L

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

Referring to the labyrinth of the inner ear which acts as an acceleration sensor.

Lacerta (abbr Lac, Lacr)

See constellation.

lag

1. The delay between change of conditions and the indication of the change on an instrument.

2. Delay in human reaction.

3. The amount one cyclic motion is behind another, expressed in degrees. The opposite is lead.

lag coefficient = time constant.

Lagrangian coordinates

1. A system of coordinates by which fluid parcels are identified for all time by assigning them coordinates which do not vary with time. Examples of such coordinates are: (a) the values of any properties of the fluid conserved in the motion; or (b) more generally, the positions in space of the parcels at some arbitrarily selected moment. Subsequent positions in space of the parcels at some arbitrarily selected moment. Subsequent positions in space of the parcels are then the dependent variables, functions of time and of the Lagrangian coordinates. Also called material coordinates. Compare Eulerian coordinates. See Lagrangian equations.

2. Same as generalized coordinates.

Lagrangian correlation

The correlation between the properties of a flow following a single parcel of fluid through its space and time variations. Compare Eulerian correlation. See correlation coefficients.

Lagrangian correlation coefficient

See correlation coefficient.

Lagrangian equations

Any of the fundamental equations of hydrodynamics expressed in Lagrangian coordinates.

Lagrangian point

One of the five solutions by Lagrange to the three-body problem in which three bodies will move as a stable configuration. In three of the solutions the bodies are in line; in the other two the bodies are at the vortices of equilateral triangles.

Lagrange predicted in 1772 that if the three bodies form an equilateral triangle revolving about one of the bodies, the system would be stable. This prediction was fulfilled in 1908 with the discovery of the asteroid Achilles approximately 60 degrees ahead of Jupiter in Jupiter's orbit. Since then other asteroids have been discovered 60 degrees ahead and 60 degrees behind Jupiter.

lambert (abbr L or l)

A unit of luminance (or brightness) equal to 1/pi candle per square centimeter. Physically, the lambert is the luminance of a perfectly diffusing white surface receiving an illuminance of 1 lumen per square centimeter.

Lambert law

A law of physics which states that the radiant intensity (flux per unit solid angle) emitted in any direction from a unit radiating surface varies as the cosine of the angle between the normal to the surface and the direction of the radiation. The radiance (or luminance) of a radiating surface is, therefore, independent of direction. Also called Lambert cosine law. Compare cosine law of illumination.

Lambert law is not obeyed exactly by most real surfaces, but an ideal black body emits according to this law. This law is also satisfied (by definition) by the distribution of radiation from a perfectly diffuse radiator and by the radiation reflected by a perfectly diffuse reflector. In accordance with Lambert law, an incandescent special black body when viewed from a distance appears to be simply a uniformly illuminated disk. This law does not take into account any effects that may alter the radiation after it leaves the source.

Lambert law of absorption = Bouguer law.

laminar boundary layer

In fluid flow, layer next to a fixed boundary. The fluid velocity is zero at the boundary but the molecular viscous stress is large because the velocity gradient normal to the wall is large. See turbulent boundary layer.

The equations describing the flow in the laminar boundary layer are the Navier-Stokes equations containing only the inertia and molecular viscous terms.

laminar flow

In fluid flow, a smooth flow in which no crossflow of fluid particles occur between adjacent streamlines; hence, a flow conceived as made up of layers - commonly distinguished from turbulent flow.

Landau damping

The damping of a space charge wave by electrons which move at the phase velocity of the wave and gain energy transferred from the wave.

landing gear

The apparatus comprising those components of an aircraft or spacecraft that support and provide mobility for the craft on land, water, or other surface. The landing gear consists of wheels, floats, skis, bogies, and treads, or other devices, together will all associated struts, bracing shock absorbers, etc.

Landing gear includes all supporting components, such as the tail wheel or tail skid, outrigger wheels or pontoons, etc., but the term is often conceived to apply only to the principle components, i.e., to the main wheels, floats, etc., and the nose gear, if any. See auxiliary landing gear.

landing skid

A skid or runner used in the main landing gear of an aerodynamic vehicle, upon which the vehicle slides over the ground.

land mile = statute mile (5280 feet).

land return = ground return.

Langevin ion = large ion.

langley

A unit of energy per unit area, equal to 1 gram-calorie per square centimeter, commonly employed in radiation theory.

The langley is almost always used, in conjunction with some time unit, to express a flux density; but the time unit has been purposely separated in order that it may be chosen in a manner convenient to each particular problem. The unit is named in honor of the American scientist, Samuel P. Langley (1834-1906), who made many contributions to the knowledge of solar radiation.

Langmuir probe

A small metallic conductor or pair of conductors inserted within a plasma in order to sample the plasma current.

In some cases, the plasma density, electron temperature, and plasma potential can be inferred from a measurement with a Langmuir probe.

Langmuir rate of evaporation = maximum evaporation rate.

language

In electronic computers:

1. A system consisting of (a) a well-defined, usually finite, set of characters; (b) rules for combining characters with one another or form words or other expressions; and (c) a specific assignment of meaning to some of the words or expressions, usually for communicating information or data among a group of people, machines, etc.

2. A system similar to the above but without any specific assignment of meanings. Such systems may be distinguished from sense 1 above, when necessary, by referring to them as formal or uninterpreted languages. See code, machine language.

Although it is sometimes convenient to study a language independently of any meanings, in all practical cases at least one set of meanings is eventually assigned.

lap belt

A safety belt that fastens across the lap. This is the usual kind of safety belt. Also called a seat belt.

