant current which, if maintained in two straight, parallel conductors of infinite length, of negligible circular cross sections, and placed 1 meter apart in a vacuum will produce between these conductors a force equal to 2*10-7 newtons per meter of length.

amplidyne

A special type of direct current generator used as a power amplifier in which the output voltage responds to changes in field excitation; used extensively in servo systems.

amplifier

A device which enables an input signal to control a source of power, and thus is capable of delivering at its output an enlarged reproduction of the essential characteristics of the signal.
Typical amplifying elements are electron tubes, transistors, and magnetic circuits.

amplitude

1. The maximum value of the displacement of a wave or other periodic phenomenon from a reference position.

2. Angular distance north or south of the prime vertical; the arc of the horizon, or the angle at the zenith between the prime vertical and a vertical circle, measured north or south from the prime vertical to the vertical circle.

The term is customarily used only with reference to bodies whose centers are on the celestial horizon, and is prefixed E or W, as the body is rising or setting, respectively; and suffixed N or S to agree with the declination. The prefix indicates the origin, and the suffix indicates the direction of measurement. Amplitude is designated as true, magnetic, compass, or grid as the reference direction is true, magnetic, compass, or grid east or west, respectively.

amplitude-modulated indicator

One of two general classes of radar indicators, in which the sweep of the electron beam is deflected vertically or horizontally from a base line to indicate the existence of an echo from a target. The amount of deflection is usually a function of the echo signal strength. Also called deflection-modulated indicator . Compare intensity modulated indicator.

amplitude modulation

1. In general, modulation in which the amplitude of a wave is the characteristic subject to variation.

2. Specifically, in telemetry those systems of modulation in which each component frequency, f, of the transmitted intelligence produces a pair of sideband frequencies at carrier frequency plus f and carrier minus f.

In special cases: (a) the carrier may be suppressed, (b) either the lower or upper sets of sideband frequencies may be suppressed; (c) the lower set of sideband frequencies may be produced by one or more channels of information and the upper set of sideband frequencies may be produced by one or more other channels of information; (d) the carrier may be transmitted without intelligence carrying sideband frequencies.

AMR = Atlantic Missile Range (definition 4)

anacoustic zone

The region above an altitude of about 100 miles where the distance between the air molecules is greater than the wavelength of sound, and sound waves can no longer be propagated.

analog

In computers, pertaining to the use of physical variables such as voltage, distance, rotation, etc. To represent numerical variable as in analog computer, analog output . Compare digital.

analog computer

A computing machine working on the principle of measuring, as distinguished from counting, in which the input data is analogous to a measurement continuum, such as linear lengths, voltages, resistances, etc., which can be manipulated by the computer.
Analog computers range in complexity from a slide rule to electrical computers used for solving mathematical problems.0

analog output

Transducer output in which the amplitude is continuously proportional to a function of the stimulus. Distinguished from digital output .

analog to digital conversion

A process by which a sample of analog information is transformed into a digital code.

analog to digital converter

(abbr ADC)

A device which will convert an analog voltage sample to an equivalent digital code of some finite resolution. Also called digitizer, encoder .

analytical photography

Photography, either motion picture or still, accomplished to determine (by qualitative, quantitative, or any other means) whether a particular phenomenon does or does not occur. See technical photography.

Differs from metric photography in that measurements are not a prime requisite.

AND

In Boolean algebra, the operation of intersection.

And, Andr.

International Astronomical Union abbreviations for Andromeda . See constellation.

AND circuit = AND gate.

AND gate, and gate

A circuit or device used in computers whose output is energized only when every input is in its prescribed state. It performs the logical function of the AND, the Boolean operation of intersection. Also called intersector , AND circuit .

AND-NOT gate = exclusive OR circuit.

Andromeda (abbr And, Andr).

See constellation.

aneroid

A thin, disk-shaped box or capsule, usually metallic, partially evacuated of air and sealed, which expands and contracts with changes in atmospheric or gaseous pressure.

The aneroid is the sensing and actuating element in various meters or gages, such as barometers, altimeters, manifold-pressure gages, etc; it is also the triggering or operating element in various automatic mechanisms. A device similar to an aneroid, but open to outside pressures, such as the capsule in an airspeed indicator, is not commonly called an aneroid .

angel

A radar echo caused by a physical phenomenon not discernible to the eye.

Angels are usually coherent echoes and sometimes of great signal strength (up to 40 decibels above the noise level). They have been ascribed to insects flying through the radar beam, but have also been observed under atmospheric conditions which indicate there must be other causes. Studies indicate that a fair portion of them are caused by strong temperature or moisture gradients, or both, such as might be found near the boundaries of bubbles of especially warm or moist air (see blob). They frequently occur in shallow layers at or near temperature inversions within the lowest few thousand feet of the atmosphere.

angle

The inclination to each other of two intersecting lines, measured by the arc of a circle intercepted between the two lines forming the angle, the center of the circle being the point of intersection.

An acute angle is less than 90°; a right angle 90 °; an obtuse angle, more than 90° but less than 180 °; a straight angle, 180°; a reflex angle, more than 180° but less than 360°; a perigon, 360°. Any angle not a multiple of 90° is an oblique angle. If the sum of two angles is 90°, they are complementary angles; if 180°, supplementary angles; if 360°, explementary angles. Two adjacent angles have a common vertex and lie on opposite sides of a common side. A dihedral angle is the angle between two intersecting planes. A spherical angle is the angle between two intersecting great circles.

angle modulation

Modulation in which the angle of a sine-wave carrier is the characteristic varied from its normal value.
Phase and frequency modulation are particular forms of angle modulation.

angle of arrival

A measure of the direction of propagation of electromagnetic radiation upon arrival at a receiver (most commonly used in radio). It is the angle between the plane of the phase front and some plane of reference, usually the horizontal, at the receiving antenna. This angle is a function of the index of refraction gradient of the medium through which the energy is traveling, and the relative positions of the transmitter and receiver. Compare angle of incidence.

Angles of arrival can be measured for both the direct and reflected components of a wave using a multiple-antenna receiving system called an interferometer .

angle of attack

The angle between a reference line fixed with respect to an airframe and a line in the direction of movement of the body.

angle of climb

The angle between the flight path of a climbing vehicle and the local horizontal.

angle of depression

The angle in a vertical plan between the local horizontal and a descending line. Also called depression angle . See angle of elevation.

angle of descent

The angle between the flightpath of a descending vehicle and the local horizontal.

angle of deviation

The angle through which a ray is bent by refraction.

angle of elevation

The angle in a vertical plane between the local horizontal and an ascending line, as from an observer to an object. Also called elevation angle.
A negative angle of elevation is usually called an angle of depression.

angle of incidence

1. The angle at which a ray of energy impinges upon a surface, usually measured between the direction of propagation of the energy and a perpendicular to the surface at the point of impingement, or incidence. Compare angle of arrival. See also angle of reflection, angle of refraction.
In some cases involving radio waves, the angle of incidence is measured relative to the surface.

2. = angle of attack. (British usage).

angle of minimum deviation

See minimum deviation.

angle of pitch

1. The angle, as seen from the side, between the longitudinal body axis of an aircraft or similar body and a chosen reference line or plane, usually the horizontal plane. This angle is positive when the forward part of the longitudinal axis is directed above the reference line.

2. Same as blade angle (in all senses).

angle of reflection

The angle at which a reflected ray of energy leaves a reflecting surface, measured between the direction of the outgoing ray and a perpendicular to the surface at the point of reflection. Compare angle of incidence.
In some cases involving radio waves, the angle of reflection is measured relative to the surface.

angle of refraction

The angle at which a refracted ray of energy leaves the interface at which the refraction occurred, measured between the direction of the refracted ray and perpendicular to the interface at the point of refraction.

angle of roll

The angle that the lateral body axis of an aircraft or similar body makes with a chosen reference plane in rolling; usually the angle between the lateral axis and a horizontal plane. The angle of roll is considered positive if the roll is to starboard.

angle of yaw

The angle, as seen from above, between the longitudinal body axis of an aircraft, rocket, or the like and a chosen reference direction. This angle is positive when the forward part of the longitudinal axis is directed to starboard. Also called yaw angle .

angstrom(abbr A, Å)

A unit of length, used chiefly in expressing short wavelengths. It equals $10-10meter\; or\; 10-8$ centimeters.

