E

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.
Greek symbols may not appear correctly in some browsers. For example a gamma may appear as γ.

earphone

An electroacoustic transducer operating from an electrical system to an acoustical system and intended to be closely coupled acoustically to the ear.

Earth

See planet, table.

earth axis

Any one of a set of mutually perpendicular reference axes established with the upright axis (the Z-axis) pointing to the center of the earth, used in describing the position or performance of an aircraft or other body in flight.

The earth axes may remain fixed or may move with the aircraft or other object.

earth current

A large-scale surge of electric charge within the earth's crust, associated with a disturbance of the ionosphere.

Current patterns of quasi-circular form and extending over areas the size of whole continents have been identified and are known to be closely related to solar-induced variations in the extreme upper atmosphere.

earthlight

The illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere. Also called earthshine.

Spectroscopic observations reveal that earthlight is relatively richer in blue light than is direct sunlight; this condition results from the fact that an appreciable part of the total earth reflection is backward-scattered light which, in accordance with Rayleigh law, is relatively rich in the blue and poor in the red.

earth point

The point where the forward straight-line projection of a meteor trajectory intersects the surface of the earth.

earth radiation = terrestrial radiation.

earth-rate unit (abbr eru)

A unit of angular drift, as of a gyro, equal to the rate of angular movement of the earth with respect to the stars, 15° per hour.

earth satellite

A body that orbits about the earth; specifically, an artificial satellite placed in orbit by man.

earth shine = earthlight.

earth's rate correction

A command rate applied to a gyro to compensate for the apparent precession of the gyro spin axis with respect to its base caused by the rotation of the earth.

earth tide

A periodic movement of the earth's crust caused by the tide-producing forces of the moon and sun.

Ebert ion counter

An ion counter of the aspiration condenser type, used for the measurement of the concentration and mobility of small ions in the atmosphere.

ebullism

The formation of bubbles, with particular reference to water vapor bubbles in biological fluids caused by reduced ambient pressure; the boiling of body fluids.

eccentric

Not having the same center; varying from a circle.

eccentric anomaly (symbol E)

See anomaly.

eccentricity (symbol e)

1. Of any conic, the ratio of the length of the radius vector through a point on the conic to the distance of the point from the directrix.

2. Of an ellipse, the ratio of the distance between the center and focus of an ellipse to its semimajor axis. Also called numerical eccentricity.
The eccentricity e of an ellipse can be computed by the formula where a is the semimajor axis and b is the semiminor axis.

3. Of an ellipse, the distance between the center and the focus. Also called linear eccentricity.

echo

1. A wave that has been reflected or otherwise returned with sufficient magnitude and delay to be detected as a wave distinct from that directly transmitted.

2. In radar, a pulse of reflected radio frequency energy; the appearance on a radar indicator of the energy returned from a target. Also called blip.

echo intensity

The brightness or brilliance of a radar echo as displayed on an intensity-modulated indicator. Echo intensity is, within certain limits, proportional to the voltage of the target signal or to the square root of its power. Compare echo power.

echo power

The electrical strength, or power, of a radar target signal. Echo power is normally measured in watts or dbm (decibels referred to a milliwatt).

echo pulse

A pulse of radio energy received at the radar after reflection from a target; that is, the target signal of a pulse radar.

echo signal = target signal.

eclipse

1. The reduction in visibility or disappearance of a nonluminous body by passing into the shadow cast by another nonluminous body.

2. The apparent cutting off, wholly or partially, of the light from a luminous body by a dark body coming between it and the observer.

1. The first type of eclipse is exemplified by a lunar eclipse, the moon passing through the shadow cast by the earth; or by the passage of a satellite into the shadow cast by its planet; but when the satellite actually passes directly behind its planet, it may properly be termed an occultation.

2. The second type of eclipse is exemplified by a solar eclipse, caused by the moon passing between the sun and the earth. If the relative positions and distances are such that at a point on the earth the sun is completely obscured, the eclipse is total; if the distances are such that, when in line with the sun, the moon is surrounded by a ring of light, the eclipse is annular; and when the moon passes to one side of a straight line from sun to observer and shows a crescent of light, it is a partial eclipse.

eclipse year

The interval between two successive conjunctions of the sun with the same node of the moon's orbit, averaging 346 days 14 hours 52 minutes 52.42 seconds in 1962, and increasing at the rate of 0.0276 second annually. See year.

ecliptic

The apparent annual path of the sun among the stars; the intersection of the plane of the earth's orbit with the celestial sphere.

The ecliptic is a great circle of the celestial sphere inclined at an angle of about 23° 27' to the celestial equator.

ecliptic longitude = celestial longitude.

ecliptic pole

On the celestial sphere, either of the two points 90° from the ecliptic.

ecliptic system of coordinates

A set of celestial coordinates based on the ecliptic as the primary great circle. See coordinate, table.

The points 90° from the ecliptic are the north and south ecliptic poles. Angular distance north or south of the ecliptic, analogous to latitude, is celestial latitude. Celestial longitude is measured eastward along the ecliptic from the vernal equinox through 360°.

ecological system

A habitable environment, either created artificially, as in a manned space vehicle, or occurring naturally, such as the environment on the surface of the earth, in which man, animals, or other organisms can live in mutual relationship with one another and the environment.

Ideally the environment furnishes the sustenance for life, and the resulting waste products revert or cycle back into the environment to be used again for the continuous support of life.

ecology

The study of the environmental relations of organisms. See environment.

economizer

A reservoir in a continuous-flow oxygen system in which oxygen exhaled by the user is collected for recirculation in the system.

ecosphere

1. = biosphere.

2. A volume of space surrounding the Sun, extending from the orbit of Venus past the orbit of Mars, in which some biologists believe conditions are favorable for the development and maintenance of life.

eddy

In a fluid, any circulation drawing its energy from a flow of much larger scale and brought about by pressure irregularities.

eddy coefficient = exchange coefficient.

eddy stresses = Reynolds stresses.

eddy velocity

The difference between the mean velocity of fluid flow and the instantaneous velocity at a point. For example u' = u - , where u' is the eddy velocity; u is the instantaneous velocity; and is mean velocity. Also called fluctuation velocity.

Over the same interval which defines the mean velocity, the average value of the eddy velocity is necessarily zero.

eddy viscosity

The turbulent transfer of momentum by eddies giving rise to an internal fluid friction, in a manner analogous to the action of molecular viscosity in laminar flow, but taking place on a much larger scale.

The value of the coefficient of eddy viscosity (an exchange coefficient) is of the order of 10⁴ square centimeters per second, or 100,000 times the molecular kinematic viscosity.

edge effect

See diffraction, note.

E-display

In radar, a rectangular display in which targets appear as blips with distance indicated by the horizontal coordinate and elevation by the vertical coordinate. Also called E-scan and E-scope.

edit

In computer terminology, to arrange, delete, select, or add to information.

