M

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 version.
Greek symbols may not appear correctly in some browsers. For example a gamma may appear as γ.
If you are using a text only browser and you come across symbols or equations that do not appear, click here for the text version

Mach = Mach number.: Some writers use Mach as a unit of speed equivalent to a Mach number of 1.00, as a speed of Mach 3.1.
Mach angle: The angle between a Mach line and the direction of movement of undisturbed flow. See Mach wave.
Mach cone: 1. The cone-shaped shock wave theoretically emanating from an infinitesimally small particle moving at supersonic speed through a fluid medium. It is the locus of the Mach Lines.; 2. The cone-shaped shock wave generated by a sharp-pointed body, as at the nose of a high-speed aircraft. See Mach wave.
Mach indicator = Machmeter.
machine error: See error, note.
machine language: 1. A language, occurring within a computer, ordinarily not perceptible or intelligible to persons without special equipment or training.; 2. A translation or transliteration of sense 1 into more conventional characters but frequently still not intelligible to persons without special training.
machine word: For a given computer, the number of information characters handled in each transfer. This number is usually fixed, but may be variable in some computers.
Mach line: A line representing a Mach wave; a Mach wave.
Machmeter: An instrument that measures and indicates speed relative to the speed of sound, i.e., that indicates the Mach number. Also called Mach indicator.
Mach number (symbols M, N_Ma): (Pronounced mock, after Ernst Mach, 1838-1916, Austrian scientist). A number expressing the ratio of the speed of a body or of a point on a body with respect to the surrounding air or other fluid, or the speed of a flow, to the speed of sound in the medium; the speed represented by this number. See Cauchy number.; If the Mach number is less than 1, the flow is called subsonic and local disturbances can propagate ahead of the flow. If the Mach number is greater than 1, the flow is called supersonic and disturbances cannot propagate ahead of the flow with the result that shock waves form.
Some authorities use mach number but engineering practice is to use a capital M in all words and combinations employing Mach.
Mach wave: 1. A shock wave theoretically occurring along a common line of intersection of all the pressure disturbances emanating from an infinitesimally small particle moving at supersonic speed through a fluid medium, with such a wave considered to exert no changes in the condition of the fluid passing through it.; The concept of the Mach wave is used in defining and studying the realm of certain disturbances in a supersonic field of flow.; 2. A very weak shock wave appearing, e.g., at the nose of a very sharp body, where the fluid undergoes no substantial change in direction.
Maclaurin series: See Taylor theorem.
macroscopic: Large enough to be visible to the naked eye or under low order of magnification.
macrosonics: The technology of sound at signal amplitudes so large that linear approximations are not valid.
magic tee: A compound waveguide or coaxial tee with four arms which exhibits directional characteristics, when properly matched, so that a signal entering one arm will be split between two of the other arms but not the third. A signal entering another arm is likewise split with half the energy entering one of the arms common to the other input but not its second arm and the other half of the energy entering the arm not used by the other input.; The magic tee is used in radar as a transmitter-receiver duplexer.
magnesyn: (A trade name, from magnetic + synchronous; often capitalized). An electromagnetic device that transmits the direction of a magnetic field from one coil to another, used to transmit measurements electrically from a point of measurement to an indicator in a remote-indicating system.
magnet: A body which produces a magnetic field around itself.
magnetic: 1. Of or pertaining to a magnet.; 2. Of or pertaining to a material which is capable of being magnetized.; 3. Related to or measured from magnetic north.
magnetic bay: A small magnetic disturbance whose magnetograph resembles an indentation of a coastline.; On earth, magnetic bays occur mainly in the polar regions and have durations on the order of a few hours.
magnetic binary core = binary magnetic core.
magnetic character figure = C-index.
magnetic core = binary magnetic core.
magnetic crotchet: A sudden change in the earth's magnetic field due to an increase in the conductivity of the lower ionosphere. See sudden ionospheric disturbance.
magnetic current sheath: See plasma sheath.
magnetic declination (symbol D, ): In terrestrial magnetism; at any given location, the angle between the geographical meridian and the magnetic meridian; that is, the angle between true north and magnetic north. Also called declination , and in navigation, variation.; Declination is either east or west according as the compass needle points to the east or west of the geographical meridian. Lines of constant declination are called isogonic lines and the one of zero declination is called the agonic line.
magnetic deviation: The angle between the magnetic meridian and the axis of a compass card, expressed in degrees east or west to indicate the direction in which the northern end of the compass card is offset from magnetic north. Also called deviation. Compare variation.
magnetic dip (symbol i): The angle between the horizontal and the direction of a line of force of the earth's magnetic field at any point. Also called magnetic inclination, magnetic latitude, inclination, dip.
magnetic dipole moment = magnetic moment.
magnetic disturbance daily variation (symbol S_D): A periodic variation of the earth's magnetic field that is in phase with solar (local) time. It is the difference between the solar daily variation (or the disturbed-day solar daily variation) and the quiet-day solar daily variation. This variation is primarily an effect of enhanced electromagnetic radiation during increased solar activity.
magnetic disturbed-day solar daily variation (symbol Sd): The solar daily variation of the earth's magnetic field obtained from the 5 most disturbed days of the month.
magnetic double refraction: The splitting, into two components, of a radio wave traveling in a region of free electrons. This is due to the interaction of the earth's magnetic field and the alternating field of the radio wave. Except for waves near the gyrofrequency, the components of the split wave, the ordinary ray and the extraordinary ray, will travel with slightly different velocities and be reflected at different heights. See magneto-ionic theory.
magnetic drum: A memory device used in computers; a rotating cylinder on which information may be stored as magnetically polarized areas, usually along several parallel tracks around the periphery.
magnetic element: 1. In terrestrial magnetism, any of the following measurements: the vector magnetic field, also called total field (symbol ); the scalar intensity of the total field ( symbol F); declination, also called variation ( symbol D); the intensity of the horizontal component of the earth's field ( symbol H); the intensity of the vertical component ( symbol Z), taken as positive downward; the inclination or dip ( symbol I); the angle between and H; the intensity of the component of the horizontal field in the geographic north direction ( symbol X); and the intensity of the component of the horizontal field in the geographic east direction.; 2. That part of an instrument producing or influenced by magnetism.
magnetic equator: That line on the surface of the earth connecting all points at which the magnetic dip is zero. Also called aclinic line. See geomagnetic equator.
magnetic equivalent amplitude indices: A linear measure of geomagnetic disturbance activity, based on the K-indices that gives an equivalent amplitude of the magnetic disturbance for the 3-hour period denoted by a_p. A daily index A_p is defined as the average of the a_p value over the 8 values of the day. See magnetic K-indices.
magnetic field: 1. A region of space wherein any magnetic dipole would experience a magnetic force or torque; often represented as the geometric array of the imaginary magnetic lines of force that exist in relation to magnetic poles.; 2. = magnetic field intensity.
magnetic field intensity: The magnetic force exerted on an imaginary unit magnetic pole placed at any specified point of space. It is a vector quantity. Its direction is taken as the direction toward which a north magnetic pole would tend to move under the influence of the field. If the force is measured in dynes and the unit pole is a cgs unit pole, the field intensity is given in oersteds. Also called magnetic intensity, magnetic field, magnetic field strength.; Prior to 1932 the oersted was called the gauss; but the latter term is now used to measure magnetic induction (within magnetic materials), whereas oersted is reserved for magnetic force. By definition, one magnetic line of force per square centimeter (in air) represents the field intensity of 1 oersted.
magnetic field strength = magnetic field intensity.