Laplace equation

1. The elliptic partial differential equation

where

is a scalar function of position, and

² is the Laplacian operator. In rectangular Cartesina coordinates x, y, z , this equation may be written

The Laplace equation is satisfied, for example, by the velocity potential in an irrotational flow, by gravitational potential in free space, by electrostatic potential in the steady flow of electric currents in solid conductors, and by the steady-state temperature distribution in solids. A solution of the Laplace equation is called a harmonic function. Compare Poisson equation.

2. An equation for the speed of sound. See Laplacian speed of sound.

Laplace operator = Laplacian operator.

Laplace transform

An integral transform of a function obtained by multiplying the given function f(t) by e^-pt, where p is a new variable, and integrating with respect to t from t = 0 to t =

. Also called Laplace transformation.

Thus, the Laplace transform of f(t) is

Fourier transform.

Laplace transformation = Laplace transform.

Laplacian operator

The mathematical operator

² =

(or sometimes written Δ) where

is the del-operator. In rectangular Cartesian coordinates, the Laplacian operator may be expanded in the form

Also called Laplace operator. See Laplace equation.

Laplacian speed of sound

The phase speed of a sound wave in a compressible fluid if the expansions and compression are assumed to be adiabatic. This speed a is given by the formula

a² = (c_p/c_v) RT

where c_p and c_v are the specific heats at constant pressure and volume, respectively; R is gas constant; and T is the Kelvin temperature. The value of this speed under standard conditions in dry air is 331 meters per second. Compare Newtonian speed of sound. See acoustic velocity.

lapse rate

The decrease of an atmospheric variable with height, the variable being temperature, unless otherwise specified.

The term applies ambiguously to the environmental lapse rate and the process lapse rate, and the meaning must often be ascertained from the context.

large calorie = kilogram calorie. See calorie.

large ion

An atmospheric ion of relatively large mass and low mobility which is produced by the attachment of a small ion to an Aitken nucleus. Also called slow ion, heavy ion, Langevin ion.

The ion density of large ions varies widely, depending upon the degree of atmospheric pollution. Representative low-altitude values might be 1000 per cubic centimeter in clean country air, 10,000 per cubic centimeter in an industrial area, and 100 per cubic centimeter over the oceans.

Larmor frequency

See cyclotron frequency.

Larmor orbit

The circular motion of a charged particle in a uniform magnetic field.

Whereas the motion of the particle is unimpeded along the magnetic field, motion perpendicular to the field is always accompanied by a force perpendicular to the direction of motion and the field. The electron or ion will orbit in a plane perpendicular to the magnetic field. By adding any arbitrary velocity along the magnetic field, the total path looks like a helix. The size of the Larmor orbit or helix is proportional to the particle velocity divided by the magnetic field. In a 1-gauss field, a 1-volt electron has an orbit of about 3 centimeters, and a 1-volt proton an orbit of about 1 meter.

laser

(From light amplication by stimulated emission of radiation). A device for producing light by emission of energy stored in a molecular or atomic system when stimulated by an input signal.

last quarter

The phase of the moon when it is near west quadrature, when the eastern half of it is visible to an observer on the earth.

latch

A device that fastens one thing to another, as a rocket to a launcher, but is subject to ready release so that the things may be separated.

latency

Of a computer: the time required to establish communication with a specific storage location, not including transfer time; equals access time less word time.

latent heat

The unit quantity of heat required for isothermal change in state of a unit mass of matter.

Latent heat is termed heat of fusion, heat of sublimation, heat of vaporization, depending on the change of state involved.

lateral

1. Of or pertaining to the side; directed or moving toward the side.

2. Of or pertaining to the lateral axis; directed, moving, or located along, or parallel to, the lateral axis.

lateral acceleration

Acceleration substantially along the lateral axis of an aircraft, rocket, etc.

lateral mirage

See mirage.

latitude

Angular distance from a primary great circle or plane. See coordinate, table.

Terrestrial latitude is angular distance from the equator, measured northward or southward through 90 degrees and labeled N or S to indicate the direction of measurement; astronomical latitude is angular distance between the direction of gravity and the plane of the equator; geodetic or topographical latitude is angular distance between the plane of the equator and a normal to the spheroid; geocentric latitude is the angle between a line to the center of the earth and the plane of the equator. Geodetic and sometimes astronomical latitude are also called geographic latitude. Geodetic latitude is used for charts. Assumed latitude is the latitude at which an observer is assumed to be located for an observation or computation. Fictitious latitude is angular distance from a fictitious equator. Grid latitude in angular distance from a grid equator. Transverse or inverse latitude is angular distance from a transverse equator. Oblique latitude is angular distance from an oblique equator. Difference of latitude is the shorter arc of any meridian between the paralles of two places, expressed in angular measure. Magnetic latitude, magnetic inclination, or magnetic dip is angular distance between the horizontal and the direction of a line of force of the earth's magnetic field at any point. Geomagnetic latitude is angular distance from geomagnetic equator. A parallel of latitude is a circle (or approximation of a circle) of the earth, parallel to the equator, and connecting points of equal latitude; or a circle of the celestial sphere, parallel to the ecliptic. Celestial latitude is angular distance north or south of the ecliptic. Galactic latitude is angular distance North or south of the galactic equator. See variation of latitude.

lattice

1. In nuclear physics, a geometric pattern, as, the pattern in which fuel and moderator are interspersed in a heterogeneous reactor.