EDP (abbr) = electronic data processing.

education

Of a computer, stored subroutines and subprograms which are available for use in automatic programming.

effective aperture = effective area.

effective area

1. In antenna design, the ratio of the received power available at the terminals of an antenna to the power per unit area in the incident wave. For all antennas, effective area A is related to gain G at a given wavelength by the equation: A / G = ² / 4Also called effective aperture. See aperture.

The effective area of an ideal antenna is equal to its physical area S. In practice, A/S for microwave antennas is always less than one, a representative value for paraboloids being 0.6.

2. Same as scattering cross section.

effective atmosphere

1. That part of the atmosphere which effectively influences a particular process or motion, its outer limits varying according to the terms of the process or motion considered.

For example, an earth satellite orbiting at 250 miles altitude remains within the ionosphere, but because the air particles are so sparse at this altitude as to cause no appreciable friction of deflection, the satellite may be considered to be outside the effective atmosphere. For movement of air vehicles the effective atmosphere ends at the aeropause (which see).

2. = optically effective atmosphere.

effective earth radius

See effective radius of the earth.

effective exhaust velocity (symbol c_e)

A fictitious exhaust velocity that would theoretically produce the observed value of jet thrust.

The effective exhaust velocity c_e is determined by the equation c_e = V + [A(p₁ - p₂) g / w]where V is the velocity of the exhaust gases; A is the nozzle exit area; p₁ is static pressure at the nozzle exit; p₂ is ambient pressure; g is the acceleration of gravity; and w is the weight flow rate of exhaust gases.

effective multiplication factor (symbol k_e)

The ratio of the neutron flux in a nuclear reactor to that supplied by a neutron source.

effective neutron cycle time

The lifetime of an average neutron within a reactor from the time it is produced to the time it is fission captured.

This average takes into account delayed as well as prompt neutrons.

effective propagation velocity

The velocity of an electromagnetic signal which, when multiplied by the transit time for a ray path, gives a value for actual path length.

effective radiation = effective terrestrial radiation.

effective radius of the earth

A fictitious value for the radius of the earth, used in place of the geometrical radius to correct for atmospheric refraction when the index of refraction in the atmosphere changes linearly with height. See modified index of refraction.

Under conditions of standard refraction the effective radius of the earth is 8.5 * 10⁶ meters, or four-thirds the geometrical radius. If the effective radius is used in ray tracing diagrams, the rays may be drawn as though they were traveling in straight lines.

effective Reynolds number

A fictitious Reynolds number applied to the flow of air about a body in a wind tunnel, equal to the free-air Reynolds number at which the effect obtained is the same as the effect obtained in the wind tunnel.

effective sound pressure

The root-mean-square value of the instantaneous pressure of sound waves, taken over a complete cycle or a period long compared with a cycle, at that point. The unit is the microbar (dynes per square centimeter).

effective temperature

1. In astrophysics, a measure of the temperature of a star deduced by means of the Stefan-Boltzmann law, from the total energy emitted per unit area. Compare brightness temperature, color temperature.

Effective temperature is always less than actual temperature.

2. In physiology, the temperature at which motionless, saturated air would induce, in a sedentary worker wearing ordinary indoor clothing, the same sensation of comfort as that induced by the actual conditions of temperature, humidity, and air movement. Compare sensible temperature, standard operative temperature, operative temperature.

Effective temperature is used as a guide in air-conditioning practice, and, on the comfort chart (American Society of Heating and Air Conditioning Engineers) it appears as a family of curves which serves as one coordinate in defining comfort zones.

effective terrestrial radiation

The amount by which outgoing infrared terrestrial radiation of the earth's surface exceeds downcoming infrared counter-radiation from the atmosphere. Also called nocturnal radiation, effective radiation. See actinometer.

It is to be emphasized that this amount is a positive quantity, of the order of several tenths of a langley per minute, at all times of day (except under conditions of low overcast clouds). It typically attains its diurnal maximum during the midday hours when high soil temperatures create high rates of outgoing terrestrial radiation. (For this reason the synonym nocturnal radiation is apt to lead to slight confusion.) However, in daylight hours the effective terrestrial radiation is generally much smaller than the insolation, while at night it typically dominates the energy budget of the earth's surface.

effective wavelength

The wavelength corresponding to the effective propagation velocity.

effector

Any device used to maneuver a rocket in flight, such as an aerodynamic surface, a gimbaled motor, or a jet.

efficiency (symbol )

Of a device with respect to a physical quantity which may be stored, transferred, or transformed by the device, the ratio of the useful output of the quantity to its total input.

Unless specifically stated otherwise, the term efficiency means efficiency with respect to power.

egads (abbr) = electronic ground automatic destruct sequencer.

egads button

A button used by the range safety officer to initiate destruction of a rocket vehicle in flight if its course, as plotted during flight, is predicted to go beyond the destruct line. See egads, impact predictor system.

EHF (abbr) = Extremely High Frequency. See frequency band.

eigenmode = normal mode of vibration.

eigenvalue

See characteristic value problem.

eight ball

Common name given to a flight attitude indicator.

ejection capsule

1. In an aircraft or manned spacecraft, a detachable compartment serving as a cockpit or cabin, which may be ejected as a unit and parachuted to the ground.

2. A satellite, probe, or unmanned spacecraft, a boxlike unit, usually containing recording instruments or records of observed data, which may be ejected and returned to earth by a parachute or other deceleration device.

ejector

A device consisting of a nozzle, mixing tube, and diffuser utilizing the kinetic energy of a fluid stream to pump another fluid from a low pressure region by direct mixing and ejecting both streams.

Ekman layer

The layer of transition between the surface boundary layer, where shearing stress is constant, and the free atmosphere, where the atmosphere is treated as an ideal fluid in approximate geostrophic equilibrium. Also called spiral layer.

In Ekman's analysis (see Ekman spiral), the coefficient of eddy viscosity is assumed constant within this layer; subsequent calculations have relaxed this assumption.

Ekman spiral

As used in meteorology, an idealized mathematical description of the wind distribution in the planetary boundary layer of the atmosphere, within which the earth's surface has an appreciable effect on the air motion. The model is simplified by assuming that within this layer eddy viscosity and density are constant, the motion is horizontal and steady, the isobars are straight and parallel, and the geostrophic wind is constant with height.

elastic collision

A collision between two particles in which no change occurs in the internal energy of the particles, or in the sum of their kinetic energies. Commonly referred to as a billiard-ball collision.

elasticity

The ability of a body which has been deformed by an applied force to return to its original shape when the force is removed.

elasticizer

An elastic substance or fuel used in a solid rocket propellant to prevent cracking of the propellant grain and to bind it to the combustion-chamber case.

elastic wave

See sound.

elastomers

Rubber-like compounds.

Elastomers are used as pliable components, as in tires, seals, or gaskets.

E-layer

A division of the ionosphere, usually found at an altitude between 100 and 120 kilometers in the E-region. It exhibits one or more distinct maximums and sharp gradients of free electron density. It is most pronounced in the daytime but does not entirely disappear at night. Also called E₁ - layer, Kennelly-Heaviside layer, Heaviside layer. See sporadic E-layer (under ionosphere), atmospheric shell, ionosphere.