magnetic giant pulsations: Magnetic micropulsations having large amplitudes.
magnetic inclinations = magnetic dip.
magnetic induction: A measure of the strength of a magnetic field existing within a magnetic medium.; The relation between the magnetic induction and magnetic field intensity is such that the magnetic induction within a small mass of material of magnetic permeability ľ is, except for possible hysteresis effects, ľ times greater than the external magnetic field intenstiy. Whereas magnetic field intensity is measured in oersteds, magnetic induction is measured in gausses.
magnetic intensity = magnetic field intensity.
magnetic K-indices: An approximately logarithmic measure of geomagnetic disturbance activity based on the range of the most disturbed magnetic element during each 3-hour interval of the day. The K-indices are assigned integers from 0 to 9.; The K-indices averaged over the observatories of the earth are called planetary indices K_p and divided into 28 grades.
magnetic latitude = magnetic dip: See geomagnetic latitude.
magnetic lines of force: Imaginary lines so drawn in a region containing a magnetic field to be everywhere tangent to the magnetic field intensity vector if in vacuum or nonmagnetic material, or parallel to the magnetic induction vector if in a magnetic medium. See electric lines of force.; As so defined, these lines of force are merely convenient artifices for delineating the geometry of a magnetic field. They are given quantitative significance in magnetic theory by associating one line of force per square centimeter normal to the force for every oersted of field intensity (in vacuum), for every gauss of magnetic induction (in magnetic media).
magnetic lunar daily variation (symbol L): A periodic variation of the earth's magnetic field that is in phase with the transit of the moon.; This variation is essentially a tidal effect. The amplitude of this variation changes with the phase of the moon, the seasons, and the sunspot cycle.
magnetic memory: 1. The ability of a material to retain magnetism after the magnetizing force is removed.; 2. = magnetic storage.
magnetic meridian: The horizontal line which is oriented, at any specified point on the earth's surface, along the direction of the horizontal component of the earth's magnetic field at that point; not to be confused with isogonic line. Also called geomagnetic meridian. Compare isoclinic line, magnetic equator.
magnetic micropulsations: Oscillations in magnetic records having periods of between a fraction of a minute and a few minutes, lasting for an hour or so.
magnetic mirror: A magnetic field so arranged that it will theoretically confine a hot plasma.
magnetic moment: 1. The quantity obtained by multiplying the distance between two magnetic poles by the average strength of the poles.; 2. A measure of the magnetic flux set up by the gyration of an electric charge in a magnetic field. The moment is negative, indicating it is diamagnetic, and equal to the energy of rotation divided by the magnetic field.; 3. (symbol m). In atomic and nuclear physics, a moment, measured in Bohr magnetons, associated with the intrinsic spin of the particle and with the orbital motion of the particle in a system. Also called magnetic dipole moment.
magnetic north: The direction north at any point as determined by the earth's magnetic lines of force; the reference direction for measurement of magnetic directions.
magnetic pole: 1. Either of the two places on the surface of the earth where the magnetic dip is 90 °, that in the Northern Hemisphere (at, approximately, latitude 73° 8 N, longitude, 101° W in 1955) being designated north magnetic pole , and that in the Southern Hemisphere (at, approximately, latitude, 68° S, longitude, 144° E in 1955) being designated south magnetic pole. Also called dip pole. See geomagnetic latitude, geomagnetic pole, magnetic latitude.; 2. Either of those two points of a magnet where the magnetic force is greatest.; 3. In magnetic theory, a fictitious entity analogous to a unit electric charge of electrostatic theory. In nature only dipoles, not isolated magnetic poles, exist.
magnetic pressure: The energy density associated with a magnetic field.; In a very real sense, there is energy stored in a magnetic field, and since energy per unit volume is equivalent to force per unit area or pressure, one may speak of the pressure exerted by a magnetic field. For plasma containment in a thermonuclear device, the magnetic pressure must be greater than the kinetic pressure of the plasma. See beta factor. A pressure of 1 atmosphere corresponds approximately to 5,000 gausses, and the pressure is proportional to the square of the field.
magnetic quiet-day solar daily variation (symbol S_q): The magnetic solar daily variation obtained from the 5 most quiet days of the month.
magnetic solar daily variation (symbol S): A periodic variation of the earth's magnetic field that is in phase with solar (local) time.
The primary source of this variation is the ionizing effect of solar electromagnetic radiation on the atmosphere coupled with the earth's rotation. The amplitude of this variation changes with the seasons and the sunspot cycle.
magnetic storage: In computer terminology, any device which makes use of the magnetic properties of materials for the storage of information.
magnetic storm: A worldwide disturbance of the earth's magnetic field. See M-region.; Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normality, which may take several days. Magnetic storms are caused by solar disturbances, though the exact nature of the link between the solar and terrestrial disturbances is not understood. They are more frequent during years of high sunspot number. Sometimes a magnetic storm can be linked to a particular solar disturbance. In these cases, the time between solar flare and onset of the magnetic storm is about 1 or 2 days, suggesting that the disturbance is carried to the earth by a cloud of particles thrown out by the sun.
When these disturbances are observable only in the auroral zones, they may be termed polar magnetic storms.
magnetic storm-time variation (symbol D_st): A nonperiodic variation determined from the onset of a magnetic storm. This variation is characterized by a rapid increase of the magnetic horizontal intensity above the normal value, remaining so for a few hours and then rapidly decreasing to below the normal value and remaining so for periods up to several days. The intensity then returns slowly to the normal value.
magnetic tape: A ribbon of paper, metal, or plastic, coated or impregnated with magnetic material on which information may be stored in the form of magnetically polarized areas.
magnetic variation: 1. Variation, definition 1.; 2. Change in a magnetic element.
magnetic wire: Wire made of magnetic material on which information may be stored in the form of magnetically polarized areas.
magnetoelectric: Of or pertaining to electricity produced by or associated with magnetism.; Electromagnetic pertains to magnetism produced by or associated with electricity.
magnetoelectric transducer: A transducer which measures the electromotive force generated by the movement of a conductor relative to a magnetic field.
magnetofluiddynamics = magnetohydrodynamics.
magnetogasdynamics = magnetohydrodynamics.
magnetograph: The trace of an instrument recording variations in the geomagnetic field.
magnetohydrodynamics (abbr MHD): The study of the interaction that exists between a magnetic field and an electrically conducting fluid. Also called magnetoplasmadynamics, magnetogasdynamics, hydromagnetics.
magnetohydrodynamic wave = Alfvén wave.
magnetoionic theory: The theory of propagation of electromagnetic radiation through a medium containing ions in the presence of an external magnetic field.; It applies to the propagation of radio waves in the ionosphere, and provides theoretical relationships among such aspects of the subject as the index of refraction, radiofrequency, free-electron density, electron collision frequency, the earth's magnetic field (components relative to the direction of propagation), the nature of polarization, etc. See magnetic double refraction.
magnetoionic wave component: Either of the two elliptically polarized wave components into which a linearly polarized electromagnetic wave, incident of the ionosphere, is separated because of the earth's magnetic field.
magnetometer: An instrument used in the study of geomagnetism for measuring a magnetic element.
magneton: See Bohr magneton.
magnetoplasmadynamics = magnetohydrodynamics.
magnetosphere: The region of the earth's atmosphere where ionized gas plays an important part in the dynamics of the atmosphere and where the geomagnetic field, therefore, plays an important role. The magnetosphere begins, by convention, at the maximum of the F layer at about 350 kilometers and extends to 10 or 15 earth radii to the boundary between the atmosphere and the interplanetary plasma.
magnetostriction: 1. The phenomenon wherein ferromagnetic materials experience an elastic strain when subjected to an external magnetic field.; 2. The converse of sense 1 in which mechanical stresses cause a change in the magnetic induction of a ferromagnetic material.
magnetostrictive delay line: In electronic computers, a device in which a wave is induced by the characteristic, possessed by nickel and certain other materials, of shortening in length when placed in a magnetic field. The wave travels at the speed of sound through the material. See delay line.
magnetron: An electron tube characterized by the interaction of electrons with the electric field of a circuit element in crossed steady electric and magnetic fields to produce alternating-current power output.
magnitude (symbol m): 1. The relative luminance of a celestial body. The smaller (algebraically) the number indicating magnitude, the more luminous the body. Also called stellar magnitude. See absolute magnitude.; The ratio of relative luminosity of two celestial bodies differing in magnitude by 1.0 is 2.512, the fifth root of 100.