2. Short for crystal lattice.

launch

1. The action taken in launching a rocket from the surface.

2. The resultant of this action, i.e., the transition from static repose to dynamic flight by the rocket.

3. The time at which this takes place.

4. The action of sending forth a rocket, probe, or other object from a moving vehicle, such as an aircraft or spacecraft. See lift-off.

launch

1. To send off a rocket vehicle under its own rocket power, as in the case of guided aircraft rockets, artillery rockets, and space vehicles.

2. To send off a missile or aircraft by means of a catapult, as in the case of the V-1, or by means of inertial force, as in the release of a bomb from a flying aircraft.

3. To give a space probe an added boost for flight into space just before separation from its launch vehicle.

This term has different connotations than those of fire and shoot. See lift-off.

launch azimuth

The initial heading of a powered vehicle at launch, commonly applied to launch vehicles.

launch complex

The site, facilities, and equipment used to launch a rocket vehicle. See launch site.

The complex differs according to the type rocket or particular rocket, or according to whether land launched or ship launched. The term is sometimes considered to include the launch crew.

launch crew

A group of technicians that prepares and launches a rocket.

launch emplacement

A launch pad with associated equipment.

launcher

1. Specifically, a structure or device, often incorporating a tube, a group of tubes, or a set of tracks, from which self-propelled missiles are sent forth and by means of which the missiles usually are aimed or imparted initial guidance - distinguished in this specific sense from a catapult.

2. Broadly, a structure, machine, or device, including the catapult, by means of which airplanes, rockets, or the like are directed, hurled, or sent forth.

launching angle

The angle between a horizontal plane and the longitudinal axis of a rocket, etc., being launched.

launching base

An area such as Cape Kennedy or Vandenberg Air Force Base that has several launch sites.

launching pad

A launch pad.

launching rack

A skeletonlike structure, usually incorporating rails, from which something is launched.

launching rail

A rail that gives initial support and guidance to a rocket launched in a nonvertical position.

launching site = launch site.

launch pad

The load-bearing base or platform from which a rocket vehicle is launched. Usually called pad.

launch point

The geographic position from which a rocket vehicle is launched.

launch site

1. A defined area from which a rocket vehicle is launched, either operationally or for test purposes; specifically, at Cape Kennedy or Vandenberg, any of the several areas equipped to launch a rocket.

2. More broadly, a launching base. Also called launching site.

launch stand

A facility or station at which a rocket vehicle is launched, normally incorporating a launch pad with launcher. Compare test stand.

launch vehicle

A rocket or other vehicle used to launch a probe, satellite, or the like.

launch window

The postulated opening in the continuum of time or of space, through which a spacecraft or missile must be launched in order to achieve a desired encounter, rendezvous, impact, or the like. See window.

Laval nozzle = de Laval nozzle.

law of conservation of momentum

See Newton laws of motion.

law of equal areas = Kepler second law.

laws of motion

See Newton laws of motion.

layer

Of the ionosphere, an apparently stratified distribution of free electrons. See ionosphere, note.

L-band

A frequency band used in radar extending approximately from 0.390 gigacycles per second to 1.55 gigacycles per second. See frequency band.

L-corona

That portion of the radiation from the solar corona consisting of coronal line emission.

L-display

In radar, a display in which a target appears as two horizontal blips, one extending to the right and one to the left, from a central vertical time base. When the radar antenna is alined in azimuth at the target both blips are of equal amplitude. When not correctly pointed the relative blip amplitude indicates the pointing error. The position of the signal along the baseline indicates target distance. The display may be rotated 90 degrees when used for elevation instead of azimuth aiming. Also called L-scan, L-scope, L-indicator.

L/D ratio = lift-drag ratio.

lead

The amount one cyclic motion is ahead of another, expressed in degrees. The opposite is lag.

leakage

In nuclear physics, loss of neutrons by outward diffusion from a reactor core; especially net loss from unreflected neutrons or escaped neutrons or by radiation through an imperfect shield.

leans

An illusion of a craft being tilted, with corresponding leaning of the crew in the opposite direction, caused by a false labyrinthine reaction uncorrected by visual cues.

leapfrogging

The process of phasing, or delaying the ranging pulse of a tracking radar in order to move, or shift (on the radarscope presentation) the target blip past the target blip from another radar.

leapfrog test

In computer operation, a check routine which eventually occupies every possible position in the memory.

leap year

See calendar year.

least squares

Any statistical procedure that involves minimizing the sum of squared differences.

left-handed polarized wave

An elliptically polarized transverse electromagnetic wave in which the rotation of the electric field vector is counterclockwise for an observer looking in the direction of propagation. Also called counterclockwise polarized wave.

Lenard effect

The separation of electric charges accompanying the aerodynamic breakup of water drops, first studied systematically by the German physicist P. Lenard. Also called spray electrification, waterfall effect. Compare Macky effect.

Experiments have shown that the degree of charge separation in spray processes depends upon the drop temperature, presence of dissolved impurities, speed of the impinging airblast, and contact with foreign surfaces. The largest fragments of the broken drops are observed to carry positive charges and the fine spray of drops carried off in the impinging air current carries a net negative charge.

length (symbol l)

Specifically, the dimension of an aircraft, rocket, etc., from nose to tail; the measure of this dimension. Compare span.

length-beam ratio = fineness ratio.

Leo (abbr Leo, Leon)

See constellation.

Leo Minor (abbr Lmi, L Min)

See constellation.

Leon

International Astronomical Union abbreviation for Leo. See constellation.

Lep, Leps

International Astronomical Union abbreviation for Lepus. See constellation.

lepton

In the classification of subatomic particles according to mass, the lightest of all particles; examples of leptons are the electron and positron. Compare meson, nucleon, hyperon.