Ångström compensation pyrheliometer

An instrument developed by K. Ångström for the measurement of direct solar radiation. The radiation receiver station consists of two identical manganin strips whose temperatures are measured by attached thermocouples. One of the strips is shaded, whereas the other is exposed to sunlight. An electrical heating current is passed through the shaded strip so as to raise its temperature to that of the exposed strip. The electric power required to accomplish this is a measure of the solar radiation. See actinometer, pyrheliometer. Compare Ångström pyrgeometer.

Ångström pyrgeometer

An instrument developed by K. Ångström for measuring the effective terrestrial radiation. It consists of four manganin strips, of which two are blackened and two are polished. The blackened strips are allowed to radiate to the atmosphere while the polished strips are shielded. The electrical power required to equalize the temperature of the four strips is taken as a measure of the outgoing radiation. See actinometer, pyrgeometer. Compare Ångström compensation pyrheliometer.

angular acceleration (symbol )

The rate of change of angular velocity.

angular distance

1. The angular difference between two directions, numerically equal to the angle between two lines extending in the given directions.

2. The arc of the great circle joining two points, expressed in angular units.

3. Distance between two points, expressed in wave lengths at a specified frequency.
It is equal to the number of waves between the points multiplied by if expressed in radians, or multiplied by 360° if measured in degrees.

angular frequency (symbol )

The frequency of a periodic quantity expressed in radians per second. It is equal to the frequency in cycles per second multiplied by . Also called circular frequency .

angular momentum

Quantity of rotational motion.

Linear momentum is the quantity obtained by multiplying the mass of a body by its linear speed. Angular momentum is the quantity obtained by multiplying the moment of inertia of a body by its angular speed. The momentum of a system of particles is given by the sum of the momentums of the individual particles which make up the system or by the product of the total mass of the system and the velocity of the center of gravity of the system. The momentum of a continuous medium is given by the integral of the velocity over the mass of the medium or by the product of the total mass of the medium and the velocity of the center of gravity of the medium.

See momentum

angular rate = angular speed, sense 1

angular resolution

Specifically, the ability of a radar to distinguish between two targets solely by the measurement of angles.
It is generally expressed in terms of the minimum angle by which targets must be spaced to be separately distinguishable. See resolution.

angular speed

1. Change of direction per unit time, as of a target on a radar screen. Also called angular rate .

2. = angular velocity.

angular velocity (symbol )

The change of angle per unit time; specifically, in celestial mechanics, the change in angle of the radius vector per unit time.

anisotropic

Exhibiting different properties when tested along axes in different directions.

annealing

Application of heat energy to a material cooling at a suitable rate to: relieve stresses, change certain properties, improve machinability, etc., or for realignment of atoms in a distorted crystal lattice as caused, for example, by radiation damage.

annual parallax.

See parallax.

annular

Pertaining to an annulus or ring; ring shaped.

annular eclipse

An eclipse in which a thin ring of the source of light appears around the obscuring body.

anode

The positive pole or electrode of any electron emitter, such as an electron tube or an electric cell.
The negative pole or electrode is called a cathode.

anomalistic month

The average period of revolution of the moon from perigee to perigee, a period of 27 days 13 hours 18 minutes 33.2 seconds.

anomalistic period

The interval between two successive perigee passages of a satellite in orbit about a primary. Also called perigee-to-perigee period .

anomalistic year

The period of one revolution of the earth about the sun from perihelion to perihelion; 365 days 6 hours 13 minutes 53.0 seconds in 1900 and increasing at the rate of 0.26 second per century.

anomalous dispersion

Dispersion of electromagnetic radiation characterized by a decrease in refractive index with increase in frequency.

anomalous propagation

The propagation of energy when it arrives at a destination via a path significantly different from the normally expected path.

The term is usually applied to the transmission of various forms of energy through the atmosphere when, in addition to the line-of-sight path, the energy is refracted by density discontinuities at one or more levels in atmosphere. Therefore, it propagates to a point that could not be reached via a line-of-sight path. In radio and radar studies, it refers to the abnormal refraction of a beam of radio energy, usually applied to superstandard propagation rather than to substandard propagation. In either case, anomalous propagation results from an unusual vertical distribution of temperature and moisture in the atmosphere.
The anomalous propagation of sound refers to the downward refraction of an oblique sound wave from an explosion, the refraction occurring in the region of increasing temperature with height in the lower mesosphere. The anomalous propagation of sound has been used as a method for determining upper air temperatures and winds.

anomaly

1. In general, a deviation from the norm.

2. In geodesy, a deviation of an observed value from a theoretical value, due to an abnormality in the observed quantity.

3. In celestial mechanics, the angle between the radius vector to an orbiting body from its primary (the focus of the orbital ellipse) and the line of apsides of the orbit, measured in the direction of travel, from the point of closest approach to the primary (perifocus).

The term defined above is usually called true anomaly v to distinguished it from the eccentric anomaly E which is measured at the center of the orbital ellipse to the projection of the body onto the auxiliary circle of the ellipse, or from the mean anomaly M which is what the true anomaly would become if the orbiting body had a uniform annular motion. The mean anomaly M can be computed by
M = n (t - T) where n is mean motion; t is time of the computation; and T is time of perifocus. The eccentric anomaly E and the mean anomaly M are related by the Kepler equation
M = E - e sin E where e is eccentricity of the ellipse. From E, the true anomaly v can be obtained by
$tan\; v/2=[(1+e)/(1-e)]1/2\; tan\; E/2.$

anoxaemia = hypoxaemia.

anoxia

A complete lack of oxygen available for physiological use within the body. Compare hypoxia.
Anoxia is popularly used as a synonym for hypoxia . This usage should be avoided.

Ant, Antl.

International Astronomical Union abbreviations for Antlia . See constellation.

antapex

See solar apex.

antenna

A conductor or system of conductors for radiating or receiving radio waves.

antenna array

A system of antennas coupled together to obtain directional effects, or to increase sensitivity.

antenna effect

A weakening of the effectiveness of the directional properties of a loop antenna by the capacitance of the loop to the ground. Also called height effect .
In usual direction-finding practice on ground waves, antenna effect would be manifested: (a) if in phase, by an angular displacement of the nulls from 180° displacement and (b), if in quandrature, by a residual signal obscuring the nulls. The in-phase effect is often used to eliminate the 180° ambiguity (i.e., to permit sense finding).

antenna field

A group of antennas placed in a geometric configuration.

antenna gain

See gain, sense 2(a).

antenna null

See null.

antenna pair

Two antennas located on a base line of accurately surveyed length.

antenna pattern = radiation pattern.

antenna temperature

In radio astronomy, a measure of the power absorbed by the antenna. In an ideal, loss-free radio telescope, the antenna temperature is equal to the brightness temperature if the intensity of the received radiation is constant within the main lobe. If the angular dimension of the source is small compared to the main lobe, the antenna temperature is equal to the brightness temperature multiplied by the ratio of the solid angle subtended by the source to the effective solid angle of the antenna.

anticyclonic

Having a sense of rotation about the local vertical opposite to the rotation of the earth; that is, clockwise in the northern hemisphere, counterclockwise in the southern hemisphere, undefined at the equator; the opposite of cyclonic.

antigravity

A hypothetical effect that would arise from cancellation by some energy field of the effect of the central force field of the earth or other body.
Antigravity is common in science fiction but has not yet been reported in scientific literature.

anti-g suit = g-suit.

anti-matter

Matter consisting of anti-particles.

antinode

1. Either of the two points on an orbit where a line in the orbit plane, perpendicular to the line of nodes, and passing through the focus, intersects the orbit.

2. A point, line, or surface in a standing wave where some characteristic of the wave field has maximum amplitude. Also called loop .