Decrease of light by a factor of 100 increases the stellar magnitude by 5.00; hence, the brightness objects have negative magnitudes (Sun: -26.8; mean full moon: -12.5; Venus at brightest: -4.3; Jupiter at opposition: -2.3; Sirius: -1.6; Vega: 0.2; Polaris: 2.1). The faintest stars visible to the naked eye on a clear dark night are of about the sixth magnitude (though on a perfectly black background the limit for a single luminous point approaches the eighth magnitude). The faintest stars visible with a telescope of aperture a (in inches) is one approximately of magnitude $9 + 5 log$ ₁₀ a. The magnitude of the faintest stars which can be photographed with the 200-inch telescope is about +22.7.
The expression first magnitude is often used somewhat loosely to refer to all bodies of magnitude 1.5 or brighter, including negative magnitudes.; 2. Amount; size; greatness. See order of magnitude.
main bang: The transmitted pulse, within a radar system.
main stage: 1. In a multistage rocket, the stage that develops the greatest amount of thrust, with or without booster engines.; 2. In a single-stage rocket vehicle powered by one or more engines, the period when full thrust (at or above 90 percent) is attained.; 3. A sustainer engine, considered as a stage after booster engines have fallen away, as in the main stage of the Atlas.
major axis: The longest diameter of an ellipse or ellipsoid.
major lobe: See lobe.
major planets: The four largest planets: Jupiter, Saturn, Uranus, and Neptune.
malfunction: Improper functioning of a component, causing improper operation of a system.
man-machine integration: The matching of the characteristics and capabilities of man and machine in order to obtain optimum conditions and maximum efficiency of the combined system. See man-machine system.
man-machine system: A system in which the functions of the man and the machine are interrelated and necessary for the operation of the system.
manned: Of a vehicle occupied by one or more persons who normally have control over the movements of the vehicle, as in a manned aircraft or spacecraft, or who perform some useful function while in the vehicle.
manometer: An instrument for measuring pressure of gases and vapors both above and below atmospheric pressure. See vacuum gage.
manometric equivalent: The length in millimeters of a vertical column of a given liquid at standard room temperature equivalent to 1 millimeter of mercury at 0° C.
many-to-few matrix = decoder.
map-matching guidance: 1. The guidance of a rocket or aerodynamic vehicle by means of a radarscope film previously obtained by a reconnaissance flight over the terrain of the route, and used to direct the vehicle by aligning itself with radar echoes received during flight from the terrain below.; 2. Guidance by stellar map matching.
March equinox = vernal equinox.
mare (pl. maria): Latin for sea. The large, dark, flat areas on the lunar surface, thought by early astronomers to be bodies of water. The term is also applied to less well-defined areas on Mars.
Mariotte law = Boyle-Mariotte law.
marmon clamp: A ring-shaped clamp, consisting of three equal length segments held together by explosive bolts, used to couple the main subsections of a rocket vehicle.
marriage = mating.
Mars: See planet, table.
maser: An amplifier utilizing the principle of microwave amplification by stimulated emission of radiation. Emission of energy stored in a molecular or atomic system by a microwave power supply is stimulated by the input signal.
mass (symbol m): A quantity characteristic of a body, which relates the attraction of this body toward another body. Since the mass of a body is not fixed in magnitude, all masses are referred to the standard kilogram, which is a lump of platinum.; Mass of a body always has the same value; weight changes with change in the acceleration of gravity.
mass-charge ratio: The ratio of the mass number of an element to the number of electronic charges gained or lost in ionization.
mass-density (symbol ): Mass per unit volume.
mass-energy equivalence: The equivalence of a quantity of mass m and a quantity of energy E, the two quantities being related by the mass-energy relation, E = mc².; This relation was proposed by Einstein as a consequence of his restricted (or special) theory of relativity; it has subsequently received abundant experimental confirmation and is regarded as the conversion factor relating units of energy and mass; various useful forms of this factor are: $c$ ² = (2.998 X 10¹⁰)² centimeters per second = 8.987 X 10²² ergs per gram = 931.1 million electron volts per atomic mass unit.
See relativity.
mass flow rate per unit area (symbol G): In aerodynamics, the product of fluid density and the linear velocity of the fluid v or
G = pv_{p = lower case Rho}
mass number: The whole number nearest the value of the atomic mass of an element as expressed in atomic mass units.; The mass number is assumed to represent the total number of protons and neutrons in the atomic nucleus of the element and is therefore equal to the atomic number plus the number of the neutrons. The mass number of an atom is usually written as a superscript to the element symbol, as in O¹⁸, an isotope of oxygen with mass number 18.
mass ratio: The ratio of the mass of the propellant charge of a rocket to the total mass of the rocket when charged with the propellant.
mass-velocity ratio: A quantity m_v /m_r expressing the relativistic variation of mass with velocity. where m_v is moving mass, m_r is rest mass, v is velocity, and c is the velocity of light.; This ratio becomes important only at speeds approaching the speed of light.
master station: In a hyperbolic navigation system, such as loran, that transmitting station which controls the transmissions of another station or of other stations. See hyperbolic navigation, slave station.
Mate (abbr): Modular automatic test equipment.
mate: To fit together two major components of a system. Also called marry.
material coordinates = Lagrangian coordinates.
material derivative = individual derivative.
materials: In general, the substances of which rockets and space vehicles are composed; specifically, the metals, alloys, ceramics, and plastics used in structural, protective, and electronic functions.
mating: 1. The act of fitting together two major components of a system as mating of a launch vehicle and a spacecraft. Also called marriage.; 2. = interface.
matrix: 1. Any rectangular array of elements composed of rows and columns; specifically, such an array consisting of numbers or mathematical symbols which can be manipulated according to certain rules.; 2. In electronic computers, any logical network whose configuration is a rectangular array of intersections of its input-output leads, with elements connected at some of these intersections. The network usually functions as an encoder or decoder . Loosely, any encoder, decoder, or translator.
Matts (abbr) = multiple airborne target trajectory system.
maximum energy density: See sound energy density.
maximum evaporation rate: The maximum rate at which molecules could emerge from a surface, deduced from measurements of saturated vapor pressure at the same temperature. Also called Knudsen rate of evaporation or Langmuir rate of evaporation.
maximum sound pressure: For any given cycle of a periodic wave, the maximum absolute value of the instantaneous sound pressure, without regard to sign, occurring during that cycle. The unit is the microbar.; In the case of a sinusoidal sound wave, the maximum sound pressure is also called the pressure amplitude.
maximum usable frequency (abbr MUF): For a given distance from a transmitter, the highest frequency at which sky waves can be received.