There is some evidence to indicate a second layer above the normal E-layer located at about 150 kilometers, and called the E₂-layer.

electrical

Involving the flow of electricity in a conductor. Compare electronic.

electrical distance

The distance between two points expressed in terms of the duration of travel of an electromagnetic wave in free space between the two points.

A convenient unit of electrical distance is the light microsecond or approximately 983 feet (300 meters). In the use of this unit, electrical distance is numerically equal to transmission time in microseconds.

electrical element

See element, sense 2.

electrical engine

A rocket engine in which the propellant is accelerated by some electrical device. Also called electric propulsion system, electric rocket.

Electrical engines can be classified as electrothermal, electrostatic, or electromagnetic, depending on the nature of the accelerating device.

electric-current element = electric dipole.

electric dipole

A pair of equal and opposite charges an infinitesimal distance apart.

In electromagnetics, the term dipole is often applied to two equal and opposite oscillating charges an infinitesimal distance apart; in this sense, it is synonymous with an electric-current element.

electric discharge

The flow of electricity through a gas, resulting in the emission of radiation that is characteristic of the gas and of the intensity of the current. Also called discharge, gaseous electric discharge, gaseous discharge. See corona discharge, point discharge, spark discharge, lightning discharge.

electric field

1. A region in which a charged particle would experience an electrical force; the geometric array of the imaginary electric lines of force that exist in relation to points of opposite charge.

An electric field is a vector field in which magnitude of the vector is the electric-field strength and the vector is parallel to the lines of force.

2. = electric-field strength. See atmospheric electric field.

electric-field intensity = electric-field strength.

electric-field strength

The electrical force exerted on a unit positive charge at a given point in space. Electric-field strength is expressed, in the practical system of electrical units, in terms of volts/centimeter. It is a vector quantity, being the magnitude of the electric-field vector. Also called electric field, electric intensity, electric field intensity, electric potential gradient, field strength.

The electric-field strength of the atmosphere is commonly referred to as the atmospheric electric field.

electric intensity = electric-field strength.

electric lines of force

Imaginary lines defined by the paths traced by unit charges placed in an electric field. Lines of force are everywhere parallel to the electric field strength vector. Their principal use is as a convenient means of picturing the geometry of an electric field. See magnetic lines of force.

electric potential

In electrostatics, the work done in moving unit positive charge from infinity to the point whose potential is being specified. Sometimes shortened to potential.

electric potential gradient = electric-field strength.

electric power level = level.

electric propulsion

A general term encompassing all the various types of propulsion in which the propellant consists of charged electrical particles which are accelerated by electrical or magnetic fields, or both; for example, electrostatic propulsion, electromagnetic propulsion, electrothermal propulsion.

electroacoustic transducer

A transducer for receiving waves from an electric system and delivering waves to an acoustic system, or vice versa.

Microphones and earphones are electroacoustic transducer.

electrochemical

See chemical energy.

electrochemical transducer

A transducer which uses a chemical change to indicate the input parameter.

electrode

1. A terminal at which electricity passes from one medium into another. The positive electrode is called anode; the negative electrode is called cathode.

2. In a semiconductor device, an element that performs one or more of the functions of emitting or collecting electrons or holes, or of controlling their movements by an electric field.

3. In electron tubes, a conducting element that performs one or more of the functions of emitting, collecting or controlling, by an electromagnetic field, the movements of electrons or ions. See anode (electron tubes) and cathode.

electrodynamics

The science dealing with the forces and energy transformations of electric currents, and the magnetic fields associated with them.

electrojet

A laterally limited relatively intense electric current located in the ionosphere.

electrokinetic transducer

A transducer that depends for its operation on the dielectric polarization in certain liquids resulting from viscous shearing stress that accompanies flow through porous materials.

electroluminescence

Emission of light caused by an application of electric fields to solids or gases.

In gas electroluminescence, light is emitted when the kinetic energy of electron or ions accelerated in an electric field is transferred to the atoms or molecules of the gas in which the discharge takes place.

electromagnetic

Of or pertaining to magnetism produced by or associated with electricity.

Magnetoelectric pertains to electricity produced by or associated with magnetism.

electromagnetic energy = electromagnetic radiation.

electromagnetic radiation

Energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields existing in space or in the media. The term radiation , alone, is used commonly for this type of energy, although it actually has a broader meaning. Also called electromagnetic energy or simply radiation. See electromagnetic spectrum.

electromagnetic rockets = plasma rockets. See electric propulsion.

electromagnetic spectrum

The ordered array of known electromagnetic radiations, extending from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other wavelengths of radio energy. See absorption spectrum.

The division of this continuum of wavelengths (or frequencies) into a number of named subportions is rather arbitrary and, with one or two exceptions, the boundaries of the several subportions are only vaguely defined. Nevertheless, to each of the commonly identified subportions there correspond characteristic types of physical systems capable of emitting radiation of those wavelengths. Thus, gamma rays are emitted from the nuclei of atoms as they undergo any of several types of nuclear rearrangements; visible light is emitted, for the most part, by atoms whose planetary electrons are undergoing transitions to lower energy states; infrared radiations are associated with characteristic molecular vibrations and rotations; and radio waves, broadly speaking, are emitted by virtue of the accelerations of free electrons as, for example, the moving electrons in a radio antenna wire.

electromagnetic theory

See electromagnetic radiation.

electromagnetic wave

A wave produced by oscillation of an electric charge. See electromagnetic radiation.

electromagnetism

1. Magnetism produced by an electric current.

2. The science dealing with the physical relations between electricity and magnetism.

electromechanical transducer

A transducer for receiving waves from an electric system and delivery waves to a mechanical system, or vice versa.

electrometeor

A visible or audible manifestation of atmospheric electricity. This includes, therefore, not only visible electric discharges (igneous meteors) but also the sounds produced by them, principally thunder.

electrometer

An instrument for measuring differences of electric potential.

electromyogram

A record of the response of a muscle to an electric stimulation.

electron

The subatomic particle that possesses the smallest possible negative electric charge (4.80298 * 10^-10 electrostatic units). See physical constants, table.

The mass of the electron is approximately equal to 1/1836 that of a hydrogen atom; its theoretical rest mass (symbol m_e) is equal to 9.109 3897(54) * 10^-31 kg and its rest energy is equal to 0.510 999 06(15) million electron-volt. The charge-to-mass ratio for the electron (symbol e/m_e) is 1.758796 cc^½ / g^½.
The term electron is usually reserved for the orbital or extranuclear particle, whereas the term beta particle refers to a nuclear electron.

*The numbers in bold type were taken from the 1987 publication of Fundamental Physical Constants: 1986 CODATA (Committee on Data for Science and Technology of the International Council of Scientific Unions) Recommended Values issued by the U. S. Department of Commerce, National Bureau of Standards.
"Digits in parentheses represent one standard deviation uncertainty in the final digits of the given value, based on a least-squares analysis with five variables and 17 degrees of freedom."

electron avalanche

The process in which a relatively small number of free electrons in a gas that is subjected to a strong electric field accelerate, ionize gas atoms by collision, and thus form new free electrons to undergo the same process in cumulative fashion.