In sense 2, the appropriate modifier should be used before the word antinode to signify the type that is intended; e.g., displacement antinode, velocity antinode, pressure antinode.

anti-particle

Any particle with a charge of opposite sign to the same particle in normal matters.

Thus, the proton has a positive charge; the antiproton, a negative charge. When a particle and its anti-particle collide, both may disappear with the creation of lighter particles; this process is called annihilation .

antipode

Anything exactly opposite to something else. Particularly, that point on the earth 180° from a given place.

antiresonance

For a system in forced oscillation, the condition existing at a point when any change, however small, in the frequency of excitation causes an increase in the response at this point.

antisolar point

The point on the celestial sphere 180° from the sun.

Antlia (abbr Ant, Antl.)

See constellation.

apareon

The point on a Mars-centered orbit where a satellite is at its greatest distance from Mars. Apareon is analogous to apogee . See geo.

apastron

That point of the orbit of one member of a binary star system at which the stars are farthest apart. That point at which they are closes together is called periastron .

aperiodic

Without a period; not cyclic; completely damped.

aperture

1. An opening; particularly, that opening in the front of a camera through which light rays pass when a picture is taken.

2. The diameter of the objective of a telescope or other optical instrument, usually expressed in inches, but sometimes as the angle between lines from the principal focus to opposite ends of diameter of the objective.

3. Of a unidirectional antenna, that portion of a plane surface near the antenna, perpendicular to the direction of maximum radiation, through which the major part of the radiation passes. See effective area.

aperture ratio

The ratio of the useful diameter of a lens to its focal length. It is the reciprocal of the f-number.
In application to an optical instrument, rather than to a lens, numerical aperture is more commonly used. The aperture ratio is then twice the tangent of the angle whose sine is the numerical aperture.

apex of the sun's motion = solar apex

apex of the sun's way = solar apex

aphelion

That point in a solar orbit which is most distant from the sun. The point nearest the sun is called perihelion.

apoapsis

That point in an orbit farthest from the center of attraction.

apocenter = apofocus

apocynthion

That point in the orbit of a moon satellite which is farthest from the moon.

apofocus

The point on an elliptic orbit at the greatest distance from the principal focus.

apogee

1. That point in a geocentric orbit which is most distant from the earth. That orbital point nearest the earth is called perigee . See geo.
By extension, apogee and perigee are also used in reference to orbits about other planets and natural satellites.

2. Of a satellite or rocket: To reach its apogee (sense 1), as in the Vanguard apogees at 2,560 miles .

apostilb

A unit of luminance equal to international candles per square centimeter. Compare stilb.

apparent

In astronomy, observed.
True values are reduced from apparent (observed) values by eliminating those factors such as refraction, light time, etc., which affected the observation.

apparent additional mass

A fictitious mass of fluid added to the mass of the body to represent the force required to accelerate the body through the fluid.
The apparent additional mass has inertia and momentum equal to the apparent increase of the inertia and momentum of the body.

apparent force

A force introduced in a relative coordinate system in order that Newton laws of motion be satisfied in this system. This force must be equal and opposite to an acceleration in an inertial coordinate system, in which Newton laws are (by definition) satisfied. Examples are the coriolis force, and the centrifugal force incorporated in gravity.

apparent gravity = acceleration of gravity.

apparent horizon

See horizon.

apparent motion

Motion relative to a specified or implied reference point which may itself be in motion. Also called relative motion . See relative movement.
In astronomy apparent motion usually refers to movement of celestial bodies as observed from the earth.

apparent position

The position on the celestial sphere at which a heavenly body (or a space vehicle) would be seen from the center of the earth at a particular time. Compare astrometric position.
The apparent position of a body is displaced from the true position at the time of observation by the motion of the body during the time it takes light to travel from the body to the earth (see planetary aberration) and by aberration.
Most ephemerides tabulate apparent position of the sun, moon, and planets.

apparent solar day

The duration of one rotation of the earth on its axis, with respect to the apparent sun. It is measured by successive transits of the apparent sun over the lower branch of a meridian. The length of the apparent solar day is 24 hours of apparent time and averages the length of the mean solar day, but varies somewhat from day to day.

apparent solar time

See solar time.

apparent stresses = Reynolds stresses.

apparent sun

The actual sun as it appears in the sky. Also called true sun . See mean sun, dynamical mean sun.

apparent time

Time based upon the rotation of the earth relative to the apparent or true sun. This is the time shown by a sundial. See equation of time.
Apparent time may be designated as either local or Greenwich, as the local or Greenwich meridian is used as the reference.

apparent wander

Apparent change in the direction of the axis of rotation of a spinning body, as a gyro, due to rotation of the earth. Often shortened to wander . See precession.

The horizontal component of apparent wander is called drift, and the vertical component is called topple .

Appleton layer = F₂-layer

See ionosphere.

approximate absolute temperature scale(abbr AA)

A temperature scale with the ice point at 273° and boiling point of water at 373°. It is intended to approximate the Kelvin temperature scale with sufficient accuracy for many sciences, notably meteorology, and is widely used in the meteorological literature. Also called tercentesimal thermometric scale .

appulse

1. The near approach of one celestial body to another on the celestial sphere, as in occultation, conjunction, etc.

2. A penumbral eclipse of the moon.

apron

Specifically, a protective device specially designed to cover an area surrounding the fuel inlet on a rocket or spacecraft.

Aps, Apus

International Astronomical Union abbreviations for Apus . See constellation.

apse = apsis

apsides

Plural of apsis.

apsis (plural apsides)

In celestial mechanics, either of the two orbital points nearest or farthest from the center of attraction. Also called apse . The apsides are the perihelion and aphelion in the case of an orbit about the sun, and the perigee and apogee in the case of an orbit about the earth. The line connecting these two points is called line of apsides . The nearest point is the lower apsis while the farthest point if the higher apsis.

APU (abbr) = auxiliary power unit.

Aql, Aqil.

International Astronomical Union abbreviations for Aquila . See constellation.

Aquarius (abbr Aqr, Aqar).

See constellation.

aqueous vapor = water vapor.

Aquila (abbr Aqr, Aqar)

See constellation.

Ara (abbr Ara, Arae)

See constellation.

Arago point

One of the three commonly detectable points along the vertical circle through the sun at which the degree of polarization of diffuse sky radiation goes to zero; a neutral point.
The Arago point, so named for its discoverer, is customarily located at about 20° above the antisolar point; but it lies at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity.

arc

1. A part of a curved line, as a circle.

2. A luminous glow which appears when an electric current passes through ionized air or gas.

3. An auroral arc. See aurora. See arc discharge.

arc discharge

A luminous, gaseous, electrical discharge in which the charge transfer occurs continuously along a narrow channel of high ion density. An arc discharge requires a continuous source of electric potential difference across the terminals of the arc.
Arc discharge is to be distinguished from corona discharge, point discharge, and spark discharge.

arc spectrum

The spectrum of a neutral atom, designated by the Roman numeral I following the symbol for the element, and He I. See spark spectrum.

arcs with ray structure

See aurora.

arctic blackout = blackout.

ARDC model atmosphere

See standard atmosphere.

areal radiant intensity = steradiancy.

areal velocity

In celestial mechanics, the area swept out by the radius vector per unit time.
The areal velocity is constant for a central force. See Kepler laws.

area rule

A prescribed method of design for obtaining minimum zero-lift drag for a given aerodynamic configuration, such as a wing-body configuration, at a given speed.
For a transonic body, the area rule is applied by subtracting from, or adding to, its cross-sectional area distribution normal to the air stream at various stations so as to make its cross-sectional area distribution approach that of an ideal body of minimum drag; for a supersonic body, the sectional areas are frontal projections of areas intercepted by planes inclined at the Mach angle.

areo

Combining form of Mars (Ares), as in areography.
Words formed with aero are considered pedantic by some. See geo.

areographic

Referring to positions on Mars measured in latitude from Mars' equator and in longitude from a reference meridian.

areography

The study of the surface features of Mars; the geography of Mars.

Ares

Mars.
Ares is seldom used except in combining forms as areocentric, apareon.