Lepus (abbr Lep, Leps)

See constellation.

level

In acoustics, the logarithm of the ratio of that quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.

Examples of kinds of levels in common use are electric power level, sound-pressure-squared level, voltage-squared level. Level as here defined is measured in units of the logarithm of a reference ratio that is equal to the base of logarithms. In symbols,

$L = log$ _r (q/q₀)

where L is level of kind determined by the kind of quantity under consideration, measured in units of log_r; r is the base of logarithms and the reference ratio; q is the quantity under consideration; and $q$ ₀ is the reference quantity of the same kind. Differences in the levels of two like quantities q₁ and q₂ are described by the same formula because, by the rules of logarithms, the reference quantity is automatically divided out:

log_r (q₁ /q₀) - log_r (q₂ /q₀) = log_r (q₁ /q₂).

level above threshold

In acoustics, the pressure level of the sound in decibels above its threshold of audibility for the individual observer or for a specified group of individuals. Also called sensation level.

level of escape

See critical level of escape.

level surface = geopotential surface.

Lib, Libr

International Astronomical Union abbreviations for Libra. See constellation.

Libra (abbr Lib, Libr)

See constellation.

library

In computer operations, a collection of programs, routines, and subroutines by which problems (and parts of problems) of many types can be solved.

libration

A real or apparent oscillatory motion, particularly the apparent oscillation of the moon.

Because of libration more than half of the moon's surface is revealed to an observer on the earth even through the same side of the moon is always toward the earth, because the moon's periods of rotation and revolution are the same. Other motions regarded as librations are long period orbital motions and periodic perturbation in orbital elements.

LIDAR

Acronym for LIght Detection And Ranging. Like RADAR, except lidar uses light (laser) instead of radio waves.

life sciences

The field of scientific disciplines encompassing biology, physiology, psychology, medicine, sociology, and other related areas.

lift (symbol L)

1. That component of the total aerodynamic force acting on a body perpendicular to the undisturbed airflow relative to the body.

2. To lift off, to take off in a vertical ascent. Said of a rocket vehicle. See lift-off.

lift coefficient (symbol CL)

A coefficient representing the lift of a given airfoil or other body.

The lift coefficient is obtained by dividing the lift by the free-stream dynamic pressure and by the representative area under consideration.

lift-drag ratio

The ratio of lift to drag obtained by dividing the lift by the drag or the lift coefficient by the drag coefficient. Also called L/D ratio.

lift-off

The action of a rocket vehicle as it separates from its launch pad in vertical ascent. Compare take-off.

Lift-off is applicable only to vertical ascent; take-off is applicable to ascent at any angle. A lift-off is action performed by a rocket; a launch is action performed upon a rocket or upon a satellite or spaceship carried by a rocket.

light

Visible radiation (about 0.4 to 0.7 micron in wavelength) considered in terms of its luminous efficiency, i.e., evaluated in proportion to its ability to stimulate the sense of sight.

light discharge

See electric discharge.

light energy = luminous energy.

light intensity = luminous intensity.

light ion = small ion.

light microsecond

The distance of a light wave travels in free space in one-millionth of a second. See electrical distance.

lightning discharge

See spark discharge, note.

lightning recorder = sferies receiver.

light time

The elapsed time taken by electromagnetic radiation to travel from a celestial body to the observer at the time of observation.

The American Ephemeris and Nautical Almanac uses a light time of 498.8 seconds for 1 astronomical unit.

light-year

A unit of length used in expressing stellar distances equal to the distance electromagnetic radiation travels in 1 year. 1 light-year = 9.460 X 10E12 kilometers = 63,280 astronomical units = 0.3068 parsecs.

limb

The edge of the apparent disk of a celestial body, as of the sun.

limb darkening

A condition, sometimes observed on celestial objects, in which the brightness of the object decreases as the edges or limbs of the object are approached. The Sun and Jupiter exhibit limb darkening.

limb of the earth

The edge of the earth at the horizon.

limen

Threshold; a psychophysical concept denoting the lowest detectable intensity of any sensory stimulus.

liminal contrast = threshold contrast.

limiter

A device whose output is constant for all inputs above a predetermined value.

L-indicator = L-display.

line absorption

See absorption spectrum.

linear

1. Of or pertaining to a line.

2. Having a relation such that a change in one quantity is accompanies by an exactly proportional change in a related quantity, such as input and output of electronic equipment.

linear acceleration (symbol a)

The rate of change of linear velocity. See acceleration.

linear accelerator

A device for accelerating charged particles employing alternate electrodes and gaps arranged in a straight line, so proportional that when their potentials are varied in the proper amplitudes and frequency, particles passing through them receive successive increments of energy.

linear array

An antenna array whose elements are equally space along a straight line.

linearly polarized sound wave = plane polarized sound wave.

linear polarization

The polarization of an electromagnetic wave radiated by an electric vector that does not rotate but that alternates so as to describe a line. Normally, the vector is oriented either horizontally or vertically. See elliptical polarization.

linear speed

Rate of motion in a straight line. See angular speed.

linear transducer

A transducer for which the pertinent measures of all the waves concerned are linearly related.

By linearly related is meant any relation of linear character whether by linear algebraic equation or by linear differential equation or by other linear connection. The term waves concerned connotes actuating waves and related output waves, the relation of which is of primary interest in the problem at hand.

line of apsides

The line connecting the two points of an orbit that are nearest and farthest from the center of attraction, as the perigee and apogee of the moon or the perihelion and aphelion of a planet; the major axis of any elliptical orbit and extending indefinitely in both directions.

line of flight

The line in air or space along which an aircraft, spacecraft, etc., flies or travels.

line of force

A line indicating the direction in which a force acts, as in a magnetic field. See electric lines of force, magnetic lines of force.

line of nodes

The straight line connecting the two points of intersection of the orbit or a planet, planetoid, or comet and the ecliptic, or the line of intersection of the planes of the orbits of a satellite and its primary.

line of position

In navigation, a line representing all possible locations of a craft at a given instant.