An avalanche cannot begin until the local electric field strength is high enough to accelerate a free electron to the minimum ionizing speed in the space and time interval corresponding to one mean free path of the electron, for upon collision, the electron usually loses its forward motion in the direction of the field.

electron beam

Specifically, a focused stream of electrons used for neutralization of the positively charged ion beam in an ion engine. Also used to melt or weld materials with externally high melting points.

electron device

A device in which electricity is conducted principally by electrons moving through a vacuum, gas, or semiconductor.

electron gun

An electrode structure which produces and may control, focus, deflect, and converge one or more electron beams.

electronic

1. Involving the flow of electrons in a vacuum or through semiconductors.

2. Of or pertaining to electronics, i.e., to that branch of physics that treats of the emission, transmission, behavior, and effects of electrons, especially as applied by means of vacuum tubes, cathode-ray tubes, photoelectric cells, and the like, together with the associated electrical devices.

electronic Bohr magneton = Bohr magneton.

electronic data processing

The use of electronic devices and systems in the processing of data so as to interpret the data and put them into usable form.

electronic missile acquisition (abbr EMA)

A crossed baseline interferometer system giving azimuth and elevation angles.

This system was designed as an acquisition aid for theodolites. The EMA equipment operates on the Dovap transponder frequency.

electronics

That branch of physics that treats of the emission, transmission, behavior, and effects of electrons. See electronic.

electronic sky screen equipment

See elsse.

electronic transducer

A unilateral transducer that depends for its operation on the generation of a voltage by the relative motion of the electrodes in a vacuum tube.

electronic work function = Helmholtz function.

electron tube

A device in which conduction by electrons takes place through a vacuum or gaseous medium within a gastight envelope.

electron-volt (abbr ev)

A unit of energy equal to the energy required to move an electron through a potential difference of 1 volt. Often shortened to volt.

One electron volt equals 1.6020 * 10^-9 joule.

electrostatic memory

The ability of a substance or device to retain an electrostatic charge after the charging force is removed.

electrostatic rocket = ion rocket. See electric propulsion, ion engine.

electrostatic storage

In a computer, storage of information in the form of electrostatic charges.

electrostatic-storage tube

A cathode-ray tube in which information is stored as positive or negative charges on a dielectric surface.

electrostriction

The phenomenon wherein some dielectric materials experience an elastic strain when subjected to an electric field, this strain being independent of polarity of the field.

electrothermal rocket = electric thermal rocket. See electric propulsion.

element

1. One of the simple parts of which a complex entity is composed.

2. In chemistry, a substance which cannot be broken down by ordinary chemical means into simpler components.

3. In an electron tube, a constituent part of the tube that contributes directly to the electrical operation of the tube.

4. In a circuit, any electrical device (such an inductor, resistor, capacitor, generator, line, electron tube) with terminals at which it may be directly connected to other electrical devices.

5. In a semiconductor device, any integral part of the semiconductor device, any integral part of the semiconductor device that contributes to its operation.

6. = orbital element.

elevated pole

The celestial pole above the horizon.

The celestial pole below the horizon is called depressed pole.

elevation = angle of elevation.

elevation angle = angle of elevation.

ELF (abbr) = extremely low frequency. See frequency band.

ellipse

A plane curve constituting the locus of all points the sum of whose distances from two fixed points called focuses or foci is constant; an elongated circle. See conic section.

The orbits of planets, satellites, planetoids, and comets are ellipses, the primary being at one focus.

ellipsoid

A surface whose plane sections (cross sections) are all ellipses or circles, or the solid enclosed by such a surface. Also called ellipsoid of revolution, spheroid.

ellipsoid of revolution = ellipsoid.

It is so named from the fact that it can be formed by revolving an ellipse about one of its axes.

elliptic

Pertaining to an ellipse, or in the form of an ellipse.

elliptically polarized sound wave

A transverse wave in an elastic medium in which the displacement vector at any point rotates about the point and has a magnitude which varies as the radius vector of an ellipse.

An elliptically polarized wave is equivalent to two superposed plane polarized waves of simple sinusoidal form in which the displacements lie in perpendicular planes and are 90° apart in phase.

elliptical polarization

The polarization of a wave radiated by an electric vector rotating in a plane and simultaneously varying in amplitude so as to describe an ellipse.

elliptical system

A tracking or navigation system where ellipsoids of position are determined from time or phase summation relative to two or more fixed stations which are the focuses for the ellipsoids.

ellipticity (symbol e)

The amount by which a spheroid differs from a sphere or an ellipse differs from a circle, calculated by dividing the difference in the length of the axes by the length of the major axis. Also called compression. See flattening.

ellipticity ratio

1. The ratio of the major axis to the minor axis of an ellipse.

2. As a measure of elliptical polarization, the power ratio of the maximum to the minimum electric vectors of an elliptically polarized antenna.

elongation

The angular distance of a body of the solar system from the sun; the angle at the earth between lines to the sun and another celestial body of the solar system. The term is usually used only in connection with inferior planets.

The greatest elongation of such a body is its maximum angular distance from the sun before it starts back toward conjunction. The direction of the body east or west of the sun is usually specified, as greatest elongation east.

elsse (abbr) = Electronic sky screen equipment.

An electronic device which indicates the departure of a rocket from a predetermined trajectory.

EMA (abbr)= electronic missile acquisition.

embolism

Large amounts of air in the blood stream which, reaching the heart, cause it to fail; small amounts are resorbed and cause no symptoms.

emissance = emittance, sense 2.

emission

1. With respect to electromagnetic radiation, the process by which a body emits electromagnetic radiation as a consequence of its temperature only. Compare reflection, transmission. See emittance, emissivity.

2. With respect to electric propulsion and energy conversion, the sending out of charged particles from a surface causing the generation of these particles; e.g., emission of ions from an ionizing surface in ion engines.

emission line

A minute range of wavelength (or frequency) in the electromagnetic spectrum within which radiant energy is being emitted by a radiating substance. See spectral line, emission spectrum.

emission spectrum

The array of wavelengths and relative intensities of electromagnetic radiation emitted by a given radiator.

Each radiating substance has a unique, characteristic emission spectrum, just as every medium of transmission has its individual absorption spectrum.

emissive power

The rate of thermal emission of radiant energy per unit area of emitting surface. Usually called thermal emissive power.

emissivity (symbol )

A property of a material, measured as the emittance of a specimen of the material that is thick enough to be completely opaque and has an optically smooth surface.

emittance (symbol E, )

1. The radiant flux per unit area emitted by a body.

2. The ratio of the emitted radiant flux per unit area of a sample to that of a black body radiator at the same temperature and under the same conditions.

Spectral emittance refers to emittance measured at a specified wavelength.
Because of the two common meanings of emittance, it should be defined when used unless the context allows no misinterpretation.

emulsion

In photography, a light-sensitive coating on a film, plate, or paper. See nuclear emulsion.

emulsion plate

A plate with a photographic emulsion specially designed to permit observation of the individual tracks of ionizing particles.

enamel

A thin ceramic coating, usually of high glass content, applied to a substrate, generally a metal.

