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.
2. In optics, a specific deviation from perfect imagery, as, for example: spherical aberration, coma, astigmatism, curvature of field, and distortion.
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.
2. An aircraft, space vehicle, or the like that aborts.
3. An act or instance of aborting.
2. Complete, as in absolute vacuum.
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.
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.
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.
2. A manometer that measures absolute 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.
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:
"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 GPM (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).
An absolute vacuum is not obtainable.
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.
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 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.
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.
(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:
)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 ILx is the flux density of radiation of wavelength L, initially of flux density IL0, 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.
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 ften referred to as line absorption. The spectrum formed by a selectively absorbing liquid or solid is typically continuous in nature (continuous absorption).
) 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.
2. The act or process of accelerating, or the state of being accelerated. Negative acceleration is called deceleration . Used for boost and G force.
By the International Gravity Formula, 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=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.
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.
) 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 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.
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.
Some acoustics Mach meters measure transit time or velocity of a sound pulse; others measure an angle, as the angle of the Mach cone.
) =speed of 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.
2. Those qualities of an enclosure that together determine its character with respect to distinct hearing.
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.
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.
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.
2. = radioactive, as active sample .
3. = fissionable, as active material .
4. Receiving energy from some source other than a signal, as active element .
Adaptation is applied particularly to evolutionary change.
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.
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.
2. An expression, usually numerical, identifying an address (sense 1).
2. = adiabatic wall temperature.
3. The final and initial temperature in an adiabatic, Carnot cycle.
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.
2. Of or pertaining to the air, atmosphere, or aviation.
The problem of the effect of reentry on the trajectory of a vehicle is a problem in aeroballistics.
The aerodynamic force is proportional to the expression
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 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.
The term is used when the context calls for discrimination from space vehicle.
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.
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 .
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.
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.
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.
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.
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.
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.
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.
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 |
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.
Air light is not be confused with airglow .
An airborne computing system which presents a continuous indication of the aircraft position on the basis of aircraft heading, airspeed, and elapsed time.
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.
Because this adjective is pronounced as the noun airspace is, it is subject to misunderstanding. Aerospace is commonly used instead.
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.
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 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.
Airspace is sometimes particularized by altitude, as the airspace above 20,000 feet.
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.
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.
Near Earth, however, the magnetic field begins to be dominated by the dipole-like form of the main field generated in the Earth's core, and the combined drift due to both electric and magnetic fields tends to separate ions from electrons, steering the former to the dusk side of earth and the latter to the dawn side. This creates Alfvén layers, regions where those motions fail to satisfy charge neutrality. Charge neutrality is then restored by electrons drawn upwards as the downward region 2 current, and electrons dumped into the ionosphere (plus some ions drawn up) to create the corresponding upward currents.
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.
The dynamics of such waves are analogous to those in a vibrating string, the phase speed C being given by
where mu is the permeability; H is the magnitude of the magnetic field; and rho
is
the fluid density. alga (plural, algae)
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.
Different algorisms have been used in the design of computing machines.
2. = algorism.
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.
The decay product is the nuclide having a mass number four units smaller and an atomic number two units smaller than the original nuclide.
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.
Often shortened to intercept.
Many writers have proposed specific definitions for this term but the definitions are highly variable.
In an altitude wind tunnel for testing engines, provision is made for exchanging fresh air for exhaust-laden air during operation.
, 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.
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.
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.
Typical amplifying elements are electron tubes, transistors, and magnetic circuits.
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.
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.
Analog computers range in complexity from a slide rule to electrical computers used for solving mathematical problems.
A device which will convert an analog voltage sample to an equivalent digital code of some finite resolution. Also called digitizer, encoder .
Differs from metric photography in that measurements are not a prime requisite.
See constellation.
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 .
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.
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.
Phase and frequency modulation are particular forms of angle modulation.
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 .
A negative angle of elevation is usually called an angle of depression.
In some cases involving radio waves, the angle of incidence is measured relative to the surface.
2. = angle of attack. (British usage).
2. Same as blade angle.
In some cases involving radio waves, the angle of reflection is measured relative to the surface.
A unit of length, used chiefly in expressing short wavelengths. It equals centimeters.
)The rate of change of angular velocity.
2. The arc of the great circle joining two points, expressed in angular units.
3. Distance between two points, expressed in wavelengths 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.
) 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 .
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
It is generally expressed in terms of the minimum angle by which targets must be spaced to be separately distinguishable. See resolution.
2. = angular velocity.