In space this concept can be extended to sphere of position, plane of position, etc.

line of sight

1. The straight line between the eye of an observer and the observed object or point. Also called optical path.

2. Any straight line between one point and another, or extending out from a particular point.

3. In radio, a direct propagation path that does not go below the radio horizon.

line printer

A printer, often used in conjunction with a computer, which is capable of printing an entire line of characters at one time.

liner

In solid rockets, a layer of inhibiter applied to the inner surface of the chamber holding the grain.

line-reversal pyrometer

A thermometer for high-temperature gases in which the temperature of a calibrated radiator is adjusted until the spectral areal radiant intensity of its continuum radiation is equal to the intensity of radiation from some suitable characteristic spectral line emitted by the gas.

The comparison is made at the wavelength of the spectral line. Seeding is often used to create such a line.

line spectra

The spontaneous emission of electromagnetic radiation from the bound electrons as they jump from high to low energy levels in an atom.

This radiation is essentially at a single frequency determined by the jump in energy. Each different jump in energy level, therefore, has its own frequency and the net radiation is referred to as the line spectra. Since these line spectra are characteristic of the atom, they can be used for identification purposes.

line spectrum

A spectrum which contains a finite number of components within a specified frequency range.

line width

The finite width, expressed either in wavelength units or frequency units, of a spectral line (e.g., an absorption line).

It is customary to employ, as a convenient measure of this quantity, the half-width, which is the width of the spectral line measured between the two points at which its intensity is just half the peak intensity of the line center. The bell-shaped profile of a spectral line is produced, in general, by the joint action of several factors. Each line is characterized, first of all, by a natural width which is related through quantum principles to the lifetime of the excited state of the emitting atom or molecule, in the case of lines in an emission spectrum. This natural width may be extended by Doppler broadening due to random thermal motions of the emitting or absorbing gas, by pressure broadening due to collisions between the particles involved in the radiation, and by electric fields, as in the Stark effect. Compare equivalent width.

Linke scale

A type of cyanometer; an instrument used to measure the blueness of the sky. The Linke scale is simply a set of eight cards are numbered 2 to 16, the odd numbers to be used by the observer if he judges the sky color to lie between any of the given shades. Also called blue-sky scale.

Sky-blueness study, or cyanometry, is a means of studying atmospheric turbidity.

Linke turbidity factor

See turbidity factor, note.

liquid

A substance in a state in which the individual particles move freely with relation to each other and take the shape of the container, but do not expand to fill the container. Compare fluid.

liquid fuel

A rocket fuel which is liquid under the conditions in which it is utilized in the rocket. See liquid propellant.

liquid level manometer

A displacement manometer employing a liquid as the movable partition and providing means for observing the change in level of one or both of the free surfaces.

liquid-metal corrosion

Corrosion of a vehicle's structural metal by an adjacent liquid metal used as a coolant.

Metals such as sodium, potassium, mercury, rubidium, etc., are used as liquid coolants.

liquid propellant (abbr LP)

Specifically, a rocket propellant in liquid form.

Examples of liquid propellants include fuels such as alcohol, gasoline, aniline, liquid ammonia, and liquid hydrogen; oxidants such as liquid oxygen, hydrogen peroxide (also applicable as a monopropellant), and nitric acid; additives such as water; and monopropellants such as nitromethane.

liquid-propellant engine

See liquid-propellant rocket engine.

liquid-propellant rocket

1. A rocket powered by a liquid-propellant rocket engine.

2. = liquid-propellant rocket engine.

liquid-propellant rocket engine

A rocket engine using a propellant or propellants in liquid form. Also called liquid-propellant rocket.

Rocket engines of this kind vary somewhat in complexity, but they consist essentially of one or more combustion chambers together with the necessary pipes, valves, pumps, injectors, etc. See liquid propellant, rocket engine.

liquid rocket = liquid-propellant rocket.

lithometeor

Solid matter suspended in the atmosphere, as smoke, dust, dry haze, etc., as contrasted with hydrometeor.

lithosphere

The solid part of the earth or other spatial body. Distinguished from the atmosphere and the hydrosphere. See geosphere, biosphere.

live testing

The testing of a rocket engine, vehicle, or missile by actually launching it. Compare static testing.

Lmi, L Min

International Astronomical Union abbreviations for Leo Minor. See constellation.

load

1. The device which receives signal power from a source.

2. The signal power delivered by a source.

The use of load in sense 2 is discouraged.

load factor

A number which yields the inertial load when multiplied by the weight of an object.

The load factor for a rocket is obtained by dividing the sum of the external forces by the weight of the rocket. For example, the longitudinal load factor n, is:

n = (F - D)/W

where F is rocket thrust; D is aerodynamic drag; and W is weight of missile. The force of gravity does not appear in the sum of external forces because, on each particle of mass, the gravity is canceled by the inertial force of free-fall acceleration.

load isolator

A waveguide or coaxial device which provides a good energy path from a signal source to a load but provides a poor energy path for reflections from a mismatched load back to the signal source.

lobe

An element of a beam of focused radio energy. Lobes define surfaces of equal power density at varying distances and directions from the radiating antenna.