Maxwellian distribution: The velocity distribution, as computed in the kinetic theory of gases, of the molecules of a gas in thermal equilibrium.; This distribution is often assumed to hold for neutrons in thermal equilibrium with the moderator (thermal neutrons).
McLeod gage: A liquid-level vacuum gage in which a known volume of gas, at the pressure to be measured, is compressed by the movement of a liquid column to a much smaller known volume, at which the resulting higher pressure is measured.; Particular designs are named after the inventors or by various trade names.
M-curve: A plot of values of M-units (modified index of refraction) as a function of height in an atmosphere. M-curves are frequently used in ray tracing studies.
MCW (abbr) = modulated continuous wave.
M-display: In radar, a display in which target distance is determined by moving an adjustable blip along the baseline until it coincides with the horizontal position of the target signal deflections. The control which moves the blip is calibrated in distance. Also called M-scan, M-scope, M-indicator.
mean = arithmetic mean.
mean anomaly: See anomaly.
mean center of moon: A central point for a lunar coordinate system; the point on the lunar surface intersected by the lunar radius that is directed toward the earth's center when the moon is at the mean ascending node and when the node coincides with the mean perigee or mean apogee.
mean deviation = average deviation.
mean distance = semimajor axis.
mean equinox: A fictitious equinox whose position is that of the vernal equinox at a particular date with the effect of nutation removed. Also called mean equinox of date.
mean equinox of date = mean equinox.
mean error = root-mean-square error.
mean free path (symbol l, , L): 1. Of any particle, the average distance that a particle travels between successive collisions with the other particles of an ensemble.
In vacuum technology, the ensemble of particles of interest comprises only the molecules in the gas phase.; 2. Specifically, the average distance traveled by the molecules of a perfect gas between consecutive collisions with one another. It may be determined roughly from either of the formulas or where l is the mean free path; u (lower case Mu) is the dynamic viscosity; v is the kinematic viscosity; p (lower case Rho) is the density; c is the molecular speed (a function of the gas temperature); n is the number of molecules per unit volume; and d is the molecule diameter.; Given the mean free path l₀ at a level where the pressure is p₀ , the temperature is T₀ (°K), and the acceleration of gravity is g₀, then its value at any other level is l = l₀ p₀ Tg / pT₀ g₀ where p, T, and g are the pressure, temperature, and acceleration of gravity, respectively, at the new level. See mixing length.; 3. For any process the reciprocal of the cross section per unit volume for that process.
mean motion (symbol n, ľ): Of an object in orbit, a measure of angular velocity, n = / P where P = period.
mean noon: The instant the mean sun is over the upper branch of the reference meridian; twelve o'clock mean time.
mean position: Of a star, the position on the celestial sphere computed from past observations plus known proper motion but not corrected for short term variations. See Besselian star numbers.
mean sea level: The average height of the surface of the sea for all stages of the tide over a 19-year period, usually determined from hourly height readings.; Mean sea level is the datum from which heights are measured. In this sense sometimes shortened to sea level. See geoid.
mean sidereal time: Sidereal time adjusted for nutation to eliminate slight irregularities in the rate.
mean solar day: The duration of one rotation of the earth on its axis, with respect to the mean sun.; The length of the mean solar days is 24 hours of mean solar time or 24 hours 3 minutes 56.555 seconds of mean sidereal time. A mean solar day beginning at midnight is called a civil day; and one beginning at noon, 12 hours later, is called an astronomical day. See calendar day.
mean solar second: Prior to 1960 the fundamental unit of time, equal to 1/86,400 of the mean solar day. Now replaced by the ephemeris second.
mean solar time: See solar time.
mean square: Referring to the arithmetic mean of the squares of the values under consideration, as mean-square amplitude, mean-square error.
mean-square error: The quantity whose square is equal to the sum of the squares of the individual errors divided by the number of those errors.
mean sun: A fictitious sun conceived to move eastward along the celestial equator at a rate that provides a uniform measure of time equal to the average apparent time; the reference for reckoning mean time, zone time, etc. See dynamical mean sun.
mean time: Time based upon the rotation of the earth relative to the mean sun.; Mean time may be designated as local or Greenwich as the local or Greenwich meridian is the reference. Greenwich mean time is also called universal time. Zone, standard, daylight saving or summer, and war time are also variations of mean time, specified meridians being used as the reference. Mean time reckoned from the upper branch of the meridian is called astronomical time. Mean time was called civil time in U.S. terminology from 1925 through 1952. See equation of time, mean sidereal time.
measurand: A physical quantity, force, property or condition which is to be measured. Also called stimulus.
mechanical equivalent of heat (symbol J) = Joule constant.
mechanical system: In the study of vibration, an aggregate of matter comprising a defined configuration of mass, mechanical stiffness, and mechanical resistance.
mechanoreceptor: A nerve ending that reacts to mechanical stimuli, as touch,, tension, and acceleration.
median: The middle term of a series, or the interpolated value of the two middle terms if the number of terms is even. Compare mean.
median lethal dose: The amount of radiation required to kill, within a specified period, 50 percent of the individuals of a group of animals or organisms.
medium frequency (abbr MF): See frequency bands.
mega (abbr M): A prefix meaning multiplied by 10⁶.
megacycle (abbr Mc, mc): One million cycles; one thousand kilocycles.; The term is often used as the equivalent of one million cycles per second.
megaparsec: One million parsecs. See parsec.
mel: A unit of acoustic pitch. By definition, a simple tone of frequency 1000 cycles per second, 40 decibels above a listener's threshold, produces a pitch of 1000 mels. The pitch of any sound that is judged by the listener to be n times that of a 1-mel tone is n mels.
membrane structure: A shell structure, often pressurized, that does not take wall bending or compression loads.
memory: The component of a computer, control system, guidance system, instrumented satellite, or the like, designed to provide ready access to data or instructions previously recorded so as to make them bear upon an immediate problem, such as the guidance of a physical object, or the analysis and reduction of data.
memory capacity: See storage capacity.
memory device: See storage.
Men, Mens: International Astronomical Union abbreviations for Mensa. See constellation.
Mensa (abbr Men, Mens): See constellation.
Mercury: See planet, table.
mercury memory: The retention of information by the propagation of a sound wave in liquid memory.
meridian: A north-south reference line, particularly a great circle through the geographical poles of the earth. The term usually refers to the upper branch, that half, from pole to pole, which passes through a given place, the other half being called the lower branch. See coordinate, table.; A terrestrial meridian is a meridian of the earth. Sometimes designated true meridian to distinguish it from magnetic meridian, compass meridian, or grid meridian, the north-south lines relative to magnetic, compass, or grid direction, respectively. An astronomical meridian is a line connecting points having the same astronomical longitude. A geodetic meridian is a line connecting points of equal geodetic longitude. Geodetic and sometimes astronomical meridians are also called geographic meridians. Geodetic meridians are shown on charts. The prime meridian passes through longitude 0°. A fictitious meridian is one of a series of great circles or lines used in place of a meridian for certain purposes. A transverse or inverse meridian is a great circle perpendicular to a transverse equator. An oblique meridian is a great circle perpendicular to an oblique equator. Any meridian used as a reference for reckoning time is called a time meridian. The meridian through any particular place or observer, serving as the reference for local time, is called local meridian, in contrast with the Greenwich meridian, the reference for Greenwich time. A celestial sphere, through the celestial poles and the zenith.