Enceladus

A satellite of Saturn orbiting at a mean distance of 238,000 kilometers.

encoder = analog to digital converter.

end-fire array

A linear antenna array whose direction of maximum radiation is along the axis of the array.

energy

Any quantity with dimensions mass * length squared divided time squared. Compare entropy.

energy conversion efficiency

The efficiency with which a nozzle converts the energy of the working substance into kinetic energy, expressed as the ratio of the kinetic energy of the jet leaving the nozzle to the kinetic energy of a hypothetical ideal jet leaving an ideal nozzle using the same working substance at the same initial state and under the same conditions of velocity and expansion.

energy density

The sound energy per unit volume in a sound wave. The unit is the erg per cubic centimeter.

energy density spectrum

The square of the amplitude of the (complex) Fourier transform of an aperiodic function. Sometimes called energy spectrum. See power spectrum.

energy equation

See thermodynamic energy equation, mechanical energy equation, total energy equation.

energy level

Any one of different values of energy which a particle, atom, or molecule may adopt under conditions where the possible values are restricted by quantizing conditions.

During transitions from one energy level to another, quanta of radiant energy are emitted or absorbed, their frequency depending on the difference between the energy levels.

energy management

In rocketry the monitoring of the expenditure of fuel for flight control and navigation.

energy spectrum = energy density spectrum.

engine

A machine or apparatus that converts energy, especially heat energy, into work. Also called motor.

engine control

Any control for regulating the power and speed of an engine, such as the throttle, mixture control, manifold-pressure regulator, fuel-pressure control, supercharger control, etc.

engine-exhaust trail = exhaust trail.

engine mount

A structure used for attaching an engine to a vehicle.

engine spray

That part of a pad deluge that is directed at cooling a rocket's engine or engines during launch.

English candle = international candle.

enhanced radiation

Increased radio wave or thermal radiation from the sun, of several hours or days duration.

Enhanced radiation is usually accompanied by many bursts.

enthalpy

A mathematically defined thermodynamic function of state h = u + pv where h is specific enthalpy; u is specific internal energy; p is pressure; and v is specific volume. Also called heat function.

The change in enthalpy measures the heat imparted to a system during a reversible isobaric process: dh = dq where dq is the heat increment per unit mass. For a perfect gas, dh = c_pdTwhere c_p is the specific heat at constant pressure and dT is the temperature increment.

entropy

1. A measure of the extent to which the energy of a system is unavailable. A mathematically defined thermodynamic function of state, the increase in which gives a measure of the energy of a system which has ceased to be available for work during a certain process: ds = (du + pdv)/T >= dq/T where s is specific entropy; u is specific internal energy; p is pressure; v is specific volume; T is Kelvin temperature; and q is heat per unit mass. For reversible processes, ds = dq/T In terms of potential temperature , ds = c_p (d/)where c_p is the specific heat at constant pressure. See third law of thermodynamics.

In an adiabatic process, the entropy increases if the process is irreversible and remains unchanged if the process is reversible. Thus, since all natural processes are irreversible, it is said that in an isolated system the entropy is always increasing as the system tends toward equilibrium, a statement which may be considered a form of the second law of thermodynamics.

2. In communication theory, average information content.

entry corridor

Depth of the region between two trajectories which define the design limits of a vehicle which will enter a planetary atmosphere.

envelope

1. Of a variable, a curve which bounds the values which the variable can assume, but does not consider possible simultaneous occurrences or correlations between different values.

2. The bounds within which a certain system can operate as a flight envelope, especially a graphic representation of these bounds showing interrelationships of operational parameters.

environment

An external condition or the sum of such conditions, in which a piece of equipment, a living organism, or a system operates as in temperature environment, vibration environment,or space environment.

Environments are usually specified by a range of values, and may be either natural or artificial.

environmental chamber

A chamber in which humidity, temperature, pressure, fluid contents, noise, and movement may be controlled so as to simulate different environments.

environmental lapse rate

The rate of decrease of temperature T with elevation z in the atmosphere, - T / z or occasionally T / p, where p is pressure. See autoconvective lapse rate, superadiabatic lapse rate.

The concept may be applied to other atmospheric variables (e.g., lapse rate of density) if these are specified. The environmental lapse rate is determined by the distribution of temperature in the vertical at a given time and place and should be carefully distinguished from the process lapse rate, which applies to an individual air parcel.

E₁-layer

See E-layer.

eosinophils

A type of white blood cell or leukocyte which stains a red color with eosin stain; normally about 2 to 3 percent of white cells in the blood but tending to decrease during stressful situations and thus usable as an index for stress.

ephemeris (plural, ephemerides)

A periodical publication tabulating the predicted positions of celestial bodies at regular intervals, such as daily, and containing other data of interest to astronomers.

A publication giving similar information useful to a navigator is called an almanac.

ephemeris day

86,400 ephemeris seconds. See ephemeris time.

ephemeris second (abbr s)

This was the fundamental unit of time of the International System of Units of 1960: 1/31556925.9747 of the tropical year defined by the mean motion of the sun in longitude at the epoch 1900 January 0 day 12 hours. See ephemeris time.

See second for the definition of the latest unit of time in the SI system or look on the WWW version of the National Institute of Standards and Technology: Physics Laboratory's International System of Units (SI).

ephemeris time (abbr E.T.)

The uniform measure of time defined by the laws of dynamics and determined in principle from the orbital motions of the planets, specifically the orbital motion of the earth as represented by Newcomb's Tables of the Sun. Compare universal time.

Beginning with the volume for 1960 the American Ephemeris and Nautical Almanac uses ephemeris time as the tabular argument in the fundamental ephemerides of the sun, moon, and planets.
A gravitational ephemeris expresses the position of a celestial body as a function of ephemeris time; and, at any instant, the measure of ephemeris time is the value of the argument at which the ephemeris position is the same as the actual position at the instant. The ephemeris time at any instant is obtained from observation by directly comparing observed position of the sun, moon, and planets with gravitational ephemerides of their coordinates; observations of the moon are the most effective and expeditious for this purpose. An accurate determination, however, requires observations over a more or less extended period; in practice, it takes the form of determining the time correction T that must be applied to universal time (U.T.) to obtain ephemeris time: E.T. = U.T. + TThe universal time at any instant may be obtained with little delay from observations of the dirunal motions.
The fundamental epoch from which ephemeris time is reckoned is the epoch that Newcomb designated as 1900 January 0, Greenwich mean noon, but which actually is 1900 January 0 day 12 hours E.T.; the instant to which this designation is assigned is the instant near the beginning of the calendar year A.D. 1900 when the geometric mean longitude of the Sun referred to the mean equinox of data was 279 degrees 41 minutes 48.04 seconds. Ephemeris time is the measure of time in which Newcomb's Tables of the Sun agree with observation.
The primary unit of ephemeris time is the tropical year, defined by the mean motion of the sun in longitude at the epoch 1900 January 0 day 12 hours E.T.; its length in ephemeris days is determined by the coefficient of T in Newcomb's expression for the geometric mean longitude of the sun L referred to the mean equinox of date, given among the elements of the sun.

epoch

A particular instant for which certain data are valid, as the data for which an astronomical catalogue is computed.