Their configuration is governed by two factors: (a) the geometrical properties of the antenna reflector and feed system; and (b) the mutual interference between the direct and reflected rays for an antenna situated above a reflecting surface. In addition to the major lobes of an antenna system, there exist side lobes (or minor lobes) that result from the unvoidable finite size of the reflector. They exist at appreciable angles from the axis of the beam, and, while objectionable, they normally contain much less energy than that in the major lobe. See radiation pattern.

lobe pattern = radiation pattern.

See lobe.

local

In astronomy, referred to a reference line passing through a particular place other than Greenwich, as local meridian.

local angular momentum

In meteorology, angular momentum about an arbitrarily located vertical axis which is fixed in reference to the earth.

local apparent time

The arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the local celestial meridian and the hour circle of the apparent or true sun, measured westward from the lower branch of the local celestial meridian through 24 hours; local hour angle of the apparent or true sun, expressed in time units, plus 12 hours.

local astronomical time

Mean time reckoned from the upper branch of the local meridian.

local civil time (abbr LCT)

See local mean time, note.

local lunar time

The arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the local celestial meridian and the hour circle of the moon, measured westward from the lower branch of the local celestial meridian through 24 hours; local hour angle of the moon, expressed in time units, plus 12 hours. See lunar time.

local mean time (abbr LMT)

The arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the local celestial meridian and the hour circle of the mean sun, measured westward from the lower branch of the local celestial meridian through 24 hours; local hour angle of the mean sun, expressed in time units, plus 12 hours. Mean time reckoned from the upper branch of the local meridian is called local astronomical time.

Local mean time at the Greenwich meridian is called Greenwich mean time, or universal time. It was called local civil time in United States terminology from 1925 through 1952.

local meridian

The meridian through any particular place or observer, serving as the reference for local time, in contrast with Greenwich meridian.

local sidereal time (abbr LST)

Local hour angle of the vernal equinox, expressed in time units; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the local celestial meridian and the hour circle of the vernal equinox, measured westward form the upper branch of the local celestial meridian through 24 hours.

local time

Time based upon the local meridian as reference, as contrasted with that based upon a zone meridian, or the meridian of Greenwich.

local velocity

The velocity of a particular point on an object relative to its surrounding fluid. See remote velocity.

lock, to lock on

1. Of a radar or other sensing and tracking device. To acquire a particular object of interest and continue tracking it automatically.

2. In phase-lock radio receivers, to adjust the frequency of the voltage controlled oscillation, to the point where it is controlled by signal power from the detector.

3. In coded ranging systems, to adjust the ground generated code until it exactly matches in time and code the transmitted code.

lockup

A pneumatic regulator which shuts off flow to a volume (tank) at the set regulation point or lockup pressure.

logarithm

The power to which a fixed number, called the base , usually 10 or e (2.7182818), must be raised to produce the value to which the logarithm corresponds.

An antilogarithm or inverse logarithm is the value corresponding to a given logarithm. A cologarithm is the logarithm of the reciprocal of a number.

logarithmic

Pertaining to logarithms; in a proportion corresponding to the logarithms of numbers, as a logarithmic scale.

logarithmic decrement

The natural logarithm of the ratio of any two successive amplitudes of like sign in the decay of a single-frequency oscillation.

logarithmic scale

A scale graduated in the logarithms of uniformly spaced consecutive numbers.

logic

See logical design, sense 3.

logical design

1. The planning of a computer or data processing system prior to its detailed engineering design.

2. The synthesizing of a network of logical elements to perform a specified function.

3. The result of 1 and 2 above, frequently called the logic of the system, machine, or network.

logical element

In a computer or data processing system, the smallest building blocks which can be represented by operators in an appropriate system of symbolic logic. Typical logical elements are the AND gate and the flip-flop, which can be represented as operators in a suitable symbolic logic.

logical operation

In computer operations, (a) any nonarithmetical operation (e.g., extract, bitwise multiplication jump, data transfer, etc.) (b) sometimes, only those mathematical operations which are expressible bitwise in terms of the propositional calculus or a two-valued Boolean algebra.

long-baseline system

A trajectory measuring system with receiving stations separated by distance in the order of magnitude of the distance to the target being traced.

longitude

1. Angular distance, along a primary great circle, from the adopted reference point; the angle between a reference plane through the polar axis and a second plane through that axis. See coordinate, table.

Terrestrial longitude is the arc of a parallel, or the angle at the pole, between the prime meridian and the meridian of a point on the earth, measured eastward or westward from the prime meridian through 180 degrees, and labeled E or W to indicate the direction of measurement. Astronomical longitude is the angle between the plane of the reference meridian and the plane through the polar axis and the normal to the spheroid. Geodetic and sometimes astronomical longitude are also called geographic longitude. Geodetic longitude is used for charts. Assumed longitude is the longitude at which an observer is assumed to be located for an observation or computation. Difference of longitude at which an observer is assumed to be located for an observation or computation. Difference of longitude is the smaller angle at the plor or the shorter arc of a parallel between the meridians of two places, expressed in angular measure. Fictitious longitude is the arc of the fictitious equator between the prime fictitious meridian and any given fictitious meridian. Grid longitude is angular distance between a prime grid meridian and any given grid meridian. Oblique longitude is angular distance between a prime oblique meridian and any given oblique meridian. Transverse or inverse longitude is angular distance between a prime transverse meridian and any given transverse meridian. Celestial longitude is angular distance east of the vernal equinox, along the ecliptic. Galactic longitude is angular distance east of sidereal hour angle 80 degrees, along the galactic equator.