Arg

International Astronomical Union abbreviation for Argo . See constellation.

Argo (abbr Arg).

See constellation.

argument

In astronomy, an angle or arc, as in argument of perigee .

argument of latitude

In celestial mechanics, the angular distance measured in the orbit plane from the ascending node to the orbiting object; the sum of the argument of perigee and the true anomaly.

argument of perigee

In celestial mechanics, the angle or arc, as seen from a focus of an elliptical orbit, from the ascending node to the closest approach of the orbiting body to the focus. The angle is measured in the orbital plane in the direction of motion of the orbiting body.

Ari, Arie

International Astronomical Union abbreviations for Aries. See constellation.

Ariel

A satellite of Uranus orbiting at a mean distance of 192,000 kilometers.

Aries (abbr Ari, Arie)

See constellation.

arithmetic element = arithmetic unit.

arithmetic mean

One of several accepted measures of central tendency, physically analogous to center of gravity. Pertaining to a set of numbers $x$₁, x₂,...x_n, the arithmetic mean, usually denoted by the symbol , is the sum x₁+x₂+...+x_n divided by n. Also called mean, average, simple average .
Since the word mean is also applied to other measures of central tendency, such as weighted means, geometric means, harmonic means, the adjective arithmetic is used for clarity. However, when used without further qualification, the term mean is understood as arithmetic mean.

arithmetic unit

That part of a computer which performs arithmetic operations. Also called arithmetic element .

array = antenna array.

arrhythmia

Absence of rhythm, as, for example, in heart beat.

arrow wing

An aircraft wing of V-shaped planform, either tapering or of constant chord, suggesting a stylized arrowhead.

artificial antenna

A device which has the equivalent impedance characteristics of an antenna and the necessary power-handling capabilities, but which does not radiate nor intercept radiofrequency energy. Also called dummy antenna .

artificial asteroid

A manmade object placed in orbit about the sun. See asteroid.

artificial earth satellite

A manmade earth satellite, as distinguished from the moon.

artificial feel

A control feel simulated by mechanisms incorporated in the control system of an aircraft or spacecraft where the forces acting on the control surfaces are not transmitted to the cockpit controls, as in the case of an irreversible control system or a power boosted system.

artificial gravity

A simulated gravity established within a space vehicle by rotation or acceleration.

artificial horizon

1. A gyro-operated flight instrument that shows the pitching and banking attitudes of an aircraft or spacecraft with respect to a reference line horizon, within limited degrees of movement, by means of the relative position of lines or marks on the face of the instrument representing the aircraft and the horizon. See attitude gyro.

2. A device, such as a spirit level, pendulum, etc., that establishes a horizontal reference in a navigation instrument.

artificial satellite

A manmade satellite.

A-scan = A-display.

ascendent

The negative of the gradient. The ascendent of a function is a vector with magnitude equal to the maximum spatial rate of change of that function at a given point at a given time. It is directed toward increasing values of the function along the line of maximum change, and is represented by , where F is the function and the del-operator.

ascending node

That point at which a planet, planetoid, or comet crosses to the north side of the ecliptic; that point at which a satellite crosses to the north side of the equatorial plane of its primary. Also called northbound node . The opposite is descending node or southbound node .

A-scope = A-display.

asdic

British term for sonar .

Askania (a trade name) = cine-theodolite.

aspect ratio

The ratio of the square of the span of an airfoil to the total airfoil area, or the ratio of its span to its mean chord.
An airfoil of high aspect ratio is of relatively long span and short chord; one of low aspect ratio is of relatively short span and long chord.

aspects

The apparent positions of celestrial bodies relative to one another; particularly, the apparent positions of the moon or a planet relative to the sun.

aspiration condenser

An ion counter collecting element consisting of a cylindrical condenser which when charged produces a radial field which collects ions from the aspirated air.

assemble

In computer terminology, to organize the subroutines into a complete program.

assisted take-off

A take-off of an aircraft using a supplementary source of power, usually rockets. See RATO.

associated corpuscular emission

The full complement of secondary charged particles (usually limited to electrons) associated with an X-ray or gamma ray beam in its passage through matter.
The full complement of electrons is obtained after the radiation has traversed sufficient matter to bring about equilibrium between the primary photons and secondary electrons. Electronic equilibrium with the secondary photons is intentionally excluded.

assumed latitude

See latitude.

assumed longitude

See longitude.

A-station

In loran, the designation applied to the transmitting station of a pair, the signal of which always occurs less than half a repetition period after the next preceding signal and more than half a repetition period before the next succeeding signal of the other station of the pair, designated a B-station.

asteroid

One of the many small celestial bodies revolving around the sun, most of the orbits being between those of Mars and Jupiter. Also called planetoid, minor planet . See planet.

The term minor planet is preferred by many astronomers but asteroid continues to be used in astronomical literature, especially attributively, as in asteroid belt.
All asteroids with determined orbits (except for a few discovered during World War II) are numbered for identification in the order of their discovery. The Ephemerides of the Minor Planets published by the U.S.S.R. Academy of Sciences lists all numbered asteroids, data concerning them, and their predicted positions. The daily positions of the first four minor planets are tabulated in the American Ephemeris and Nautical Almanac. Orbits have been determined for approximately 1700 asteroids. Asteroids have names as well as numbers, see Table I. The names are usually feminine but masculine names have been used for asteroids closer to or farther away from the Sun than the majority. The first asteroid to be given a masculine name, Eros (number 443) was the first to be discovered inside the orbit of Mars. The Trojan asteroids, named for heroes of the Trojan war, are in the orbit of Jupiter.

astral dome = astrodome.

astre fictif

A point on the celestial sphere used as a reference in measuring time intervals. See day.

astro

A prefix meaning star or stars and, by extension, sometimes used as the equivalent of celestial , as in astro nautics.

astroballistics

The study of the phenomena arising out of the motion of a solid through a gas at speeds high enough to cause ablation; for example, the interaction of a meteoroid with the atmosphere.
Astroballistics uses the data and methods of astronomy, aerodynamics, ballistics, and physical chemistry.

astrobiology

The study of living organisms on celestial bodies other than the earth.

astrocompass

An instrument used to determine direction by sighting heavenly bodies of known position.

astrodome

A transparent dome in the fuselage or body of an aircraft or spacecraft intended primarily to permit taking celestial observations in navigating. Also called a navigation dome, astral dome .

astrodynamics

The practical application of celestial mechanics, astroballistics, propulsion theory, and allied fields to the problem of planning and directing the trajectories of space vehicles.
Astrodynamics is sometimes used as a synonym for celestial mechanics. This usage should be discouraged.

astrogation

Contraction of astronavigation .

astrographic position = astrometric position.

astrolabe

1. In general, any instrument designed to measure the altitudes of celestial bodies.

2. Specifically, an instrument designed for very accurate celestial altitude measurements, as in survey work.

astrometric position

The position of a heavenly body (or space vehicle) on the celestial sphere corrected for aberration but not for planetary aberration. Compare apparent position.
Astrometric positions are used in photographic observations where the position of the observed body can be measured in reference to the positions of comparison stars in the field of the photograph.

astrometry

The branch of astronomy dealing with geometrical relations of the celestial bodies and their real and apparent motions. The techniques of astrometry, especially the determination of accurate position by photographic means, are used in tracking satellite and space probes.

astronaut

1. A person who rides in a space vehicle.

2. Specifically, one of the test pilots selected to participate in Project Mercury, Project Gemini, Project Apollo, or any other United States program for manned space flight.

astronautic centrifuge

See centrifuge.

astronautics

1. The art, skill, or activity of operating spacecraft.

2. In a broader sense the science of space flight.

astronavigation

The plotting and directing of the movement of a spacecraft from within the craft by means of observations on celestial bodies. Sometimes contracted to astrogation or called celestial navigation .

astron machine

An experimental thermonuclear device where a magnetic filed is generated by a relativistic ring of electrons and shaped into a magnetic mirror configuration. The hot electrons serve as a heat source to heat the ions.

astronomic = astronomical.
In any combination, such as astronomic coordinates, astronomic is equivalent to astronomical.

astronomical

Of or pertaining to astronomy or to observations of the celestial bodies. Also called astronomic .
Astronomers have long preferred astronomical. Geodesists usually use astronomic as an intended parallel to geodetic. The Coast and Geodetic Survey uses astronomic in their publications insofar as is compatible with established practice.

astronomical constants

1. The elements of the orbits of the bodies of the solar system, their masses relative to the sun, their size, shape, orientation, rotation, and inner constitution, and the velocity of light.

