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**NACA (abbr)**- National Advisory Committee for Aeronautics. The predecessor of NASA.
**NACA Standard Atmosphere**- See standard atmosphere.
**nadir**- That point on the celestial sphere vertically below the observer, or 180 degrees from the zenith.
**nano (abbr n)**- A prefix meaning multiplied by 10
^{-9}. **nanosecond (abbr nsec)**- 10E-9 second. Also called
*millimicrosecond.* **Napierian base**- The logarithmic base, e.
**NASA (abbr)**- National Aeronautics and Space Administration.
**NASC (abbr)**- National Aeronautics and Space Council.
**natural coordinates**- An orthogonal, or mutually perpendicular, system of curvilinear coordinates for the description of fluid motion, consisting of an axis T tangent to the instantaneous velocity vector and an axis N normal to this velocity vector to the left in the horizontal plane, to which a vertically directed axis Z may be added for the description of three-dimensional flow.
**natural frequency**- 1. The frequency of free oscillation of a system. For a multiple-degree-of-freedom system, the natural frequencies are the frequencies of the normal modes of vibration.
- 2. The undamped resonance frequency of a physical system. It is expressed in cycles per unit time. The system may be mechanical, pneumatic, or electrical.
**natural mode**= normal mode of vibration.**natural year**= tropical year.**nausea**- A feeling of discomfort in the region of the stomach, with aversion to food and a tendency to vomit.
**Nautical Almanac**- An annual publication of the U.S. Naval Observatory and H. M. Nautical
Almanac Office, Royal Greenwich Observatory, listing the Greenwich hour angle
and declination of various celestial
bodies to a precision of 0.1 minute of arc at hourly intervals; time of
sunrise, sunset, moonrise, moonset; and other astronomical information useful
to navigators. Prior to 1960 separate publications were issued by the two
observatories entitled the
*American Nautical Almanac*and the*Abridged Nautical Almanac.*See American Ephemeris and Nautical Almanac. **nautical mile**- A unit of distance used principally in navigation. For practical
navigation it is usually considered the length of 1 minute of any great
circle of the earth, the meridian being
the great circle most commonly used. Also called
*sea mile.* *Because of various lengths of the nautical mile in use throughout the world, due to differences in definition and the assumed size and shape of the earth, the International Hydrographic Bureau in 1929 proposed a standard length of 1852 meters, which is known as the international nautical mile. This has been adopted by nearly all maritime nations. The U.S. Departments of Defense and Commerce adopted this value on July 1, 1954. With the yard-meter relationship then in use, the international nautical mile was equivalent to 6076.10333 feet. Using the yard-meter conversion factor effective July 1, 1959, the international nautical mile is equivalent to 6076.11549 international feet.***nautical twilight**- That period when the upper limb of the sun is below the visible horizon and the center of the sun is not more than 12 degrees below the celestial horizon.
**nautical year**= tropical year.**Navier-Stokes equations**- The equations
of motion for a viscous
fluid which may be written
where *p*is the pressure; ρ is the density;*t*is the time;*F*is the total external force;*N*is the fluid velocity; and*v*is the kinematic viscosity. For an incompressible fluid, the term in^{.}N (divergence) vanishes and the effects of viscosity then play a role analogous to that of temperature in thermal conduction and to that of density in simple diffusion. See viscosity, Ekman spiral. *Solutions of the Navier-Stokes equations have been obtained only in a limited number of special cases. The equations are derived on the basis of certain simplifying assumptions concerning the stress tensor of the fluid; in one dimension they represent the assumption referred to as the Newtonian friction law.***navigation**- The practice or art of directing the movement of a craft from one point to another.
*Navigation usually implies the presence of a human, a navigator, aboard the craft. Compare guidance.***navigational planets**- The four planets commonly used in celestial surface and air navigation: Venus, Mars, Jupiter, and Saturn.
**navigational stars**- The 57 stars included in the main listing of the
*Nautical Almanac*and Polaris. *The navigational stars include almost all of the stars with common names. See table VII.***navigational triangle**- The spherical triangle solved in computing altitude and azimuth. See celestial triangle.