Eppley pyrheliometer

A pyrheliometer of the thermoelectric type. Radiation is allowed to fall on two concentric silver rings, the outer covered with magnesium oxide and the inner covered with lampblack. A system of thermocouples (thermopile) is used to measure the temperature difference between the rings. Attachments are provided so that measurements of direct and diffuse solar radiation may be obtained.
This instrument has been adopted by the U.S. Weather Bureau for station use.

_{For instrumentation of NOAA's Surface Radiation Research Branch see} http://www.srrb.noaa.gov/instrument/instpage.htm

Equ, Equl

International Astronomical Union abbreviations for Equuleus. See constellation.

equation

In astronomy, a small correction to observed values to remove the effects of systematic errors in an observation.

equation of state

An equation relating temperature, pressure, and volume of a system in thermodynamic equilibrium.

A large number of such equations have been devised to apply equally to gaseous and liquid phases throughout a wide range of temperatures and pressures. Of these, the simplest are the perfect gas law and Van der Waal equation.

equation of time

Prior to 1965, the difference between mean time and apparent time, usually labeled + or - as it is to be applied to mean time to obtain apparent time. After 1965, the correction to be applied to 12 hours + local mean time (LMT) to obtain the local hour angle (LHA) of the sun.

equation of wave motion = wave equation.

equations of motion

A set of equations which give information regarding the motion of a body or of a point in space as a function of time when initial position and initial velocity are known. See Newton laws of motion, Eulerian coordinates.

equator

The primary great circle of a sphere or spheroid, such as the earth, perpendicular to the polar axis; or a line resembling or approximating such a circle.

The terrestrial equator is 90° from the earth's geographical poles; the celestial equator or equinoctial is 90 ° from the celestial poles; the galactic equator or galactic circle is 90° from the galactic poles. The astronomical equator is a line connecting points having 0° astronomical latitude; the geodetic equator connects points having 0° geodetic latitude. The expression terrestrial equator is sometimes applied to the astronomical equator. The geodetic equator is shown on charts. A fictitious equator is a reference line serving as the origin for measurement of fictitious latitude. A transverse or inverse equator is a meridian the plane of which is perpendicular to the axis of a transverse projection. An oblique equator is a great circle the plane of which is perpendicular to the axis of an oblique projection. A grid equator is a line perpendicular to a prime grid meridian at the origin. The magnetic equator or aclinic line is that line on the surface of the earth connecting all points at which the magnetic dip is zero. The geomagnetic equator is the great circle 90° from the geomagnetic poles of the earth.

equatorial bulge

The excess of the earth's equatorial diameter over the polar diameter.

equatorial electrojet

See electrojet.

equatorial satellite

A satellite whose orbit plane coincides, or almost coincides, with the earth's equatorial plane.

equatorial system

A set of celestial coordinates based on the celestial equator as the primary great circle; usually declination and hour angle or sidereal hour angle. Also called equinoctial system of coordinates, celestial equator system of coordinates.

equigeopotential surface = geopotential surface.

equilibrium flow

Gas flow in which energy is constant along streamlines and composition of the gas at any point is not time dependent.

equilibrium glide

Gliding flight in which the sum of the vertical components of the aerodynamic lift and centrifugal force is equal to the force of gravity.

equilibrium spheroid

The shape that the earth would attain if it were entirely covered by a tideless ocean of constant depth. Compare geoid.

equilibrium vapor pressure

The vapor pressure of a system in which two or more phases of a substance coexist in equilibrium. See vapor tension.

In meteorology the reference is to water substance unless otherwise specified.

equinoctial = celestial equator.

equinoctial colure

That great circle of the celestial sphere through the celestial poles and equinoxes; the hour circle of the vernal equinox.

equinoctial day = sidereal day.

equinoctial point

One of the two points of intersection of the ecliptic and the celestial equator. Also called equinox.

equinoctial system of coordinates = celestial equator system of coordinates.

equinoctial year = tropical year.

equinox

1. One of the two points of intersection of the ecliptic and the celestial equator, occupied by the sun when its declination is 0°. Also called equinoctial point.

That point occupied on or about March 21, when the sun's declination changes from south to north, is called vernal equinox, March equinox, or first point of Aries; that point occupied on or about September 23, when the declination changes from north to south, is called autumnal equinox, September equinox, or first point of Libra.
Equinox is often used to mean vernal equinox, when referring to the origin of measurement of right ascension and celestial longitude.

2. That instant the sun occupies one of the equinoctial points.

equivalent-barotropic atmosphere

See equivalent-barotropic model.

equivalent-barotropic model

A model atmosphere characterized by (a) frictionless and adiabatic flow (b) hydrostatic and quasigeostrophic equilibrium, and in which (c) the vertical shear of the horizontal wind is assumed to be proportional to the horizontal wind itself.

An equivalent-barotropic atmosphere is, accordingly, an atmosphere in which the wind does not change direction with height and consequently one in which the contours and isotherms (on isobaric surfaces, for example) are everywhere parallel. In such an atmosphere, the vertically averaged motions are presumably equivalent to those at some intermediate level, the equivalent-barotropic level. In terms of the motion at this level, assumed to be an isobaric surface, the behavior of the equivalent-barotropic model may be described by a single equation (the vorticity equation) in a single unknown (the height of the isobaric surface). See barotropic vorticity equation.

equivalent binary digits

1. The number of binary places required to handle the largest quantity which can be handled in some other notation. For instance 3.323 binary digits are required to convey information equivalent to that conveyed by one decimal digit.

2. The number of places required to express in binary notation a quantity in some other notation.

equivalent foot-candle = foot-lambert.

equivalent pendulum

A device, usually incorporating accelerometers and gyros, which has the same response to acceleration as a pendulum with a specific period. See Schuler tuning.

equivalent potential temperature

The potential temperature corresponding to the adiabatic equivalent temperature: _e = T_{a, e} (1000 / p)^0.286where _e is the equivalent potential temperature; T_{a, e} is the adiabatic equivalent temperature; and p is the pressure in millibars. This temperature is conservative with respect to dry-adiabatic and pseudoadiabatic processes.

equivalent temperature

1. Isobaric equivalent temperature; the temperature that an air parcel would have if all water vapor were condensed out at constant pressure, the latent heat released being used to heat the air, T_{i, e} = T [1 + (Lw / c_p T)]where T_{i, e} is the isobaric equivalent temperature; T is the temperature; w is the mixing ratio; L is the latent heat; and c_p is the specific heat of air at constant pressure.