)The change of angle per unit time; specifically, in celestial mechanics, the change in angle of the radius vector per unit time.
The negative pole or electrode is called a cathode.
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.
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
Anoxia is popularly used as a synonym for hypoxia . This usage should be avoided.
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).
Antigravity is common in science fiction but has not yet been reported in scientific literature.
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.
Thus, the proton has a positive charge; the antiproton, a negative charge. When a particle and its antiparticle collide, both may disappear with the creation of lighter particles; this process is called annihilation .
See constellation.
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.
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.
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 .
international candles per square centimeter. Compare stilb.
True values are reduced from apparent (observed) values by eliminating those factors such as refraction, light time, etc., which affected the observation.
The apparent additional mass has inertia and momentum equal to the apparent increase of the inertia and momentum of the body.
In astronomy apparent motion usually refers to movement of celestial bodies as observed from the earth.
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 time may be designated as either local or Greenwich, as the local or Greenwich meridian is used as the reference.
The horizontal component of apparent wander is called drift, and the vertical component is called topple .
See ionosphere.
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 .
2. A penumbral eclipse of the moon.
See constellation.
See constellation.
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.
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 is to be distinguished from corona discharge, point discharge, and spark discharge.
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.
The areal velocity is constant for a central force. See Kepler laws.
Words formed with aero are considered pedantic by some. See geo.
Ares is seldom used except in combining forms as areocentric, apareon.
See constellation.
2. A device, such as a spirit level, pendulum, etc., that establishes a horizontal reference in a navigation instrument.
, where F is the function and 
the del-operator.
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.
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.
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.
Astroballistics uses the data and methods of astronomy, aerodynamics, ballistics, and physical chemistry.
Astrodynamics is sometimes used as a synonym for celestial mechanics. This usage should be discouraged.
2. Specifically, an instrument designed for very accurate celestial altitude measurements, as in survey work.
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.
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.
2. In a broader sense the science of space flight.
In any combination, such as astronomic coordinates, astronomic is equivalent to astromonical.
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.
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.
When the astronomical equator is corrected for station error, it becomes the geodetic equator.
Astronomical latitude corrected for the meridional component of station error becomes geodetic latitude .
Astronomical longitude corrected for the prime-vertical component of station error divided by the cosine of the latitude becomes geodetic longitude .
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.
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.
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.
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 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.
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,, 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.
The ramjet is an athodyd; the pulsejet, especially the earlier type, is usually not considered an athodyd.
2. = standard atmosphere.
3. (abbr atm) A unit of pressure equal to 14.7 pounds per square inch.
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 ducts may act as waveguides for radio and radar waves.
2. The distribution of electrical potential in the atmosphere regarded merely from a geometric point of view as a typical scalar field (rarely used).
2. The study of electrical processes occurring within the atmosphere.
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.
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.
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 , 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.
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.
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.
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.
See atomic mass, note.
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 has an atomic number of 6 and an atomic mass of 14.
The weight of an atom according to a scale of atomic weight units, awu, valued as one-twelfth the mass of the carbon atom ( = 12.00000). See Table IV.
Thus expressed, the atomic weight to the nearest integer is identical with the mass number.
See atomic weight, note.
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.
) A measure of the space rate of attenuation of any transmitted electromagnetic radiation. The attenuation coefficient is defined by
or
I = I0e-ax
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.
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.
The reciprocal of the attenuation coefficient.
2. A device or system that automatically regulates and corrects attitude, especially of a pilotless vehicle.
This instrument is similar to the artificial horizon, but has greater angular indication.
2. Broadly, any gyro-operated instrument that indicates attitude.
The jet may be continuous or intermittent. A vernier engine is sometimes used to produce it.
The word audio may be used as a modifier to indicate a device or system intended to operate at audiofrequencies, e.g., audioamplifier.
The term is used principally in reference to the moon.
See constellation.
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.
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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.
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For more information,about auroras, visit Auroras: Paintings in the Sky.
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.
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.
An arrangement whereby the frequency of an oscillator is automatically maintained within specified limits.
A process by which gain is automatically adjusted as a function of input or other specified parameter.
2. That instant the sun reaches the point of zero declination when crossing the celestial equator from north to south.
Aniline is used as an auxiliary fluid with nitric acid and some organic fuels to initiate combustion.
A power unit carried on an aircraft or spacecraft which can be used in addition to the main sources of power of the craft.
The number of molecules in 1 mole of gas ( 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.
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.
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 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.
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.
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.
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.