*The celestial triangle is formed on the celestial sphere by the great circles connecting the elevated pole, zenith of the assumed position of the observer, and a celestial body. The terrestrial triangle is formed on the earth by the great circles connecting the pole and two places on the earth. The term astronomical triangle applies to either the celestial or terrestrial triangle used for solving celestial observations.***navigation dome**= astrodome.**N-display**- In radar, a display similar to the K-display in
which the target appears
as a pair of vertical deflections or blips from the
horizontal time base. Direction is indicated by the relative amplitude of the
vertical deflections; target distance is determined by moving an adjustable
signal along the baseline until it coincides with the horizontal position of
the vertical deflections. The horizontal control is calibrated in distance.
Also called
*N-scan, N-scope, N-indicator.* **negative acceleration**= deceleration.**negative altitude**- See altitude.
**negative divergence**= convergence.**negative feedback**- Feedback which results in decreasing the amplification.
**negative g**- In designating the direction of acceleration on a body, the opposite of
*positive g*, for example, the effect of flying an outside loop in the upright seated position. See physiological acceleration. **negative temperature coefficient**- 1. The partial derivative of any physical variable with respect to temperature, when the value of the variable decreases as temperature increases.
- 2. The decrease in reactivity of a nuclear reactor with increase in temperature.
*Increasing temperature within the reactor increases the average neutron energy. Since the cross section of the fissionable material decrease with increased neutron energy, the net effect of increased temperature is to decrease the number of fissions.***negaton**- Variant spelling
*negatron.*See electron. **negatron**- A negative electron. Sometimes shortened to
*negaton.*See electron. **negentropy**= average information content.**neper (abbr n)**- A measure of power equal to one-half the
*Napierian logarithm*(logarithm to the base*e*) of the power ratio. 1 neper is equal to 8.686 decibels. *Under certain conditions the neper is a measure of the ratio of amplitudes of such physical quantities as voltage, current, particle velocities, and sound pressure. The neper is not commonly used in English-speaking countries.***nephelometer**- 1. General name for instruments which measure, at more than one angle, the scattering function of particles suspended in a medium. Information obtained from such instruments may be used to determine the size of the suspended particles an the visual range through the medium.
- 2. An instrument for chemical analysis by measuring the light-scattering properties of a suspension.
- 3. = nephometer. See visibility meter.
**nephelometry**- 1. The study of suspensoids by means of light-scattering techniques. See nephelometer.
- 2. The study of the scattering properties of small samples of air and its suspensoids.
- 3. Chemical analysis by use of the nephelometer.
**nepheloscope**- 1. An instrument for demonstrating the temperature changes which occur in air that is rapidly expanded or compressed.
- 2. A laboratory instrument for the production of clouds by the condensation process.
- 3. = nephoscope.
**nephometer**- A general term for instruments designed to measure the amount of
cloudiness. An early type consists of a convex hemispherical mirror mapped
into six parts. The amount of cloud coverage on the mirror is noted by the
observer. Also called
*nephelometer.*Compare nephoscope. **nephoscope**- An instrument for determining the direction of cloud motion. There are two
basic designs of nephoscope: the direct-vision nephoscope and the mirror
nephoscope. Also called
*nepheloscope.* **Neptune**- See planet, table.
**Nereid**- A satellite of Neptune orbiting at a mean distance of 5,570,000 kilometers.
**net radiation factor**- The fraction of the total energy emitted by one surface or volume that is absorbed by another surface or volume directly and indirectly.
**net thrust**- The gross thrust of a jet engine minus the drag due to the momentum of the incoming air.
**network**- 1. A combination of electrical elements.
- 2. A group of parts or systems combined to provide a closed information loop, i.e., one that provides for inquiry or command, response, and interpretation of response in relation to inquiry or command.
**neurology**- The study of the anatomy, physiology, and pathology of the nervous system.
**neuromuscular**- Pertaining jointly to nerves and muscles, as
*neuromuscular junction.* **neutral**- Without an electrical charge; neither positive nor negative.