2. = system of astronomical constants.

The astronomical constants used in the calculations of The American Ephemeris and Nautical Almanac, as well as other national ephemerides, were adopted at various times between 1896 and 1930. Although the system was known to contain many inconsistencies, the International Astronomical Union recommended their continued use in 1952. Space-related research has provided data for the computation of a more accurate system, and in January 1964 The Working Group on the System of Astronomical Constants recommended a new system of constants to be introduced into the national and international ephemerides at the earliest practicable date. Both the conventional and revised systems are given in Table II. The constants in Table III were recommended for use in trajectory calculations for NASA programs by the Ad Hoc NASA Standards Constants Committee May 16, 1963.

astronomical coordinates

Coordinates defining a point on the surface of the earth, or of the geoid, in which the local direction of gravity is used as a reference. Sometimes called geographic coordinates. See astronomical equator, astronomical latitude, astronomical longitude.

astronomical day

A mean solar day beginning at mean noon, 12 hours later than the beginning of the civil day of the same date. Astronomers now generally use the civil day. See Julian day, astronomical time.

astronomical equator

A line on the surface of the earth connecting points having 0° astronomical latitude. Sometimes called terrestrial equator.
When the astronomical equator is corrected for station error, it becomes the geodetic equator.

astronomical latitude

Angular distance between the direction of gravity and the plane of the celestial equator. Sometimes called geographic latitude .

Astronomical latitude corrected for the meridional component of station error becomes geodetic latitude .

astronomical longitude

The angle between the plane of the reference meridian and the plane of the celestial meridian. Sometimes called geographic longitude .

Astronomical longitude corrected for the prime-vertical component of station error divided by the cosine of the latitude becomes geodetic longitude .

astronomical meridian

A line connecting points having the same astronomical longitude. Also called terrestrial meridian .
Because the deflection of the vertical varies from point to point, the astronomical meridian is an irregular line. When the astronomical meridian is corrected for station error, it becomes the geodetic meridian.

astronomical parallel

A line connecting points having the same astronomical latitude.
Because the deflection of the vertical caries from point to point, the astronomical parallel is an irregular line. When the astronomical parallel is corrected for station error, it becomes the geodetic parallel.

astronomical position

1. A point on the earth whose coordinates have been determined as a result of observation of celestial bodies.
The expression is usually used in connection with position on land determined with great accuracy for survey purposes.

2. A point on the earth, defined in terms of astronomical latitude and longitude.

astronomical refraction

1. The angular difference between the apparent zenith distance of a celestial body and its true zenith distance, produced by refraction effects as the light from the body penetrates the atmosphere. Also called atmospheric refraction, astronomical refraction error. See Bemporad formula.

For bodies near zenith the astronomical refraction is only about 0.1 minute, but for bodies near the horizon it becomes about 30 minutes or more and contributes measurably to the length of the apparent day.

2. Any refraction phenomenon observed in the light originating from a source outside of the earth's atmosphere; as contrasted with terrestrial refraction. This is applied only to refraction caused by inhomogeneities of the atmosphere itself, and not to that caused by ice crystals suspended in the atmosphere.

astronomical refraction error = astronomical refraction, sense 1.

astronomical scintillation

Any scintillation phenomena, such as irregular oscillatory motion, variation of intensity, and color fluctuation observed in the light emanating from am extraterrestrial source; to be distinguished from terrestrial scintillation primarily in that the light source for the latter lies somewhere within the earth's atmosphere. Also called stellar scintillation . See seeing.

Astronomical scintillation is typically strongest for celestial objects lying at large zenith distances and is not easily observed by eye for objects whose zenith distances are under 30°. Nonperiodic vibratory motions of stellar images with frequencies of the order of 1 to 10 cycles per second create a troublesome problem of seeing in astronomical work. The size of the schlieren producing vibratory scintillations has been estimated to be of the order of centimeters, and chromatic scintillations of celestial objects appear to be produced by parcels whose dimensions are of the order of decimeters or, perhaps, meters. Hence, astronomical scintillation is primarily a consequence of the high-frequency, short-wavelength type of atmospheric turbulence.

astronomical seeing

See seeing.

astronomical solar time

See solar time.

astronomical time

Mean time reckoned from the upper branch of the meridian. See astronomical day.

astronomical triangle

The navigational triangle, either terrestrial or celestial, used in the solution of celestial observations.

astronomical twilight

See twilight.

astronomical unit (abbr AU)

1. A unit of length, usually defined as the distance from the earth to the sun, 149,599,000 kilometers.

This value for the AU was derived from radar observations of the distance of Venus. The value given in astronomical ephemerides, 149,500,000 kilometers, was derived from observations of the minor planet Eros.

2. The unit of distance in terms of which, in the Kepler Third Law,$n2a3=\; k2(1+m)$, the semimajor axis a of an elliptical orbit must be expressed in order that the numerical value of the Gaussian constant k may be exactly 0.01720209895 when the unit of time is the ephemeris day.

In astronomical units, the mean distance of the earth from the sun, calculated by the Kepler law from the observed mean motion n and adopted mass m, is 1.00000003.

astronomical year= tropical year.

astronomy

The science concerning the location, magnitudes, motions, and constitution of celestial bodies and structures.

astrophysics

A branch of astronomy concerning the physical properties of celestial bodies, such as luminosity, size, mass, density, temperature, and chemical composition.

astrotracker = star tracker.

asynchronous computer

An automatic computer in which succeeding operations are started by signals indicating the completion of the previous operation, rather than by signals from a master synchronizer. Contrast to synchronous computer. See variable cycle.

atelectasis

Collapsed or airless state of all or part of a lung. Also called apneumatosis .

athodyd

A type of jet engine consisting essentially of a duct or tube of varying diameter and open at both ends, which admits air at one end, compresses it by the forward motion of the engine, adds heat to it by the combustion of fuel, and discharges the resulting gases at the other end to produce thrust.

The ramjet is an athodyd; the pulsejet, especially the earlier type, is usually not considered an athodyd.

atmosphere

1. The envelope of air surrounding the earth; also the body of gases surrounding or comprising any planet or other celestial body. Compare biosphere, geosphere, hydrosphere, lithosphere. See atmospheric shell.

2. = standard atmosphere.

3. (abbr atm) A unit of pressure equal to 14.7 pounds per square inch.

atmospheric boil = terrestrial scintillation.

atmospheric boundary layer = planetary boundary layer.

atmospheric braking

The action of slowing down an object entering the atmosphere of the earth or other planet from space, by using the drag exerted by air or other gas particles in the atmosphere; the action of the drag so exerted.

atmospheric duct

An almost horizontal layer in the troposphere, extending from the level of a local minimum of the modified refractive index as a function of height, down to the level where the minimum value is again encountered, or down to earth's surface if the minimum value is not encountered again.
Atmospheric ducts may act as waveguides for radio and radar waves.

atmospheric electric field

1. The electric field strength of the atmosphere at any specified point in space and time.

2. The distribution of electrical potential in the atmosphere regarded merely from a geometric point of view as a typical scalar field (rarely used).

atmospheric electricity

1. Electrical phenomena, regarded collectively, which occur in the earth's atmosphere.

2. The study of electrical processes occurring within the atmosphere.

atmospheric entry

The penetration of any planetary atmosphere by any object from outer space; specifically, the penetration of the earth's atmosphere by a manned or unmanned capsule or spacecraft.

atmospheric interference = atmospherics.

atmospheric ion

See ion.

atmospheric layer = atmospheric shell.

atmospheric noise = atmospherics.

atmospheric optics

The study of the optical characteristics of the atmosphere and of the optical phenomena produced by the atmosphere's suspensoids and hydrometeors. It embraces the study of refraction, reflection, diffraction, scattering, and polarization of light, but is not commonly regarded as including the study of any other kinds of radiation. Also called meteorological optics.

atmospheric oscillation = atmospheric tide.

atmospheric physics = physical meteorology.

atmospheric pressure

The pressure at any point in an atmosphere due solely to the weight of the atmospheric gases above the point concerned. See station pressure, sea-level pressure.

atmospheric radiation

Infrared radiation emitted by or being propagated through the atmosphere. See insolation.