meridian angle: Angular distance east or west of the local celestial meridian; 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 a celestial body, measured eastward or westward from the local celestial meridian through 180°, and labeled E or W to indicate the direction of measurement. See hour angle.
meridian transit: See transit.
meridional: Referring to a meridian.
mesh: A set of branches forming a closed path in a network, provided that if any one branch is omitted from the set, the remaining branches of the set do not form a closed path.; The term loop is sometimes used in the sense of mesh.
meson: In the classification of a subatomic particles by mass, the second lightest of such particles. Its mass is intermediate between that of the lepton and the nucleon (see hyperon).; Mesons are highly unstable, very short-lived particles; they carry positive, negative, or no charge, and, in a vacuum, move with velocities approaching the speed of light. All of these particles have extremely short lifetimes and the heavier more unstable mesons tend to decay into lighter ones.
mesopause: The base of the inversion at the top of the mesosphere, usually found at 80 to 85 kilometers. See atmospheric shell.
mesopeak: The temperature maximum at about 50 kilometers in the mesosphere. See atmospheric shell.
mesosphere: 1. The atmospheric shell, in which temperature generally decreases with heights, extending from the stratopause at about 50 to 55 kilometers to the mesopause at about 80 to 85 kilometers.; 2. The atmospheric shell between the top of the ionosphere (the top of this region has never been clearly defined) and the bottom of the exosphere. (This definition has not gained general acceptance.)
message: 1. An ordered selection from an agreed set of symbols, intended to communicate information.; 2. The original modulating wave in a communication system.; The term in sense 1 is used in communication theory; the term in sense 2 is often used in engineering practice.
metabolic reserves: The energy source stored in chemical form, such as carbohydrates, that can be efficiently mobilized and utilized by the body, particularly for muscular activity and work beyond the normal level of activity of an individual.
metachemical: Pertaining to the chemistry of subatomic particles.
metagalaxy: The entire system of galaxies including the Milky Way.
metallic fuels: Of or pertaining to nuclear fuels which are a mixture, a pressed powder, or an alloy of a fissionable material, such as uranium-235 or plutonium-239, and a metal such as aluminum, zirconium, or stainless steel.
metastable atom: An atom with an electron excited to an energy level where simple radiation is forbidden and thus the atom is momentarily stable. See forbidden line.; The presence of these metastable atoms in a discharge is the cause of several anomalous effects since in essence they are storing energy which can be released to other particles upon collision. The Penning effect is a result of the presence of metastable atoms.
metastable compound: A chemical compound of comparative stability which, however, becomes unstable under a particular set of conditions.
metastable propellant: A metastable compound used as a propellant.; Nitromethane (CH₃NO₂), for example, may be used as a monopropellant at chamber pressure above 500 pounds per square inch. At lower pressure, it requires an oxidizer for stable combustion.
meteor: In particular, the light phenomenon which results from the entry into the earth's atmosphere of a solid particle from space; more generally, any physical object or phenomenon associated with such an event. See meteoroid.
meteoric: Of or pertaining to meteors and meteoroids.
meteorite: Any meteoroid which has reached the surface of the earth without being completely vaporized.
meteoritic: Of or pertaining to meteorites.
meteoritics: The study of meteorites and meteoric and meteoritic phenomena.
meteoroid: A solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom or molecule.
meteorological optics = atmospheric optics.
meteorological rocket: A rocket designed primarily for routine upper air observation (as opposed to research) in the lower 250,000 feet of the atmosphere, especially that portion inaccessible to balloons, i.e., above 100,000 feet. Also called rocketsonde.
meteorology: The study dealing with the phenomena of the atmosphere. This includes not only the physics, chemistry, and dynamics of the atmosphere, but is extended to include many of the direct effects of the atmosphere upon the earth's surface, the oceans, and life in general.; A distinction can be drawn between meteorology and climatology, the latter being primarily concerned with average, not actual, weather conditions. Meteorology may be subdivided, according to the methods of approach and the applications to human activities, into a large number of specialized sciences. The following are of interest to space science: aerology, aeronomy, dynamic meteorology, physical meteorology, radio meteorology.
meteor path: The projection of the trajectory of a meteor in the celestial sphere as seen by the observer.
meteor shower: A number of meteors with approximately parallel trajectories.
meteor stream: A group of meteoric bodies with nearly identical orbits.
meteor trail = meteor train.
meteor train: Anything, such as light or ionization, left along the trajectory of the meteor after the head of the meteor has passed.
meteor wake: Meteor train phenomena of very short duration, in general much less than a second.
meter: 1. (abbr m) The basic unit of length of the metric system, defined as 1,650,763.73 wavelengths in vacuum of the unperturbed transition 2p₁₀ - 5d₅ in krypton.; Effective 1 July 1959 in U.S. customary system of measures, 1 yard = 0.9144 meter, exactly, or 1 meter = 1.094 yards = 39.37 inches. The standard inch is exactly 25.4 millimeters.; 2. A device for measuring, and usually indicating, some quantity.
metering jet: A jet in a fuel-injection system.
method of attributes: In reliability testing, measurement of quality by noting the presence or absence of some characteristic (attribute) in each of the units in the group under consideration and counting how many do or do not possess it.; An example of this method is go and no-go gaging of a dimension.
method of characteristics: See characteristics.
method of small perturbation = perturbation method.
Metonic cycle: A period of 19 years, after which the various phases of the moon fall on approximately the same days of the year as in the previous cycle.; The Metonic cycle is the basis for the golden numbers used to determine the data of Easter. Four such cycles form a Callippic cycle.
metric photography: The recording of events by means of photography (either singly or sequentially), together with appropriate coordinates, to form the basis for accurate measurements.
metric system: The international decimal system of weights and measures based on the meter and the kilogram.; The use of the metric system in the United States was legalized by Congress in 1866 but was not made obligatory.
metric wave: See frequency bands.
metrology: The science of dimensional measurement; sometimes includes the science of weighing.
Mev = million electron volt.
Mic, Micr: International Astronomical Union abbreviations for Microscopium. See constellation.
Michaelson actinograph: A pyrheliometer of the bimetallic type used to measure the intensity of direct solar radiation in terms of the angular deflection of a blackened bimetallic strip exposed to the direct solar beams.
Micr.: International Astronomical Union Abbreviation for Microscopium. See constellation.
micro (abbr): 1. (abbr ľ). A prefix meaning divided by 10⁶.; 2. A prefix meaning very small, as in micrometeorite.
microbar (abbr ľb): The unit of pressure in the CGS system and equal to 1 dyne per square centimeter; the unit of sound pressure.; In British literature the term barye has been used.
The term bar properly denotes a pressure of 10⁶ dynes per square centimeter. Unfortunately, the bar was once used in acoustics to mean 1 dyne per square centimeter, but this is no longer correct.
microenvironment: The environment created and maintained within a very small space, such as a pressurized capsule or space suit, and sufficient to support life in a reasonably normal manner.
microlock: A satellite telemetry system which uses phase-lock techniques in the ground receiving equipment to achieve extreme sensitivity.
micromanometer: A manometer capable of measuring very small pressure changes or differences.
micrometeorite: A very small meteorite or meteoritic particle with a diameter in general less than a millimeter.
micrometeorite penetration: Penetration of the thin outer shell (skin) of space vehicles by small particles traveling in space at high velocities.
micrometer: One of a class of instruments for making precise linear measurements in which the displacements measured correspond to the travel of a screw of accurately known pitch.
micron (abbr ľ): 1. A unit of length equal to one-millionth of a meter or one-thousandth of a millimeter.; The micron is a convenient length unit for measuring wavelengths of infrared radiation, diameters of atmospheric particles, etc.; 2. = micron of mercury.
micron liter: One liter of gas at a pressure of one micron of mercury.
micron of mercury (abbr ľ of Hg or ľ Hg): A unit of pressure equal to a pressure of 1/1000th of 1 millimeter of mercury pressure at ° C and the standard acceleration of gravity; a millitorr (10^-3 torr approximately). See torr.
microphone: An electroacoustic transducer which receives an acoustic signal and delivers a corresponding electric signal.