2. Adiabatic equivalent temperature; The temperature that an air parcel would have after undergoing the following (physically unrealizable) process: dry-adiabatic expansion until saturated; pseudoadiabatic expansion until all moisture is precipitated out; dry-adiabatic compression to the initial pressure. This is the equivalent temperature as read from a thermodynamic chart and is always greater than the isobaric equivalent temperature: T_{a, e} = T exp (Lw / c_p T)where T_{a, e} is the adiabatic equivalent temperature. Also called pseudoequivalent temperature.

equivalent width

In spectrography, a measure of the total absorption of radiant energy as indicated by an absorption line or absorption band. Compare line width.

The formula for equivalent width W is where A is the fraction of incident radiation which is absorbed at any wavelength, and ₁ and ₂ are wavelengths, on opposite sides of the line or band, where the absorption has dropped to zero.
Thus, in a plot of A against , the equivalent width represents the area under the curve, or the width of a fictitious line or band which absorbs completely throughout its extent but which absorbs the same total amount of energy as the actual line or band.

Equl

International Astronomical Union abbreviation for Equuleus. See constellation.

Equuleus (abbr Equ, Equl)

See constellation.

eradiation

1. Same as radiation, with respect to emission.

2. = terrestrial radiation.

erase

In computer terminology, to expunge, wipe out, or destroy stored information, usually without destroying the storage media, as in demagnetizing a magnetic tape.

erector

A vehicle used to support a rocket for transportation and for placing the rocket in an upright position within a gantry.

E-region

The region of the ionosphere in which the E-layer tends to form. See atmospheric shell.

The E-layer has been observed to be subdivided into two or more layers, and these are then assigned the designation, E₁, E₂, etc. Patchy and intermittent clouds of fairly high ionization, known as sporadic E-layers, also form in the same general region.

erf = probability integral.

erg

The unit of energy or work in the centimeter-gram-second system; the work performed by a force of 1 dyne acting through a distance of 1 centimeter.

ergometer

An instrument for measuring muscular work.

Eri

International Astronomical Union abbreviation for Eridanus. See constellation.

Erid

International Astronomical Union abbreviation for Eridanus. See constellation.

Eridanus (abbr Eri, Erid)

See constellation.

erosion gage

An instrument for measuring the effect of dust and micrometeors on materials exposed to space environment.

erratic error

An error caused by an incomplete element in an instrument.

An example of an erratic error is backlash in a gear train.

error

1. In mathematics, the difference between the true value and a calculated or observed value.

A quantity (equal in magnitude to the error) added to a calculated or observed value to obtain the true value is called a correction.

2. In a computer or data-processing system, any incorrect step, process, or result.

In addition to the mathematically usage, in the computer field the term is also commonly used to refer to machine malfunctions as machine errors and to human mistakes as human errors. It is frequently helpful to distinguish between these as follows: errors result from approximations used in numerical methods; mistakes result from incorrect programming, coding, data transcription, manual operation, etc.; malfunction result from failures in the operation of machine components such as gates, flip-flops, amplifiers, etc.

error band

An error value, usually expressed in percent of full scale, which defines the maximum allowable error permitted for a specified combination of transducer parameters.

error coefficients

Partial derivatives showing the variation of a function of several variables, with one of these variables. These derivatives are used as coefficients of the variables being changed in a series representation of the total variation of the function.

error function = probability integral.

error signal

A voltage the magnitude of which is proportional to the difference between an actual and a desired position.

In radar use, error signals are obtained from selsyns and from automatic gain control circuits and are used to control a servo system so that the resultant motions tend to correct the error.

ertor

The effective temperature (radiational) of the earth's ozone layer.

erythema

A reddening of the skin due to capillary dilation.

Several forms of erythema can be caused by undue exposure of the human body to weather elements. The most common is sunburn.

E-scan = E-display.

escape

Of a particle or larger body: to achieve an escape velocity and a flightpath outward from a primary body so as neither to fall back to the body nor to orbit it.

escape rocket

A small rocket engine attached to the leading end of an escape tower, which may be used to provide additional thrust to the capsule to obtain separation of the capsule from the booster vehicle in an emergency.

escape speed = escape velocity.

escape tower

A trestle tower place on top of a space capsule, which during liftoff connects the capsule to the escape rocket.

The escape tower is of such length as to protect the capsule from the heat of the escape rocket in case the rocket is used to separate the capsule from the booster vehicle during ascent. The tower is ultimately separated from the capsule if ascent is normal.

escape velocity

The radial speed which a particle or larger body must attain in order to escape from the gravitational field of a planet or star. When friction is neglected, the escape velocity is where G is the universal gravitational constant (see gravitation); m is the mass of the planet or star; and r is the radial distance from the center of the planet or star. Also called escape speed.

Escape velocity from Earth is 7 miles/sec; from Mars it is 3.2 miles/sec; and from the Sun it is 390 miles/sec. In order for a celestial body to retain an atmosphere for long periods of time, the mean velocity of the atmospheric molecules must be considerably below the escape velocity.

E-scope = E-display.

E.T. (abbr) = ephemeris time.

etiology

The doctrine of causes, particularly the causes and reasons for diseases.

E₂-layer

See E-layer note.

Eulerian angles

A system of three angles which uniquely define with reference to one coordinate system (e.g., earth axes), the orientation of a second coordinate system (e.g., body axes). Any orientation of the second system is obtainable from that of the first by rotation through each of the three angles in turn, the sequence of which is important.

Eulerian coordinates

Any system of coordinates in which properties of a fluid are assigned to points in space at each given time, without attempt to identify individual fluid parcels from one time to the next. See equations of motion. Compare Lagrangian coordinates.

Eulerian coordinates are to be distinguished from Lagrangian coordinates. The particular coordinate system used to identify points in space (Cartesian, cylindrical, spherical, etc.) is quite independent of whether the representation is Eulerian or Lagrangian.

Eulerian correlation

The correlation between the properties of a flow at various points in space at a single instant of time. Sometimes called synoptic correlation. Compare Lagrangian correlation.

Eulerian equations

Any of the fundamental equations of hydrodynamics expressed in Eulerian coordinates. These are so commonly used that the designation Eulerian is often omitted.

Europa

A satellite of Jupiter orbiting at a mean distance of 671,000 kilometers. Also called Jupiter II.

evaporation

The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation. Also called vaporization.

In meteorology, evaporation usually is restricted in use to the change of water from liquid to gas, while sublimation is used for the change from solid to gas as well as from gas to solid.
According to the kinetic theory of gases, evaporation occurs when liquid molecules escape into the vapor phase as a result of the chance acquisition of above average, outward-directed, translational velocities at a time when they happen to lie within about one mean free path below the effective liquid surface. It is conventionally stated that evaporation into a gas ceases when the gas reaches saturation. In reality, net evaporation does cease, but only because the numbers of molecules escaping from and returning to the liquid are equal, that is, evaporation is counteracted by condensation.
Energy is lost by an evaporating liquid; and, when no heat is added externally, the liquid always cools. The heat thus removed is termed the latent heat of vaporization.

evaporation coefficient

The ratio of the actual evaporation rate to the maximum or Knudsen rate of evaporation.

evaporation rate

1. The mass of material evaporated per unit from unit surface of a liquid or solid.

2. The number of molecules of a given substance evaporated per second per square centimeter from the free surface of the condensed phase.

evection

A perturbation of the moon in its orbit due to the attraction of the sun. This results in an increase in the eccentricity of the moon's orbit when the sun passes the moon's line of apsides and a decrease when perpendicular to it. See lunar inequality.