Atmospheric radiation, lying almost entirely within the wavelength interval of from 3 to 80 microns, provides one of the most important mechanisms by which the heat balance of the earth-atmosphere system is maintained. Infrared radiation emitted by the earth's surface (terrestrial radiation) is partially absorbed by the water vapor of the atmosphere which in turn remits it, partly upward, partly downward. This secondarily emitted radiation is then, in general, repeatedly absorbed and reemitted, as the radiant energy progresses through the atmosphere. The downward flux, or counterradiation, is of basic importance in the greenhouse effect; the upward flux is essential to the radiative balance of the planet.

atmospheric region = atmospheric shell.

atmospheric refraction

Refraction resulting when a ray of radiant energy passes obliquely through an atmosphere.

It may be called "astronomical refraction" if the ray enters the atmosphere from outer space, or "terrestrial refraction" if it emanates from a point on or near the surface of the earth.

atmospherics

The radiofrequency electromagnetic radiation originating, principally, in the irregular surges of charge in thunderstorm lightning discharges. Atmospherics are heard as a quasi-steady background or crackling noise (static) in ordinary amplitude-modulated radio receivers. Also called atmospheric interference, strays, sferics . See sferics.

Since any acceleration of electric charge leads to emission of electromagnetic radiation, and since the several processes involved in propagation of lightning lead to very large charge accelerations, the lightning channel acts like a huge transmitter, sending out broad band radiation; the 10-kilocycle range propagates best and is used in detecting atmospherics. Atmospherics may occasionally be detected at distances in excess of 2000 miles from their source. Advantage has been taken of this in using radio direction-finding equipment to locate active thunderstorm areas in remote regions and in between weather reporting stations.

atmospheric scintillation = terrestrial scintillation.

atmospheric shell

Any one of a number of strata or layers of the earth's atmosphere. Also called atmospheric layer, atmospheric region .

Temperature distribution is the most common criterion used for denoting the various shells. The troposphere (the region of change) is the lowest 10 or 20 kilometers of the atmosphere, characterized by decreasing temperature with height. The top of the troposphere is called the tropopause. Above the tropopause, the stratosphere, a region in which the temperature generally increases with altitude, extends to the stratopause, the top of the inversion layer, at about 50 to 55 kilometers. Above the stratosphere, the mesosphere, a region of generally decreasing temperatures with height extends to the mesopause, the base of an inversion layer at about 80 to 85 kilometers. The region above the mesopause, in which temperature generally increases with height, is the thermosphere.
The distribution of various physicochemical processes is another criterion. The ozonosphere, lying roughly between 10 and 50 kilometers, is the general region of the upper atmosphere in which there is an appreciable ozone concentration and in which ozone plays an important part in the radiative balance of the atmosphere; the ionosphere, starting at about 70 or 80 kilometers, is the region in which ionization of one or more of the atmospheric constituents is significant; the neutrosphere is the shell below this which is, by contrast, relatively unionized; and the chemosphere, with no very definite height limits, is the region in which photochemical reactions take place.
Dynamic and kinetic processes are a third criterion. The exosphere is the region at the top of the atmosphere, above the critical level of escape, in which atmospheric particles can move in free orbits, subject only to the earth's gravitation.
Composition is a fourth criterion. The homosphere is the shell in which there is so little photodissociation or gravitational separation that the mean molecular weight of the atmosphere is sensibly constant; the heterosphere is the region above this, where the atmospheric composition and mean molecular weight are not constant. The boundary between the two is probably at the level at which molecular oxygen begins to be dissociated, and this occurs in the vicinity of 80 or 90 kilometers.
The term mesosphere has been given another definition which does not fit into any logical set of criteria, i.e., the shell between the exosphere and the ionosphere. This use of mesosphere has not been widely accepted.
For further subdivisions, see ionosphere, troposphere, geocorona.

atmospheric shimmer = terrestrial scintillation.

atmospheric tide

Defined in analogy to the oceanic tide as an atmospheric motion on a worldwide scale, in which vertical accelerations are neglected (but compressibility is taken into account). Also called atmospheric oscillation .

Both the sun and moon produce atmospheric tides; and there exist both gravitational tides and thermal tides. The harmonic component of greatest amplitude, the 12-hour or semidiurnal solar atmospheric tide, is both gravitational and thermal in origin, the fact that it is greater than the corresponding lunar atmospheric tide being ascribed usually to a resonance in the atmosphere with a free period very close to the tidal period. Other tides of 6, 8, and 24 hours have been observed.

atmospheric transmissivity

See transmission coefficient.

atomic clock

A timekeeping device controlled by the frequency of the natural vibrations of certain atoms.

atomic mass

The mass of a neutral atom of a nuclide usually expressed in atomic mass units. See atomic weight, mass number.

The atomic mass unit, amu, is exactly one-twelfth of the mass of a neutral atom of the most abundant isotope of carbon, C¹²=12.0000.

atomic mass unit (abbr amu)

See atomic mass, note.

atomic number (symbol Z)

An integer that expresses the positive charge of the nucleus in multiples of the electronic charge e. It is the number of electrons outside the nucleus of a neutral (un-ionized) atom and, according to widely accepted theory, the number of protons in the nucleus. See atomic weight, Table IV.

An element of atomic number Z occupies the Zth place in the periodic table of the elements. Its atom has a nucleus with a charge +Ze, which is normally surrounded by Z electrons, each of charge -e.
For example, the carbon isotope $$₆C¹⁴ has an atomic number of 6 and an atomic mass of 14.

atomic particle

One of the particles of which an atom is constituted, as an electron, neutron, or a positively charged nuclear particle.

atomic rocket

A projected rocket engine in which the energy for the jetstream is to be generated by atomic fission or fusion.

atomic weight (abbr at. wt.)

The weight of an atom according to a scale of atomic weight units, awu, valued as one-twelfth the mass of the carbon atom ($C12$ = 12.00000). See Table IV.
Thus expressed, the atomic weight to the nearest integer is identical with the mass number.

atomic weight unit (abbr awu)

See atomic weight, note.

A-trace

The first trace of an oscilloscope, as the upper trace of a loran indicator.

attached shock = attached shock wave.

attached shock wave

An oblique or conical shock wave that appears to be in contact with the leading edge of an airfoil or the nose of a body in a supersonic flow field. Also called attached shock.

attachment

The process in which two particles collide and stick together forming a single complex particle. The most common attachment process is the formation of a negative ion from electron attachment to an atom or molecule. Some negative ions are unstable, however, and cannot survive.

The usual measure for this process is the attachment coefficient, which on the average is the fraction of a large number of collisions that result in attachment. Typical values of this fraction run from 1 in 10,000 to 1 in 1,000.

attachment coefficient

See attachment, note.

attenuation

Reduction in intensity.

attenuation coefficient (symbol )

A measure of the space rate of attenuation of any transmitted electromagnetic radiation. The attenuation coefficient is defined by
$dI\; =\; -aI$₀dx
or
I = I₀e^-ax where I is the flux density at the selected point in space; $I$₀ is the flux density at the source; x is the distance from the source; and a is the attenuation coefficient.

In general, the attenuation coefficient is specified only when the attenuation is known to be due to both absorption and scattering, or when it is impossible to determine which is the cause. See absorption coefficient, scattering coefficient.

attenuation constant

1. A measure of the rate of attenuation per unit length; the rate of flux-density (or power) reduction as energy (visual, electromagnetic, acoustic) propagates from its source. Also called attenuation factor, decay constant . Compare attenuation coefficient.