**neutral line**= Busch lemniscate.**neutral point**- 1. In atmospheric optics, one of several points in the sky for which the degree of polarization of diffuse sky radiation is zero.
*In an ideal Rayleigh atmosphere there would be just two such points, the solar point and the antisolar point. Because of effects of multiple scattering and because of the action of scattering particles larger than molecules, the actual atmosphere is typically found to have quite different neutral points. In the vertical plane containing the observer's zenith and the sun, three neutral points are commonly found: the Arago point, the Babinet point, and the Brewster point.*- 2. In aircraft, that location of the center of gravity at which the aircraft would exhibit neutral aerodynamic stability.
- 3. = Lagrangian point.
**neutrino**- A subatomic particle of zero, or near zero, rest mass, having no electric charge, postulated by Fermi (1934) in order to explain apparent contradictions to the law of conservation of energy in beta-particle emission.
*According to Fermi theory, the atomic nucleus in beta decay releases energy partly in the form of electrons (beta particles) and partly in the form of neutrinos.***neutron**- A subatomic particle with no electric charge, and with a mass of 1.67482 X 10E-24 gram.
*Protons and neutrons comprise atomic nuclei; and they are both classed as nucleons.***neutron flux**- The sum of the distances traveled by all the neutrons in 1 cubic centimeter in 1 second. Normally the figure must be energy qualified, e.g., thermal, intermediate, or fast neutron flux.
**neutrosphere**- The atmospheric shell from the earth's surface upward in which the atmospheric constituents are for the most part not ionized, i.e., it is electrically neutral.
*The region of transition between the neutrosphere and the ionosphere is somewhere between 70 and 90 kilometers, depending on latitude and season.***Newcomb tables of the sun**- See ephemeris time.
**new moon**- The moon at conjunction,
when little or none of it is visible to an observer on the earth because the
illuminated side is away from him. Also called
*change of the moon.*See phases of the moon. **newton**- The unit of force in the MKSA system; that force which gives to a mass of 1 kilogram an acceleration of 1 meter per second squared.
**Newton equations of motion**= Newton laws of motion.**Newton formula for the stress**= Newtonian friction law.**Newtonian friction law**- The statement that the tangential force (i.e., the force in the direction
of the flow)
per unit area acting at an arbitrary level within a fluid contained
between two rigid horizontal plates, one of which is motionless and the other
of which is in steady motion, is proportional to the shear of the fluid motion
at that level. Mathematically, the law is given by
Τ = μ( u / z) where Τ is the tangential force per unit area, usually called the shearing stress; μ is a constant of proportionally called the dynamic viscosity; and u/z is the shear of the fluid flow normal to the resting plate. Also called*Newton formula for the stress.* **Newtonian mechanics**- The system of mechanics based upon Newton laws of motion in which mass and energy are considered as separate, conservative, mechanical properties, in contrast to their treatment in relativistic mechanics.
**Newtonian speed of sound**- An approximation to the speed of sound
*a*in a perfect gas given by the relation where p is pressure and ρ is density. Compare Laplacian speed of sound. See acoustic velocity.*a*^{2}= p / ρ *Newton derived this expression by assuming the propagation of sound to be an isothermal process. It leads to values about 16 percent below those observed.***Newtonian telescope**- A reflecting telescope in which a small plane mirror reflects the convergent beam from the objective to an eyepiece at one side of the telescope. After the second reflection the rays travel approximately perpendicular to the longitudinal axis of the telescope. See Cassegrain telescope.
**Newtonian universal constant of gravitation**= gravitational constant.**Newton law of cooling**- See thermal conductivity.
**Newton law of gravitation**- Every particle of matter in the universe attracts every other particle
with a force, F, acting along the line joining the two particles, proportional
to the product of the masses M and m of the particles and inversely proportional to
the square of the distance r between the particles, or F = G(M * m) / r
^{2}, where G = gravitational constant. **Newton laws of motion**- A set of three fundamental postulates forming the basis of the mechanics of rigid bodies, formulated by Newton in 1687.