2. Of a planet in solar system, the sum of two angles: the celestial longitude of the ascending node of the planetary orbit, and the angle measured eastward from the ascending node along the orbit to the position of the planet.

longitudinal axis

The fore-and-aft line through the center of gravity of a craft.

longitudinal wave

A wave in which the direction of displacement at each point of the medium is normal to the wave front. Compare transverse wave.

long-range accuracy (abbr Lorac)

A two-dimensional radio navigation system using continuous-wave transmission to provide hyperbolic lines of position through radiofrequency phase comparison techniques from four transmitters.

The system is used for surveying or ship-positioning. Frequency band, 1.7 to 2.5 megacycles. Similar to Raydist system in principle.

long-range navigation (abbr loran)

A two-dimensional pulse-synchronized radio navigation system to determine hyperbolic lines of position through pulse-time differencing from a master compared to two slave stations.

Loran uses the frequency band 1.7 to 2.0 megacycles; loran C (Cytac) uses transmission at 100 kilocycles and phase compares the continuous wave in the pulse envelopes for greater accuracy using pulse technique for resolving ambiguities.

long-wave radiation

In meteorology, = infrared radiation.

long-wire antenna

A linear antenna which, by virtue of its considerable length in comparison with the operating wavelength, provides a directional radiation pattern.

look angles

The elevation and azimuth at which a particular satellite is predicted to be found at a specified time.

Look angles are used in satellite tracking and data acquisition to minimize the amount of searching needed to acquire the satellite in the telescope field of view or the antenna beam.

looming

A mirage effect produced by greater-than-normal refraction in the lower atmosphere, thus permitting objects to be seen that are usually below the horizon. This occurs when the air density decreases more rapidly with height than in the normal atmosphere.

If the rate of decrease of density with height is greater in the region followed by the ray from the top of the object than for the ray from the bottom of the object, the image will be stretched vertically. This stretching is often called looming but is more properly termed towering. The antonym of looming is sinking and that of towering is stooping.

loop

1. = antinode.

2. = mesh.

3. = loop antenna.

4. = feedback control loop.

loop antenna

An antenna consisting of a conducting coil, of any convenient cross section (generally circular), which emits or receives radio energy. The principal lobe of the radiation pattern is wide and is in the direction perpendicular to the plane of the coil. Also called loop.

loop range

The total distance from a transmitter to a target to a receiver.

Lorac (abbr) = long-range accuracy.

loran (abbr) = long-range navigation.

loran C

See long-range navigation, note.

Lorentz force

The force affecting a charged particle due to the motion of the particle in a magnetic field. The Lorentz force is

F_L = q(v X B)

where q is the charge on the moving object; v is the velocity of the object; and B is the magnetic induction vector.

Lorin tube = ramjet engine.

Loschmidt number

The number of molecules of an ideal gas per unit volume. Loschmidt number = 2.6870 X 10E19 molecules per cubic centimeter. See Avogadro number.

loss

A decrease in signal power in transmission from one point to another. Loss is usually expressed in decibels. Also called transmission loss.

loudness

The intensive attribute of an auditory sensation, in terms of which sounds may be ordered on a scale extending from soft to loud. Loudness is measured in sones.

Loudness depends primary upon the sound pressure of the stimulus, but it also depends upon the frequency and waveform of the stimulus.

lower atmosphere

Generally, and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the troposphere and lower stratosphere); hence, used in contrast to the common meaning for the upper atmosphere.

lower branch

That half of a meridian or celestial meridian from pole to pole which passes through the antipode or nadir of a place.

lower culmination = lower transit.

lower limb

That half of the edge of the apparent disk of a celestial body having the least altitude; in contrast with the upper limb, that half having the greatest altitude.

lower transit

Transit of the lower branch of the celestial meridian. Also called inferior transit, lower culmination.

low frequency (abbr LF)

See frequency bands.

low-pass filter

A wave filter having a single transmission band extending from zero frequency up to some critical or bounding frequency, not infinite.

low vacuum

The condition in a gas-filled space at pressures less than 760 torr and greater than some lower limit. It is recommended that this lower limit be chosen as 25 torr corresponding approximately to the vapor pressure of water at 25 degrees C and to 1 inch of mercury.

lox

1. Liquid oxygen. Used attributively as in lox tank, lox unit. Also called loxygen.

2. To load the fuel tanks of a rocket vehicle with liquid oxygen. Hence, loxing.

lox-hydrogen engine

An engine using liquid hydrogen as fuel and liquid oxygen as oxidizer.

loxing

See lox.

loxygen = lox.

loz

Liquid ozone.

LP (abbr) = liquid propellant.

L-scan = L-display.

L-scope = L-display.

lumen

A unit of luminous flux equal to the luminous flux radiated into a unit solid angle (steradian) from a point source having a luminous intensity of 1 candela.

An ideal source possessing an intensity of 1 candela in every direction would radiate a total of 4 pi lumens.

luminance

In photometry, a measure of the intrinsic luminous intensity emitted by a source in a given direction; the illuminance produced by light from the source upon a unit surface area oriented normal to the line of sight at any distance from the source, divided by the solid angle subtended by the source at the receiving surface. Also called brightness but luminance is preferred. See Lambert law. Compare luminous emittance.

It is assumed that the medium between source and receiver is perfectly transparent; therefore, luminance is independent of extinction between source and receiver. The source may or may not be self-luminous. Luminance is a measure only of light; the comparable term for electromagnetic radiation in general is radiance.

luminance contrast

See contrast, sense 2.

luminescence

Light emission by a process in which kinetic heat energy is not essential for the mechanism of excitation.