For free-space transmission of radar frequency energy, the attenuation constant is usually expressed in decibels per mile or kilometer (db/mi or db/km).

2. Specifically, of a traveling plane wave at a given frequency, the relative rate of decrease of amplitude of a field component (or of voltage or current) in direction of propagation in nepers per unit length.

attenuation factor = attenuation constant.

attenuation length

The reciprocal of the attenuation coefficient.

attenuation ratio

The magnitude of the propagation ratio.

attitude

The position or orientation of an aircraft, spacecraft, etc., either in motion or at rest, as determined by the relationship between its axes and some reference line or plane or some fixed system of reference axes.

attitude control

1. The regulation of the attitude of an aircraft, spacecraft, etc.

2. A device or system that automatically regulates and corrects attitude, especially of a pilotless vehicle.

attitude gyro

1. A gyro-operated flight instrument that indicates the attitude of an aircraft or spacecraft with respect to a reference coordinate system throughout 360° of rotation about each axis of the craft.

This instrument is similar to the artificial horizon, but has greater angular indication.

2. Broadly, any gyro-operated instrument that indicates attitude.

attitude jet

A jetstream used to correct or alter the attitude of a flying body either in the atmosphere or in space; the nozzle that directs this jetstream.

The jet may be continuous or intermittent. A vernier engine is sometimes used to produce it.

attribute

A characteristic of a thing which can be appraised only in terms of whether it does or does not exist. See method of attributes.

attributes testing

A reliability test procedure where the items under test are classified according to qualitative rather than quantitative characteristics.

AU (abbr) = astronomical unit.

audible sound

Sound containing frequency components lying between about 15 and 20,000 cycles per second.

audio

Pertaining to the audiofrequency range.

The word audio may be used as a modifier to indicate a device or system intended to operate at audiofrequencies, e.g., audioamplifier.

audio frequency

Any frequency corresponding to a normally audible sound wave. See audiofrequency range.

audio frequency range

The range of frequencies to which the human ear is sensitive, approximately 15 cycles per second to 20,000 cycles per second. Also called audiorange.

audiorange = audio frequency range.

auditory sensation area

In acoustics, the frequency region enclosed by the curves defining the threshold of pain and the threshold of audibility.

Auger shower

A very large cosmic-ray shower. Also called extensive air shower .

augmentation

The apparent increase in the semidiameter of a celestial body, as observed from the earth, as its altitude increases, due to reduced distance from the observer.
The term is used principally in reference to the moon.

augmentation correction

A correction due to augmentation, particularly that sextant altitude correction due to the apparent increase in the semidiameter of a celestial body as its altitude increases.

augmenter tube

A tube or pipe, usually one of several, through which the exhaust gases from an aircraft reciprocating engine are directed especially to provide additional thrust.

Aur, Auri

International Astronomical Union abbreviations for Auriga . See constellation.

aural null

See null.

Auriga (abbr Aur, Auri)

See constellation.

aurora

The sporadic radiant emission from the upper atmosphere over middle and high latitudes. It is believed to be due primarily to the emission from nitrogen - atomic N I and N II, molecular, N₂, and ionic N₂+; atomic oxygen (O I and O II); atomic sodium (Na I); the hydroxyl radical (OH); and hydrogen. Compare airglow.

According to various theories, auroras seem definitely to be related to magnetic storms and the influx of charged particles from the sun. The exact details of the nature of the mechanisms involved are still being investigated, but release of trapped particles from Van Allen belt apparently plays an important part. The aurora is most intense at times of magnetic storms (when it is also observed farthest equatorward), and shows a periodicity which is related to the sun's 27-day rotation period and the 11-year sunspot cycle. The distribution with height shows a pronounced maximum near 100 kilometers. The lower limit is probably near 80 kilometers.
The aurora can often be clearly seen, and it assumes a variety of shapes and colors which are characteristic patterns of auroral emission.
The following is the general classification and abbreviations of the forms of the auroras adopted by the International Union of Geodesy and Geophysics in 1930 for reporting of visual observations. The classification was modified slightly and expanded in 1963. The new classification is described in the International Auroral Atlas , Aldine Pub. Co., Chicago, 1963.

I. Forms without ray structure:
HA (abbr for homogeneous quiet area). These can appear near the horizon, and between the arc and the horizon a dark segment is often seen. These arcs can be narrow or broad, and are very often diffuse along the upper border but sharp along the lower one.
HB (abbr for homogeneous bands). These forms do not have the regular shape of the arcs; they are more rapidly moving phenomena. The lower border is often irregular and sharp. The breadth can vary from a very narrow band to a band which is so large that it resembles a curtain hanging down. These bands very often turn into bands with ray structure.
PA (abbr for pulsating arcs). Parts of an arc flash up and disappear regularly within a period of about 20 seconds. This form quite often stands isolated in the sky without other auroras.
DS (abbr for diffuse luminous surfaces). These either appear like a diffuse veil or glow over great parts of the heavens without distinct boundaries, often appearing after intense displays of rays and curtains, or as more isolated feeble luminous streaks which sometimes bear a striking resemblance to clouds. Sometimes large areas of the heavens can be discolored by a green, violet, or red diffuse light.
PS (abbr for pulsating surfaces). Diffuse patches appear and disappear rhythmically at the same place, retaining the same irregular shape, When the patches are lying near the magnetic zenith the contours can be more sharp, and form a sort of corona. These forms appear often in connection with flaming auroras.
G (abbr for feeble glow near the horizon resembling the dawn). Of white or redlike color, this form is often the upper part of an arc whose lower border is below the horizon.

II. Forms with ray structure: These forms consist of short or long rays which can be arranged in different ways.
RA (abbr for arcs with ray structure). A homogeneous arc which has remained quiet and unaltered for a rather long time may become sharp and luminous along the lower border and they very rapidly change into an arc of rays. The rays can be short or long.
RBI (abbr for bands with ray structure). These resemble the bands mentioned under HB but are constituted of a series of rays which are arranged close to each other along the band, or they can appear more scattered. Often a series of parallel bands appear. When a band is near the magnetic zenith is may have the form of a corona.
D (abbr for draperies). If the ray become very long the band appears like a curtain or drapery whose lower border is often more luminous. Several parallel curtains frequently appear at the same time. Near the zenith the curtain may have a fanlike form on account of the perspective.
R (abbr for rays). The rays can be isolated, narrow or broad, short or long. They may appear in great segments or like masses or rays, very often resembling curtains.
C (abbr for corona). When the rays approach the magnetic zenith they seem, on account of the perspective, to converge to this point and form a corona. This may be formed by long rays or by short ones, it may be completed or incomplete. A corona can also be formed by bands, draperies, or more diffuse forms near the magnetic zenith.

III. Flaming auroras (abbr F). A characteristic, rapidly moving form, consisting of strong waves of light which move upwards, one after the other, in the direction of the magnetic zenith. The waves have the form of detached arcs which move upwards normally to the direction of the arc; they can be compared to invisible waves illuminating broad rays and patches which appear and disappear rhythmically when the waves pass them. The flaming aurora frequently appears after strong displays of rays and curtains and is often followed by the formation of a corona.

For more information,about auroras, visit Auroras: Paintings in the Sky.

aurora australis

The aurora of the Southern Hemisphere.

aurora borealis

The aurora of northern latitudes. Also called aurora polaris, northern lights .

auroral zone

A roughly circular band around either geomagnetic pole above which there is a maximum of auroral activity. It lies about 10 to 15° of geomagnetic latitude from the geomagentic poles.