*The first law is concerned with the principle of inertia and states that if a body in motion is not acted upon by an external force, its momentum remains constant (law of conservation of momentum). The second law asserts that the rate of change of momentum of a body and is in the direction of the applied force. A familiar statement of this is the equation F = ma where F is vector sum of the applied forces, m is the mass, and a is the vector acceleration of the body. The third law is the principle of action and reaction, stating that for every force acting upon a body there exists a corresponding force of the same magnitude exerted by the body in the opposite direction.***N-indicator**= N-display.**nitrogen cycle**- The exchange of nitrogen between animals and plants, in which plants convert urea or nitrates to protein, animals digest protein and excrete its nitrogen content an urea, which is taken up again by plants.
**nitrogen desaturation**- The reduction of the nitrogen content of the tissues of the body by breathing gases not containing nitrogen.
**noctilucent clouds**- Clouds of unknown composition which occur at great heights, 75 to 90
kilometers. They resemble thin cirrus, but usually with a bluish or silverish
color, although sometimes orange to red, standing out against a dark night
sky. Sometimes called
*luminous clouds.* *These clouds have been seen rarely, and then only during twilight, especially with the sun between 5 and 13 degrees below the horizon; they have been observed only during summer months in both hemispheres (between latitudes 50 to 75 degrees N and 40 to 60 degrees S), and only in some parts of these latitude belts.***nocturnal radiation**= effective terrestrial radiation.**nodal point**= node.**node**- 1. One of the two points of intersection of the orbit of a
planet, planetoid, or comet with the ecliptic, or
of the orbit of a satellite
with the plane of the orbit of its primary. Also
called
*nodal point.*See regression of the nodes. *That point at which the body crosses to the north side of the reference plane is called the ascending node; the other, the descending node. The line connecting the nodes is called line of nodes.*- 2. A point, line, or surface in a standing wave where some characteristic of the wave field has essentially zero amplitude.
*The appropriate modifier should be used before the word node to signify the type that is intended; e.g., displacement node, velocity node, pressure node.*- 3. A terminal of any branch of a network or a terminal common to two or
more branches of a network. Also called
*junction point, branch point*, or*vertex.* **nodical month**= dracontic month.**nodical period**- The interval between successive passages of a satellite through the ascending node.
**noise**- 1. Any undesired sound. By extension, noise is any unwanted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device.
*When caused by natural electrical discharges in the atmosphere, noise may be called static.*- 2. An erratic, intermittent, or statistically random oscillation.
- 3. In electrical circuit analysis, that portion of the unwanted signal which is statistically random, as distinguished from hum, which is an unwanted signal occurring at multiples of the power-supply frequency.
*If ambiguity exists as to the nature of the noise, a phrase such as acoustic noise or electric noise should be used. Since the above definition are not mutually exclusive, it is usually necessary to depend on context for the distinction. See white noise.***noise level**- See level.
**nonadiabatic process**- See diabatic process.
**noncoherent echo**- See coherent echo.
**noncondensable gas**- A gas whose temperature is above its critical temperature, so that it cannot be liquefied by increase of pressure alone.
**nondimensional number**- A pure number not involving any physical dimensions, e.g., a ratio of two velocities or two lengths.
*Such numbers are fundamental descriptive quantities of a physical system. Nondimensional numbers involving several variables often are interpreted as estimates of characteristics velocity ratios, force ratios, heat transfer ratios, frequency ratios, etc. Usually several different ratio interpretations are possible and useful for the same number. (See Mach number, Reynolds number, Boussinesq number, Cauchy number, Prandtl number, Peclet number, Rayleigh number, Strouhal number, Richardson number, Nusselt number, Grashof number, Taylor number, Froude number.)***nondimensional parameter**- Any parameter of a problem which has the dimensions of a pure number, usually rendered so deliberately. See nondimensional number.
**nonimpinging injector**- An injector used in rocket engines which employs parallel streams of propellant usually emerging normal to the face of the injector.