Electroluminescence is luminescence from electrical discharges - such as sparks or arcs. Excitation in these cases results mostly from electron or ion collision by which the kinetic energy of electrons or ions, accelerated in an electric field, is given up to the atoms or molecules of the gas present and causes light emission. Chemiluminescence results when energy, set free in a chemical reaction, is converted to light energy. The light from many chemical reactions and from many flames is of this type. Photoluminescence, or fluorescence, results from excitation by absorption of light. The term phosphorescence is usually applied to luminescence which continues after excitation by one of the above methods has ceased. Compare incandescence.

luminosity = luminous efficiency.

luminous

1. In general, pertaining to the emission of visible radiation.

2. In photometry, a modifier used to denote that a given physical quantity, such a luminous emittance, is weighted according to the manner in which the response of the human eye varies with the wavelength of the light. See luminous efficiency.

luminous density

The instantaneous amount of luminous energy contained in a unit volume of the propagating medium; to be distinguished from radiant density in that it is weighted in accordance with the characteristics of the human eye in its nonuniform response to different wavelengths of light. See luminous efficiency. Compare flux density, illuminance.

luminous efficiency

For a given wavelength of visible radiation, the ratio of the flux that is effectively sensed by the human eye to the flux that is intrinsic in the radiation. It may be represented as a dimensionless ratio, e.g., lumens per watt. Also called luminosity.

Thus, luminous efficiency is a weighting factor which is applied to radiation quantities so that they are related physiologically to the response of the human eye, which varies as a function of wavelength. All quantities which are weighted in this manner should be modified by the term luminous (e.g., luminous emittance, luminous flux, etc.)

luminous emittance

The emittance of visible radiation weighted to take into account the different response of the human eye to different wavelengths of light. See luminous efficiency.

In photometry, luminous emittance is always used as a property of a self-luminous source, and therefore should be distinguished from luminance.

luminous energy (symbol Q)

The energy of visible radiation, weighted in accordance with the wavelength dependence of the response of the human eye. See luminous efficiency. Also called light energy.

luminous flux (symbol F)

Luminous energy per unit time; the flux of visible radiation, so weighted as to account for the manner in which the response of the human eye varies with the wavelength of radiation. See luminous efficiency. The basic unit for luminous flux is lumen.

luminous flux density

See illuminance.

luminous intensity

Luminous energy per unit time per unit solid angle; the intensity (flux per unit solid angle) of visible radiation weighted to take into account the variable response of the human eye as a function of the wavelength of light; usually expressed in candles. Also called candlepower, light intensity. Compare luminance, illuminance. See luminous efficiency, light intensity.

lunar

Of or pertaining to the moon.

lunar atmospheric tide

An atmospheric tide due to the gravitational attraction of the moon. The only detectable components are the 12-lunar-hour or semidiurnal, as in the oceanic tides, and two others of very nearly the same period. The amplitude of this atmospheric tide is so small that it is detected only by careful statistical analysis of a long record, being about 0.06 millibar in the tropics and 0.02 millibar in the middle latitudes. See tide.

lunar crater

A depression, usually circular, on the surface of the moon, usually with a raised rim called a ringwall.

Craters range in size up to 250 kilometers in diameter. The largest craters are sometimes called walled plains. The smaller, 15 to 30 kilometers across, are often called craterlets and the very smallest, a few hundred meters across, beads. Craters are named after people, mainly astronomers.

lunar cycle

Any cycle related to the moon, particularly the Callipic cycle or the Metonic cycle. See saros.

lunar day

1. The duration of one rotation of the earth on its axis, with respect to the moon. Its average length is about 24 hours 50 minutes of mean solar time. Also called tidal day.

2. The duration of one rotation of the moon on its axis, with respect to the sun.

lunar distance

The angle, at an observer on the earth, between the moon and another celestial body.

This was the basis of a method formerly used to determine longitude at sea.

lunar eclipse

The phenomenon observed when the moon enters the shadow of the earth.

A lunar eclipse is called penumbral if the moon enters only the penumbra of the earth, partial if the moon enters the umbra without being totally immersed, and total if the moon is entirely immersed in the umbra.

lunar equation

A factor used to reduce observations of the positions of celestial bodies to the barycenter of the earth-moon system.

lunar gravity

The force imparted by the moon to a mass which is at rest relative to the moon. It is approximately 1/6 of the earth's gravity.

lunar inequality

Variation in the moon's motion in its orbit, due to attraction by other bodies of the solar system. See evection, perturbation.

lunar interval

The difference in time between the transit of the moon over the Greenwich meridian and a local meridian.

lunar month

The period of revolution of the moon about the earth, especially a synodical month.

lunar noon

The instant at which the sun is over the upper branch of any meridian of the moon.

lunar orbit

Orbit of a spacecraft around the moon.

lunar parallax

The horizontal parallax or the geocentric parallax of the moon.

lunar probe

A probe for exploring and reporting on conditions on or about the moon.

lunar satellite

A manmade satellite that would make one or more revolutions about the moon. See selenoid.

lunar time

1. Time based upon the rotation of the earth relative to the moon. Lunar time may be designated as local or Greenwich as the local or Greenwich meridian is used as the reference.

2. Time on the moon.

lunation = synodical month.

lune

That part of the surface of a sphere bounded by halves of two great circles.

lunicentric = selenocentric.

Lunik

Russian term for a space probe launched to the moon's vicinity or to impact on the moon.

lunisolar precession

That component of general precession caused by the combined effect of the sun and moon on the equatorial protuberance of the earth, producing a westward motion of the equinoxes along the ecliptic. See precession of the equinoxes.

Lup, Lupi