The auroral zone broadens and extends equatorward during intense auroral displays. The northern auroral zone is centered along a line passing near Point Barrow, Alaska, through the lower half of Hudson Bay, slightly off the southern tip of Greenland, through Iceland, northern Norway and northern Siberia. Along this line auroras are seen on an average of 240 nights a year. The frequency of auroras falls off both to the north and to the south of this line but more rapidly to the south. The most severe blackouts occur in the auroral zone.

aurora polaris = aurora borealis.

austausch coefficient = exchange coefficient.

australite

See tektite.

authorized carrier frequency

A specific carrier frequency authorized for use, from which the actual carrier frequency is permitted to deviate, solely because of frequency instability, by an amount not to exceed the frequency tolerance.

autoconvection gradient = autoconvective lapse rate.

autoconvective lapse rate

The environmental lapse rate of temperature in an atmosphere in which the density is constant with height (homogenous atmosphere), equal to g/R, where g is the acceleration of gravity and R the gas constant. For dry air the autoconvective lapse rate is approximately $+3.4\; x\; 10-4$°C per centimeter. Also called autoconvection gradient.

autocorrelation

In statistics the simple linear internal correlation of members of a time series (ordered in time or other domains).

autocorrelation function

Autocorrelation for variable lag.

autoigniting propellant

Any propellant that ignites by itself without external stimulation.

autoignition temperature

The temperature at which combustible materials ignite spontaneously in air.

autokinetic illusion

The illusion of a fixed object or light moving when gazed at steadily.

automatic celestial navigation = celestial guidance.

automatic computer

A computer which can automatically perform a comprehensive sequence of operations.

automatic control

Control of devices and equipment, including aerospace vehicles, by automatic means.

automatic data processing system

An electronic system that includes an electronic data processing system plus auxiliary and connecting communications equipment.

automatic direction finder (abbr ADF)

A radio direction finder which automatically and continuously provides a measure of the direction of arrival of the received signal. Data are usually displayed visually.

automatic frequency control (abbr AFC)

An arrangement whereby the frequency of an oscillator is automatically maintained within specified limits.

automatic gain control (abbr AGC)

A process by which gain is automatically adjusted as a function of input or other specified parameter.

automatic pilot

Equipment which automatically stabilizes the attitude of a vehicle about its pitch, roll, and yaw axes. Also called autopilot .

automatic stability

Stability achieved with the controls operated by automatic devices, as by an automatic pilot.

automatic tracking

Tracking in which a servomechanism automatically follows some characteristic of the signal; specifically, a process by which tracking or data acquisition systems are enabled to keep their antennas continually directed at a moving target without manual operation.

autopilot= automatic pilot.

autosyn

(A trade name, from autosynchronous, often capitalized). A remote-indicating instrument or system based upon the synchronous-motor principle, in which the angular position of the rotor of one motor at the measuring source is duplicated by the rotor of the indicator motor, used, e.g., in fuel-quantity or fuel-flow measuring systems, position-indicating systems, etc.

autosynchronous

Full term of autosyn .

autumnal equinox

1. That point of intersection on the celestial sphere of the ecliptic and the celestial equator occupied by the sun as it changes from north to south declination, on or about September 23. Also called September equinox, first point of Libra .

2. That instant the sun reaches the point of zero declination when crossing the celestial equator from north to south.

auxiliary circle

In celestial mechanics, a circumscribing circle to an orbital ellipse with a radius a, the semimajor axis. The auxiliary circle is related to the ellipse by $QN\; =\; Q\text{'}N(1\; -\; e2)1/2$ where e is the eccentricity; Q is any point on the ellipse; N is the foot of the perpendicular through Q to the line of apsides; and Q' is the intersection of the perpendicular and the auxiliary circle.

auxiliary fluid ignition

A method of ignition of a liquid-propellant rocket engine in which a liquid which is hypergolic with either the fuel or the oxidizer is injected into the combustion chamber to initiate combustion.

Aniline is used as an auxiliary fluid with nitric acid and some organic fuels to initiate combustion.

auxiliary landing gear

That part or parts of a landing gear, as an outboard wheel, which is intended to stabilize the craft on the surface but which bears no significant part of the weight.

auxiliary power unit (abbr APU)

A power unit carried on an aircraft or spacecraft which can be used in addition to the main sources of power of the craft.

avalanche

The cumulative process in which charged particles accelerated by an electric field produce additional charged particles through collision with natural gas molecules or atoms. See Townsend ionization coefficient.

average = arithmetic mean.

average deviation

In statistics, the average or arithmetic mean of the deviations, taken without regard to sign, from some fixed value, usually the arithmetic mean of the data. Also called mean deviation . See standard deviation.

average information content

The average of the information content per symbol emitted from a source. Also called entropy and negentropy .

aviation medicine

See aerospace medicine.

Avogadro constant = Avogadro number.

Avogadro law

See Avogadro number.

Avogadro number, Avogadro constant (symbol $N$_A)

The number of molecules in 1 mole of gas ($6.02252\; X\; 1023$ per mole).

That this number is a constant for permanent gases is the Avogadro law: under normal conditions, i.e., pressure of 1 standard atmosphere and temperature of 0° C, the volume occupied by 1 mole of gas is the same for all permanent gases (22,414 cubic centimeters). See Loschmidt number.

axial flow compressor

A rotary compressor having interdigitated rows or stages of rotary and of stationary blades through which the flow of fluid is substantially parallel to the rotor's axis of rotation. Compare centrifugal compressor.

axis (plural axes)

1. A straight line about which a body rotates, or along which its center of gravity moves (axis of translation).

2. A straight line around which a plane figure may rotate to produce a solid; a line of symmetry.

3. One of a set of reference lines for a coordinate system.

axis of freedom

Of a gyro, an axis about which a gimbal provides a degree of freedom.

axis of thrust = thrust axis.

azimuth

1. Horizontal direction or bearing. Compare azimuth angle.

2. In navigation, the horizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from a reference direction, usually measured from 0° at the reference direction clockwise through 360°.

An azimuth is often designated as true, magnetic, compass, grid, or relative as the reference direction is true, magnetic, compass, grid north, or heading, respectively. Unless otherwise specified, the term is generally understood to apply to true azimuth, which may be further defined as the arc of the horizon, or the angle at the zenith, between the north part of the celestial meridian or principal vertical circle and a vertical circle, measured from 0° at the north part of the principal vertical circle clockwise through 360°.

3. In astronomy, the direction of a celestial point from a terrestrial point measured clockwise from the north or the south point of the meridian plane. See horizon system.

4. In surveying, the horizontal direction of an object measured clockwise from the south point of the meridian plane.
In surveying, an azimuth of a celestial body is called an astronomic azimuth.

azimuth angle

1. Azimuth measured from 0° at the north or south reference direction clockwise or counterclockwise through 90° or 180°.

Azimuth angle is labeled with the reference direction as a prefix and the direction of measurement from the reference direction as a suffix. Thus, azimuth angle S 144° W is 144° west of south, or azimuth 324°. When azimuth angle is measured through 180°, it is labeled N or S to agree with the latitude and E or W to agree with the meridian angle.

2. In surveying, an angle in triangulation or in traverse through which the computation of azimuth is carried.

azimuth error

An error in the indicated azimuth of a target detected by radar, resulting from horizontal refraction. Compare range error.

Inasmush as significant gradients of index of refraction are very uncommon in the atmosphere, these errors almost invariably are negligible. Seacoast areas may give rise on occasion to appreciable horizontal bending of radio waves because of the contrast of refractive index values between the air over land and the air over water.

azimuth marker

1. A scale encircling the plan position indicator (PPI) scope of a radar on which the azimuth of a target from the radar may be measured.

2. Reference limits inserted electronically at 10° or 15° intervals which extend radially from the relative position of the radar on an off center PPI scope. These are employed for target azimuth determination when the radar position is not at the center of the PPI scope and hence the fixed azimuth scale on the edge of the scope cannot be employed.

On such markers north is usually 0°, east 90°, etc. Occasionally, on ship or airborne radars, 0° is used to indicate the direction in which the craft is heading, in which cases the relative bearing, not azimuth, of the target is indicated.

azran

Azimuth and range.

This term was coined in the field of radar, and has since been extended in application to the locating of any object (or target) by means of polar coordinates.

Azusa

A short-baseline, continuous wave, phase comparison, single-station, tracking system operating at C-band and giving two direction cosines and slant range which can be used to determine space position and velocity.