Microscopium (abbr Mic, Micr): See constellation.
microsecond (abbr ľsec): One-millionth of a second.
microtorr: A unit of pressure equal to 10^-6 torr. See torr.
microwave: Of, or pertaining to, radiation in the microwave region.
microwave refractometer: A device for measuring the refractive index of the atmosphere at microwave frequencies - usually in the 3-centimeter region.
microwave region: Commonly, that region of the radio spectrum between approximately 1000 megacycles and 300,000 megacycles. See frequency band.; Corresponding wavelengths are 30 centimeters to 1 millimeter. The limits of the microwave region are not clearly defined but in general it is considered to be the region in which radar operates.
microwave turbulence: Irregular and fluctuating gradients of microwave refractive index in the atmosphere. See optical turbulence.; Microwave turbulence may be due either to blobby distribution of water vapor, or to thermal turbulence.
Midas: A two-object trajectory measuring system whereby two complete Cotar antenna systems and two sets of receivers at each station, with the multiplexing done after phase comparison, are utilized in tracking more than one object at a time.
midcourse guidance: Guidance of a rocket from the end of the launching phase to some arbitrary point or at some arbitrary time when terminal guidance begins. Also called incourse guidance. See guidance.
Midot (abbr) = multiple interferometer determination of trajectories.
Mie particle: See Mie theory.
Mie scattering: Any scattering produced by spherical particles without special regard to comparative size of radiation wavelength and particle diameter. See Mie theory.
Mie theory: A complete mathematical-physical theory of the scattering of electromagnetic radiation by spherical particles, developed by G. Mie in 1908. In contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter to wavelength.; The Mie theory is very important in meteorological optics, where diameter-to-wavelength ratios of the order of unity and larger are characteristic of many problems regarding haze and cloud scattering. Scattering of radar energy by raindrops constitutes another significant application of the Mie theory.
mil: 1. One-thousandth of an inch.; 2. A unit of angular measurement, 1/6400 of a circle.
mile: A unit of distance. See statute mile, nautical mile.
military grid: Any grid specified for use on a particular map, or series of maps, by military authorities.
Milky Way: The galaxy to which the sun belongs.; As seen at night from the earth, the Milky Way is a faintly luminous belt of faint stars.
milli (abbr m): A prefix meaning multiplied by 10^-3.
millibar: A unit of pressure equal to 1000 dynes per square centimeter, or 1/1000 of a bar.; The millibar is used as a unit of measure of atmospheric pressure, a standard atmosphere being equal to 1,013.25 millibars or 29.92 inches of mercury.
milligal: A unit of acceleration equal to 1/1000 of a gal, or 1/1000 centimeter per second per second.; This unit is used in gravity measurements, being approximately one-millionth of the average gravity at the earth's surface.
millimeter (abbr mm): One-thousandth of a meter; one-tenth of a centimeter; 0.039370 U.S. inch.
millimeter of mercury (abbr mm Hg): A unit of pressure corresponding to a column of mercury exactly 1 millimeter high at 0° C under standard acceleration of gravity of 980.665 centimeters per second square. See torr.; By mercury at 0° C is meant a hypothetical fluid having an invariable density exactly 13.5951 grams per cubic centimeter.
millimetric wave: See frequency band.
millimicron of mercury (abbr mľ Hg): A unit of pressure equal to 10^-6 millimeters of mercury or 10^-6 torr.
millimicrosecond (abbr mľsec) = nanosecond.
million electron volt (abbr Mev): A unit of energy equal to 1.603 X 10^-8 ergs.
millisecond (abbr msec): One-thousandth of a second.
millitorr: A proposed new unit of pressure equal to 10^-3 torr. See torr.
Mimas: A satellite of Saturn orbiting at a mean distance of 186,000 kilometers.
M-indicator = M-display
mini: A contraction of miniature used in combination, as in minicomponent, miniradio, minitransistor.
miniature: Used attributively in reference to equipment, such as gimbals, gyroscopes, computers, etc., made small to fit into confined spaces, as within an earth satellite or rocket vehicle.
miniaturization: See miniaturize.
miniaturize: To construct a functioning miniature of a part or instrument. Said of telemetering instruments or parts used in an earth satellite or rocket vehicle, where space is at a premium. Hence, miniaturized, miniaturization.
minimum deviation: The least total refraction experienced by radiation passing through a prismatic refractive medium.; It is important to note that the refractive deviation is minimal, in general, only with respect to adjacent light paths, for there may exist a number of path directions through a single object, each one of which yields a local minimum deviation. When radiation has undergone minimum deviation, the angular difference in path directions before and after total refraction is termed the angle of minimum deviation.
minimum ionizing speed: The speed with which a free electron must move through a given gas to be able to ionize gas atoms or molecules by collision. In air at standard conditions, this speed is about 10⁷ centimeters per second. See electron avalanche.
minitrack: A satellite tracking system consisting of a field of separate antennas and associated receiving equipment interconnected so as to form interferometers which track a transmitting beacon in the payload itself.
minor axis: The shortest diameter of an ellipse or ellipsoid.
minor lobe: See lobe.
minor planet = asteroid.: See planet.
minute (abbr min or ´ ): 1. The sixtieth part of an hour.; 2. The sixtieth part of a degree of arc.
mirage: 1. (Optics) definition submitted by Andy Young, June 30, 1997
An apparently reflected image of an object, caused by abnormal atmospheric refraction. The most commonly seen is the inferior mirage over heated surfaces, which often looks like a pool of water because it reflects the sky. Superior mirages, which appear above the direct image of the object, are due to strong thermal inversions above eye level. Other mirage-like images can be produced by thermal inversions below eye level, and by hot air adjacent to a wall heated by the Sun (lateral mirage).; 2. (Radar) A refraction phenomenon in the atmosphere wherein an image of some object is made to appear displaced from its true position. See radio duct, note.
Simple mirages may be any one of three types, the inferior mirage, the superior mirage, or the lateral mirage, depending, respectively, on whether the spurious image appears below, above, or to one side of the true position of the object. Of the three, the inferior mirage is the most common, being usually discernible over any heated street in daytime during summer. The abnormal refraction responsible for mirages is invariably associated with abnormal temperature distributions that yield abnormal spatial variations in the refractive index.
Miran (abbr) = missile ranging.