*In this injector, mixing is usually obtained by turbulence and diffusion. The V-2 used a nonimpinging injector.***nonlinear damping**- Damping due to a damping force that is not proportional to velocity.
*The words force and velocity should be treated in the generalized sense. For example, they can be replaced by voltage and current, respectively.***nonlinear distortion**- Distortion caused by a deviation from a proportional relationship between specified measures of the output and input of a system.
*The related measures need not be output and input values of the same quantity; e.g., in a linear detector, the desired relation is between the output signal voltage and the input modulation envelope.***nonvolatile**- Of a computer or
computer component. The ability to retain information in the absence of power
as
*nonvolatile memory*,*nonvolatile storage.* **nonvolatile storage**= permanent memory.**noon**- The instant at which a time reference is over the upper branch of the reference meridian.
*Noon may be solar or sidereal as either the sun or vernal equinox is over the upper branch of the reference meridian. Solar noon may be further classified as mean or apparent as the mean or apparent sun is the reference. Noon may also be classified according to the reference meridian, either the local or Greenwich meridian or additionally in the case of mean noon, a designated time zone meridian.***Nor, Norm**- International Astronomical Union abbreviations for Norma. See constellation.
**Norma (abbr Nor, Norm)**- See constellation.
**normal**- 1. Equivalent to usual, regular, rational, or standard conditions.
- 2. Perpendicular.
*A line measured to another line or a plane when it is perpendicular to it. A line is normal to a curve or curved surface when it is perpendicular to the tangent line or plane at the point of tangency.*- 3. Referring to thermal radiation properties, in a direction perpendicular to the surface.
- 4. The line normal, sense 2, to a surface.
**normal dispersion**- Dispersion of an electromagnetic wave characterized by an increase in refractive index with increase in frequency.
**normal distribution**- The fundamental frequency distribution of statistical analysis. A
continuous variety
*x*is said to have a normal distribution or to be normally distributed if it possesses a density function*f(x)*which satisfies the equationwhere μ is the arithmetic mean (or first moment) and σ is the standard deviation. Also called *Gaussian distribution.* *About two-thirds of the total area under the curve is included between x = μ - σ and x = μ + σ. The corresponding frequency distribution of vectors is the normal circular distribution in which the frequencies of vector deviations are represented by a series of circles centered on a vector mean. When applied to error distribution, this function is the normal law of errors, and the distribution is called the normal curve of error.***normal emittance**- Emittance in a direction perpendicular to the surface or in a small solid angle whose axial ray is perpendicular to the surface.
**normal functions**- See orthogonal functions.
**normal gravity**- See acceleration of gravity.
**normalize**- 1. To change in scale so that the sum of squares, or the integral of the squares of the transformed quantity is unity. See orthogonal functions".
- 2. To transform a random variable so that the resulting random variable so that the resulting random variable has a normal distribution.
- 3. In computer operations, to adjust the exponent and coefficient of a floating-point
result so that the coefficient is in the prescribed normal range. Also called
*standardize.* **normal mode of vibration**- A mode of
free vibration of an undamped system. In general, any composite motion of a
vibrating system ban be analyzed into a summation of its normal modes. Also
called
*natural mode, characteristic mode*, and*eigenmode.* **normal plane**- In aerodynamics, a plane at right angles to the longitudinal axis of an aerodynamic vehicle.
**normal shock**= normal shock wave.**normal shock wave**- A shock
wave perpendicular, or substantially so, to the direction of flow in a supersonic
flow field. Sometimes shortened to
*normal**shock.* **northbound node**=ascending node.**northern lights**= aurora borealis.**north polar sequence**- A list of stars near the north celestial pole arranged in order of photographic magnitude, used as reference stars in stellar photometry.
**north pole**- 1. In astronomy, that end of the axis of rotation of a celestial body at which, when viewed from above, the body appears to rotate in a clockwise direction. See celestial pole, ecliptic pole, geographical pole, geomagnetic pole, magnetic pole.
- 2. The north-seeking end of a magnet.