Evection amounts to 1 degree 15 minutes in the moon's longitude at maximum.

exchange coefficients

Coefficients of eddy flux (e.g., of momentum, heat, water vapor, etc.) in turbulent flow, defined in analogy to those of the kinetic theory of gases (see eddy). Also called austausch coefficients, eddy coefficients, interchange coefficients.

The exchange-coefficient hypothesis states that the mean eddy flux per unit area of a conservative quantity (suitability expressed) is proportional to the gradient of the mean value of this quantity, that is, Mean flux per unit area = -C_e (d/ dN)where C_e is the exchange coefficient; E is the mean value of the quantity; and N is the direction normal to the surface. In strict analogy to molecular properties; C_e would be constant, for turbulent flow C_e turns out to depend on time and location. See eddy viscosity diffusivity.

excitation

1. An external force, or other input, applied to a system that causes the system to respond in some way. Also called stimulus.

2. The increase in the internal energy of an atomic or molecular system caused by a collision with another particle of greater energy.

For atoms in a discharge, excitation usually refers to increasing the energy level of a bound electron.

excited atom

An atom with one or more of its bound electrons in an increased energy level.

exclusive OR circuit

A circuit which produces an output signal when any one, but not more than one, input is in its prescribed state. Also called AND-NOT gate.

exhaust deflecting ring

A type of jetavator consisting of a ring so mounted at the end of a nozzle as to permit it to be rotated into the exhaust stream.

exhaust stream

The stream of gaseous, atomic, or radiant particles that emit from the nozzle of a rocket or other reaction engine.

exhaust trail

A condensation trail that forms when the water vapor of an aircraft exhaust is mixed with and saturates (or slightly supersaturates) the air in the wake of the aircraft. Exhaust trails are of more common occurrence and of longer duration than aerodynamic trails. Also called engine-exhaust trail.

exhaust velocity

The velocity of gaseous or other particles (exhaust stream) that exhaust through the nozzle or a reaction engine, relative to the nozzle.

exobiology

That field of biology which deals with the effects of extraterrestrial environments on living organisms and with the search for extraterrestrial life.

exosphere

The outermost, or topmost, portion of the atmosphere. Its lower boundary is the critical level of escape, variously estimated at 500 to 1000 kilometers above the earth's surface. Also called region of escape. See atmospheric shell.

In the exosphere, the air density is so low that the mean free path of individual particles depends upon their direction with respect to the local vertical, being greatest for upward moving particles (see cone of escape, fringe region). It is only from the exosphere that atmospheric gases can, to any appreciable extent, escape into outer space.

exospheric

Of or pertaining to the exosphere.

exotic fuel

Any fuel considered to be unusual, as a boron-base fuel.

exotic material

Any structural material which is not presently used in great quantities in conventional applications. Usually, materials with melting points above 3000° F.

expandable space structure

A structure which can be packaged in a small volume for launch and then erected to its full size and shape outside the earth's atmosphere.

expansion wave

A simple wave or progressive disturbance in the isentropic flow of a compressible fluid, such that the pressure and density of a fluid particle decrease on crossing the wave in the direction of its motion. Also called rarefaction wave. See compressional wave.

expiratory reserve

The volume of air that can be expelled from the lungs after a normal expiration.

explement

An angle equal to 360° minus a given angle. Thus, 150 ° is the explement of 210° and the two are said to be explementary. See complement, supplement.

explementary angles

Two angles whose sum is 360°.

explosion

1. The sudden production of a large quantity of gas, usually hot, from a much smaller amount of a gas, liquid, or solid.

2. Specifically, an explosion, sense 1, produced by combustion of a fuel and an oxidizer.

The distinction between an explosion, sense 2. and a detonation is that in an explosion the heat release rate and the number of molecules per unit volume increase with time more or less uniformly, whereas a detonation is propagated by an advancing shock front behind which exothermic reactions take place and thus is (spatially) nonuniform.

explosion turbine

A turbine rotated by gases from an intermittent combustion process taking place in a constant-volume chamber.

explosive bolt

A bolt incorporating an explosive which can be detonated on command, thus destroying the bolt. Explosive bolts are used, for example, in separating a satellite from a rocket.

explosive decompression

A very rapid reduction of air pressure inside a cabin, coming to a new static condition of balance with the external pressure.

exponential atmosphere

1. = isothermal atmosphere.

2. An atmosphere in which the density is given by = _o e^-h/Hwhere is density, _o is density at the datum plane, h is altitude, and H is scale height.

exposure suit

A suit designed to protect a person from a harmful natural environment, such as cold water.

extended range Dovap (abbr Extradop)

A baseline extension of the Dovap system to provide a coherent reference to the ground transmitter and all Dovap receivers located beyond line-of-sight to the ground transmitter.

The coherent reference is supplied by a cable and is multiplied up to the proper reference frequency.

extensive air shower = Auger shower.

exterior ballistics

That branch of ballistics that deals with the motion of projectiles in flight.

external storage

In computer terminology, storage media separate from the machine but capable of retaining information in a form acceptable to the machine, as floppy disks, removable reels of magnetic tape or decks of punched cards.

extinction

The attenuation of light; that is, the reduction of illuminance of a collimated beam of light as the light passes through a medium wherein absorption and scattering occur.

extinction coefficient

In meteorology, a measure of the space rate of diminution, or extinction, of any transmitted light; thus, it is the attenuation coefficient applied to visible radiation. The extinction coefficient is identified as dI = -I dx or I = I₀ e^-x where I is the illuminance (luminous flux density) at the selected point in space, I₀ is the illuminance at the light source; and x is the distance from the source.

When so used, the extinction coefficient equals the sum of the medium's absorption coefficient and scattering coefficient, each computed as a weighted average over all wavelengths in the visible spectrum. As long as scattering effects are primary, as in the lower atmosphere, the value of the extinction coefficient is a function of the particle size of atmospheric suspensoids. It varies in order of magnitude from 10 per kilometer with very low visibility to 0.01 per kilometer in very clear air.

extinction cross section = scattering cross section.

Extradop (abbr) = extended range Dovap.

extragalactic

Outside our galaxy, which is the Milky Way.

extraordinary ray

The refracted component of a beam of radiation split by having passed through a doubly refracting substance. The other component is called the ordinary ray. See magnetic double refraction.

extraterrestrial life

Life forms evolved and existing outside the terrestrial biosphere.

extraterrestrial radiation

In general, solar radiation received just outside the earth's atmosphere.

extremely high frequency (abbr EHF)

See frequency bands.

extremely low frequency (abbr ELF)

See frequency bands.

extreme value

In statistics, the upper or lower bound of the random variable which is not expected to be exceeded by a specified percentage of the population within a given confidence interval.

eyeballs in, eyeballs out, eyeballs down, eyeballs up, eyeballs left, eyeballs right. See physiological acceleration.