Miranda: A satellite of Uranus orbiting at a mean distance of 124,000 kilometers.
mirror altitude: The altitude above the earth at which electrically charged corpuscular radiation impinging upon the earth is reflected by the geomagnetic field.
mirror ratio: See magnetic mirror.
mirror reflection = specular reflection.
missile: Any object thrown, dropped, fired, launched, or otherwise projected with the purpose of striking a target. Short for ballistic missile, guided missile.; Missile should not be used loosely as a synonym for rocket or spacecraft.
missile ranging (abbr Miran): A trajectory measuring system that measures loop ranges from a transmitter to a beacon to remote slave stations and back to the transmitter through comparison of time differences of pulses.; The transmitter interrogates at 600 megacycles and the beacon replies at 580 megacycles.
missilry: The art or science of designing, developing, building, launching, directing, and sometimes guiding a rocket missile; any phase or aspect of this art or science.; This term is sometimes spelled missilery, but is then pronounced as a three-syllable word.
mistake: An error, usually large, resulting from a human failing or an equipment malfunction.
mixed-base notation: A system of positional notation used in computers in which two or more bases are arranged according to a plan. See biquinary notation.
mixed-flow compressor: A rotary compressor through which the acceleration of fluid is partly radial and partly axial.
mixed icing: Mixed aircraft icing is composed of both glaze and rime ice. Typically the ice will be clear near the stagnation line of the wing with the remainder rime. It represents about 15% of icing reports.
mixed reflection = spread reflection.
mixing length: A mean length of travel, characteristic of a particular motion in a fluid over which an eddy maintains its identity; analogous to the mean free path of a molecule.; Physically, the idea implies that mixing occurs by discontinuous steps, that fluctuations which arise as eddies with different characteristics wander about, and that the mixing is done almost entirely by the small eddies.
mixing ratio: In a system of moist air, the dimensionless ratio of the mass of water vapor to the mass of dry air. For many purposes, the mixing ratio may be approximated by the specific humidity. In terms of the pressure p and vapor pressure e, the mixing ratio w is w = (0.6222 e) / (p - e) Compare absolute humidity, relative humidity, dew point.
mixture ratio: In liquid-propellant rockets, the relative mass flow rates to the combustion chamber of oxidizer and fuel.
MKSA system: A system of units based on the meter, kilogram, second, and ampere. Also called Giorgi system, international system.
MKS system: A system of units based on the meter, the kilogram, and the second.
mobility: 1. The average velocity or drift velocity that a charged particle in a plasma will acquire in response to a unit applied electric field when restrained by collisions with other particles.; 2. = drift mobility.; 3. = Hall mobility.; In general the electron mobility is considerably larger than any ion mobility.
mock test: An operational test of a complete rocket system without actually firing a rocket.
mockup: A full-sized replica or dummy of something, such as a spacecraft, often made of some substitute material such as wood, and sometimes incorporating actual functioning pieces of equipment, such as engines.
mode: A functioning position or arrangement that allows for the performance of a given task.; Said of a spacecraft, which may move, for example, from a cruise mode to an encounter mode; or said of controls that permit the selection of a mode, such as a reentry mode.
model atmosphere: 1. Any theoretical representation of the atmosphere, particularly of vertical temperature distribution. See adiabatic atmosphere, homogeneous atmosphere, isothermal atmosphere, thermotropic model, equivalent barotropic model, barotropic model.; 2. = standard atmosphere, sense 1.
mode of vibration: In a system undergoing vibration, a characteristic pattern assumed by the system in which the motion of every particle is simple harmonic with the same frequency.; Two or more modes of vibration may exist concurrently in a multiple-degree-of-freedom system.
moderator: A material that has a high cross section for slowing down fast neutrons with a minimum of absorption, e.g., heavy water, beryllium, used in reactor cores.; Moderators are used to improve the neutron utilization by slowing the neutrons to low energies, thereby increasing the probability of fission capture in the nuclear fuel.
mode shape = mode of vibration.
modified index of refraction: An atmospheric index of refraction mathematically modified so that when its gradient is applied to energy propagation over a hypothetical flat earth it is substantially equivalent to propagation over the true curved earth with the actual index of refraction. Also called refractive modulus, modified refractive index. Compare potential index of refraction.; The modified index of refraction is usually expressed in M-units; mathematically where n is the index of refraction at a point in the atmospheric; h is the height above mean sea level of that point; a is the radius of the earth; and N is the index of refraction in N-units. In ray tracing problems, the vertical gradient dM/dh can be used directly to obtain a ray path curvature that is relative to the curvature of the earth, i.e., where k is a value by which the earth's radius is multiplied to get the radius of curvature of the ray path; ka is called the effective earth radius.
modified refractive index = modified index of refraction.
modulated continuous wave (abbr MCW): A form of emission in which the carrier is modulated by a constant audiofrequency tone.
modulated wave: A wave which varies in some characteristic in accordance with the variations of a modulating signal. Compare continuous wave. See modulation.
modulating wave: See modulation.
modulation: 1. The variation in the value of some parameter characterizing a periodic oscillation.; 2. Specifically, variation of some characteristic of a radio wave, called the carrier wave, in accordance with instantaneous values of another wave, called the modulating wave.; Variation of amplitude is amplitude modulation, variation of frequency is frequency modulation, and variation of phase is phase modulation. The formation of very short bursts of a carrier wave, separated by relatively long periods during which no carrier wave is transmitted, is pulse modulation.
modulation index = ratio deviation.
modulator: A device to effect the process of modulation.
module: 1. A self-contained unit of a launch vehicle or spacecraft which serves as a building block for the overall structure. The module is usually designated by its primary function as command module, lunar landing module, etc.; 2. A one-package assembly of functionally associated electronic parts, usually a plug-in unit, so arranged as to function as a system or subsystem; a black box.; 3. The size of some one part of a rocket or other structure, as the semidiameter of a rocket's base, taken as a unit of measure for the proportional design and construction of component parts.
modulus (plural moduli): 1. A real, positive quantity which measures the magnitude of some number, as the modulus of a complex number is the square root of the sum of squares of its components.; 2. A coefficient representing some elastic property of body, such as the modulus of elasticity or the modulus of resilience.
modulus of elasticity (symbol E) = Young modulus.
Mögel-Dellinger effect = fadeout.
moist adiabatic lapse rate = saturation adiabatic lapse rate.
molar: Pertaining to a mole, or measured in moles.
moldavite: See tektite.
mole (abbr mol): The amount of substance containing the same number of atoms as 12 grams of pure carbon¹² (C¹²).; The gram-mole or gram-molecule is the mass in grams numerically equal to the molecular weight.
molecular drag gage: A vacuum gage in which tangential momentum is transported (viscous transport) by gas molecules from a rapidly rotating member (usually in the form of a disk or cylinder) to a nearby movable member restrained by a restoring torque which can be correlated with gas pressure. Also called molecular gage, rotating disk gage, rotating cylinder gage.
molecular effusion: The passage of gas through a single opening in a plane wall of negligible thickness where the largest dimension of the hole is smaller than the mean free path.
molecular flow: The flow of gas through a duct under conditions such that the mean free path is greater than the largest dimension of a transverse section of the duct.
molecular flux: The net number of gas molecules crossing a specified surface in unit time, those having a velocity component in the same direction as the normal to the surface at the point of crossing being counted as positive and those having a velocity component in the opposite direction being counted as negative.
molecular gage = molecular drag gage .
molecular scale temperature (symbol T_M ): An atmospheric parameter defined by T_M = (M₀ / M)T where M₀ is the mean molecular weight at sea level, M is the mean molecular weight at altitude, and T is air temperature at altitude.; Up to an altitude of about 90 kilometers, the molecular composition of air is constant; thus M₀ / M = 1 and T _M = T. Above 90 kilometers, M₀ / M is greater than unity, and T _M is greater than T.
molecular weight: The weight of a given molecule expressed in atomic weight units.
molecule: An aggregate of two or more atoms of a substance that exists as a unit.