**north-upward plan position indicator**- See plan position indicator.
**nose cone**- The cone-shaped leading end of a rocket vehicle, consisting (a) of a chamber or chambers in which a satellite, instruments, animals, plants, or auxiliary equipment may be carried, and (b) of an outer surface built to withstand high temperatures generated by aerodynamic heating.
*In a satellite vehicle, the nose cone may become the satellite itself after separating from the final stage of the rocket or it may be used to shield the satellite until orbital speed is accomplished, then separating from the satellite.***nose gear**- That part of a landing gear which is located at the forward end of the vehicle.
**notation**- A manner of representing quantities. See positional notation, binary notation, biquinary notation, decimal notation, fixed point, floating point, hexidecimal notation, octal notation, sexidecimal notation, ternary notation.
**NOT circuit**- In computers, a device or circuit which
inverts the polarity of a
pulse. Also
called
*inverter.* **nova (plural novae)**- A star which suddenly becomes many times brighter than previously, and then gradually fades.
**nozzle (symbol n used as subscript)**- 1. A duct, tube, pipe, spout, or the like through which a fluid is directed and from the open end of which the fluid is discharged, designed to meter the fluid or to produce a desired direction, velocity, or shape of discharge. See de Laval nozzle, jet nozzle, supersonic nozzle.
- 2. Specifically, that part of a rocket thrust chamber assembly in which the gases produced in the chamber are accelerated to high velocities.
**nozzle blade**- Any one of the blades or vanes in a nozzle
diaphragm. Also called a
*nozzle vane.* **nozzle-contraction area ratio**- (symbol
*ε*). Ratio of the cross-sectional area for gas flow at the nozzle inlet to that at the throat._{c} **nozzle diaphragm**- A ring of stationary, equally spaced blades or vanes, forming an annulus
of nozzles through which fluid is directed onto a turbine wheel. Sometimes
called a
*nozzle ring.* **nozzle-divergence loss factor**- (symbol λ). The ratio between the momentum of the gases in
a nozzle with an angle 2a and the momentum of an ideal
nozzle. In mathematical form
λ = 1/2(1 + cos .*a*) **nozzle efficiency**- The efficiency with which a nozzle converts potential energy into kinetic energy, commonly expressed as the ratio of the actual change in kinetic energy to the ideal change at the given pressure ratio.
**nozzle exit area**- The cross-sectional area of a rocket nozzle available for gas flow measured at the nozzle exit.
**nozzle-expansion area ratio**(symbol ε_{e})- Ratio of the cross-sectional area for gas flow at the exit of a nozzle to the cross-sectional area available for gas flow at the throat.
**nozzle ring**= nozzle diaphragm.**nozzle throat**- The portion of a nozzle with the smallest cross section.
**nozzle throat area**- The area of the minimum cross section of a rocket nozzle.
**nozzle thrust coefficient**- (symbol C
_{F}). A measure of the amplification of thrust due to gas expansion in a particular nozzle as compared with the thrust that would be exerted if the chamber pressure acted only over the throat area. Also called*thrust coefficient.* **nozzle vane**= nozzle blade.**N-scan**= N-display.**N-scope**= N-display.**nuclear Bohr magneton**= nuclear magneton.**nuclear cross section (symbol σ)**- A measure of the probability that the reaction will take place which is
defined by
*dI = In σ dx*where*I*is the intensity of the particle beam;*n*is the number of target nuclei per cubic centimeter of target; σ is the cross section for the specified process, expressed in square centimeters; and*x*is the target thickness in centimeters. See barn. **nuclear-electric rocket engine**- A rocket engine in which a nuclear reactor is used to generate electricity which is used in an electric propulsion system or as a heat source for the working fluid.
**nuclear emulsion**- A very thick photographic emulsion used in the study of cosmic rays and other energetic particles. The path of the particle through the thick emulsion is recorded in three dimensions.
**nuclear fuel**- Fissionable material of reasonable long life, used or usable in producing energy in a nuclear reactor.
**nuclear magneton (symbol M**_{N})- A unit of magnetic moment of the proton equal to 5.0505 X 10
^{ -24}erg per gauss. **nuclear radiation**- Corpuscular emissions, such as alpha and beta particles, or electromagnetic radiation, such as gamma rays, originating in the nucleus of an atom.