Moll thermopile: A thermopile used in some types of radiation instruments. Alternate junctions of series-connected thermocouples are imbedded in a shielded nonconducting plate having a large heat capacity. The remaining junctions, which are blackened, are exposed directly to the radiation. The voltage developed by the thermopile is proportional to the intensity of radiation. See solarimeter.
moment (symbol M): A tendency to cause rotation about a point or axis, as of a control surface about its hinge or of an airplane about its center of gravity; the measure of this tendency, equal to the product of the force and the perpendicular distance between the point of axis of rotation and the line of action of the force.
moment of inertia (symbol I): Of a body about an axis, , where m is the mass of a particle of the body and r is its distance from the axis.
momentum: Quantity of motion.; Linear momentum is the quantity obtained by multiplying the mass of a body by its linear speed. Angular momentum is the quantity obtained by multiplying the moment of inertia of a body by its angular speed.
The momentum of a system of particles is given by the sum of the momentums of the individual particles which make up the system or by the product of the total mass of the system and the velocity of the center of gravity of the system.
The momentum of a continuous medium is given by the integral of the velocity over the mass of the medium or by the product of the total mass of the medium and the velocity of the center of gravity of the medium.
momentum thrust = thrust.
momentum-transport hypothesis: The hypothesis that momentum is conserved in turbulent eddy transfer.; This hypothesis is to be compared with the vorticity transport hypothesis, the respective results being identical only if the eddy viscosity is constant.
Mon, Mono: International Astronomical Union abbreviations for Monoceros. See constellation.
monitor: To observe, listen in on, keep track of, or exercise surveillance over by any appropriate means, as, to monitor radio signals; to monitor the flight of a rocket by radar; to monitor a landing approach.
Monoceros (abbr Mon, Mono): See constellation.
monochromatic: Pertaining to a single wavelength, or, more commonly, to a narrow band of wavelengths.
monocoque: A type of construction, as of a rocket body, in which all or most of the stresses are carried by the skin.; A monocoque may incorporate formers but not longitudinal members such as stringers.
monopropellant: A rocket propellant consisting of a single substance, especially a liquid, capable of producing a heated jet without the addition of a second substance.; Used attributively in phrases, such as monopropellant rocket engine or motor, monopropellant rocket fuel, monopropellant system, etc.
monostatic reflectivity: The characteristic of a reflector which reflects energy only along the line of the incident ray (incidence), e.g., a corner reflector. See bistatic reflectivity.
month: 1. The period of the revolution of the moon around the earth.; The month is designated as sidereal, tropical, anomalistic, dracontic, or synodical, according to whether the revolution is relative to the stars, the vernal equinox, the perigee, the ascending node, or the sun.; 2. The calendar month, which is a rough approximation to the synodical month.
month of the phases = synodical month.
moon: 1. The natural satellite of the earth.; 2. A natural satellite of any planet. See planet table.
moonrise: The crossing of the visible horizon by the upper limb of the ascending moon.
moonset: The crossing of the visible horizon by the upper limb of the descending moon.
MOPTAR (abbr) = multiple object phase tracking and ranging.
motion: The act, process, or instance of change of position. Also called movement, especially when used in connection with problems involving the motion of one craft relative to another.; Absolute motion is motion relative to a fixed frame of reference. Actual motion is motion of a craft relative to the earth. Apparent or relative motion is change of position as observed from a reference point which may itself be in motion. Diurnal motion is the apparent daily motion of a celestial body. Direct motion is the apparent motion of a planet eastward among the stars; retrograde motion, the apparent motion westward among the stars. Motion of a celestial body through space is called space motion, which is composed of two components: proper motion, that component perpendicular to the line of sight; and radial motion, that component in the direction of the line of sight.
motion sickness: The syndrome of pallor, sweating, nausea, and vomiting which is induced by unusual accelerations.
motor: See engine.
motorboating: Oscillation in a system or component, usually manifested by a succession of pulses occurring at a subaudio or low-audio repetion frequency.
moving target indicator (abbr MTI): A device which limits the display of radar information primarily to moving targets.
M-region: See M-region magnetic storm, note.
M-region magnetic storm: A magnetic storm that is independent of visible solar disk features; it begins gradually and shows a strong tendency to recur within a period of 27 days.; The hypothetical region on the solar disk assumed to be the source of the incident corpuscular radiation is called the M-region.
M-scan = M-display.
M-scope = M-display.
MTI (abbr) = moving target indicator.
MUF (abbr) = maximum usable frequency.
multi (combining form): More than one. Used in contexts where a category of two or more is distinguished from a category of one, as in a multipropellant fuel system is more complicated than a monopropellant system.
multicoupler: A device for connecting several receivers to one antenna and properly matching the impedances of the receivers and the antenna.
multipath = multipath transmission.
multipath transmission: The process, or condition, in which radiation travels between source and receiver via more than one path. Since there can be only one direct path, some process of reflection, refraction, or scattering must be involved. See fading, Fresnel zone. Also called multipath.
multiple airborne target trajectory system (abbr Matts): A long-baseline angle-measuring system consisting of two crossed-baseline angle-measuring-equipment (AME) stations. Each AME station simultaneously tracks three airborne targets by means of frequency sharing.
multiple-degree-of-freedom system: A mechanical system for which two or more coordinates are required to define completely the position of the system at any instant.
multiple interferometer determination of trajectories (abbr Midot): A trajectory measurement system with multiple-object-tracking capability utilizing two or more short-baseline stations and a data output consisting of a series of amplitude nulls that represent direction cosines at given times in the flight.
multiple object phase tracking and ranging (abbr Moptar): A short-baseline continuous-wave phase comparison, trajectory measuring system, similar to the Cotar which consists of a crossed-baseline angle-measuring-equipment (AME) system and a distance-measuring-equipment (DME) system, wherein time sequencing of the ground station and transponders is used to track multiple targets.
multiple scattering: In contrast to primary scattering, and scattering in which radiation is scattered more than once before reaching the eye, antenna, or other sensing element.
multiple-stage compressor = multistage compressor.
multiple-stage rocket = multistage rocket.
multiple-unit steerable antenna: See musa antenna.
multiplexer: A mechanical or electrical device for time sharing or a circuit.
multiplexing: The simultaneous transmission of two or more signals within a single channel.; The three basic methods of multiplexing involve the separation of signals by time division, frequency division, and phase division.
multiplier: 1. A device which has two or more inputs and whose output is a representation of the product of the quantities represented by the input signals.; 2. = multiplier phototube.
multiplier phototube: A phototube with one or more dynodes between its photocathode and the output electrode.
multipropellant: A rocket propellant consisting of two or more substances fed separately to the combustion chamber. See bipropellant.
multistage compressor: An axial-flow compressor having two or more, usually more than two, stages of rotor and stator blades; a radial-flow compressor having two or more impeller wheels. Also called a multiple-stage compressor.
multistage rocket: A vehicle having two or more rocket units, each unit firing after the one in back of it has exhausted its propellant. Normally, each unit, or stage, is jettisoned after completing its firing. Also called a multiple-stage rocket or, infrequently, a step rocket.
multivibrator: A two-stage regenerative circuit with two possible states and an abrupt transition characteristic. See bistable multivibrator.; Multivibrators are used in digital computer for computation in binary notation.
M-unit: See modified index of refraction.
muon = mu meson.: See meson.
Mus, Musc: International Astronomical Union abbreviations for Musca. See constellation.
musa antenna: A multiple-unit steerable antenna consisting of a number of stationary antennas, the composite major lobe of which can be aimed electrically.
Musca (abbr Mus, Musc): See constellation.
myria: A prefix meaning multiplied by 10⁴.
myriameter = ten-thousand meters.
myriametric wave: See frequency bands.

Back to Table of Contents