**nuclear reactor**- An apparatus in which nuclear fission may be
sustained in a self supporting chain
reaction. Commonly called
*reactor.*Formerly called*pile.* **nuclear rocket engine**- A rocket engine in which a nuclear reactor is used as a power source or as a source of thermal energy.
**nucleon**- In the classification of subatomic particles according to mass, the second heaviest type of particle; its mass is intermediate between that of the meson and the hyperon.
*Examples of the nucleon are the proton and neutron.***nucleus**- 1. The positively charged core of an atom with which is associated practically the whole mass of the atom but only a minute part of its volume.
*A nucleus is composed of one or more protons and an approximately equal number of neutrons. The atomic number Z of the element indicates the number of protons in the nucleus. The mass number A of the element is the sum of the protons and neutrons.*- 2. In biology, a definitely delineated body within a cell containing the chromosomes.
**nuclide**- An individual atom of given atomic
number Z and mass number
A, for example, $$
_{92}U^{235}. *A nuclide is any species of atom that exist for a measurable length of time and has a nuclear structure distinct from that of any other species of atom.***null**- In direction-finding systems wherein the output amplitude is a function of the direction of arrival of the signal, the minimum output amplitude (ideally zero).
*The null is frequently employed as a means of determining bearing. The term minimum is often used to indicate an imperfect null.***number**- In computer operations, (a) amount of units by count, (b) a magnitude or quantity represented by group of digits.
*The term quantity is preferred to number in sense (b).***number system**- A scheme for representing magnitudes or quantities by a group of digits. See numeric coding, positional notation.
**numeric**- Composed wholly or partly of digits. See alphanumeric, number.
**numerical aperture**- The quantity n sin θ, the product of the index of refraction of the object medium, usually air, multiplied by the sine of the slope angle of the outermost ray from an axial point on the object.
**numeric coding**- A system of coding in which information is represented by digits. See alphabetic coding, alphanumeric.
**N-unit**- A unit of index of refraction in the atmosphere; a mathematical simplification designed to replace rather awkward numbers involved in the values of the index of refraction n for the atmosphere. It is defined by the relation N = (n - 1) 10E6
*Thus, if n = 1.000326, then N = 326. Correspondingly, M-units are used for modified index of refraction, and B-units for potential index of refraction.***Nusselt number (symbol N**_{Nu})- (After Wilhelm Nusselt, 1882- , German engineer). A number expressing the
ratio of convective to conductive heat
transfer between a solid boundary and a moving fluid, defined as
*hl/k*where*h*is the heat-transfer coefficient,*l*is the characteristic length, and*k*is the thermal conductivity of the fluid. **nutating feed**- In a tracking radar an oscillating antenna feed for producing an oscillating deflection of the beam in which the plane of polarization remains fixed.
**nutation**- 1. The oscillation of the axis of any rotating body, as a gyroscope rotor.
- 2. Specifically, in astronomy, irregularities in the precessional motion of the equinoxes because of varying positions of the moon and, to a lesser extent, of other celestial bodies with respect to the ecliptic.
- Because of nutation, the earth's axis nods like a top, describing a slightly wavy circle about the ecliptic pole. The maximum displacement is about 9.21 seconds (constant of nutation) and the period of a complete cycle is 18.60 tropical years (period of moon's node, nutation period).
**nutation period**- See nutation, note.
**nutator**- A mechanical device for gyrating the antenna feed horn or dipole or a radar about the axis of the reflector without changing its polarization.
**Nyquist frequency**- The highest frequency which can be determined in a Fourier
analysis of a discrete sampling of data. If a time series is sampled at
interval
*Δt*, this frequency is 1/2*Δt*cycles per second. Also called*turnover frequency.* **nystagmus**- An involuntary oscillation of the eyeballs, especially occurring as a result of eye fixations and stimulations of the inner ear during rotation of the body.