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- NACA (abbr)
- National Advisory Committee for Aeronautics. The predecessor of NASA.
- NACA Standard Atmosphere
- See standard
- 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
- 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.
- 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
- 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
- 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
- The practice or art of directing the movement of a craft from one point to
- 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,
- navigational stars
- The 57 stars included in the main listing of the Nautical Almanac
- 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
- 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.
- 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
- negative temperature coefficient
- 1. The partial derivative of any physical variable with respect to
temperature, when the value of the variable decreases as temperature
- 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.
- Variant spelling negatron. See electron.
- 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
- 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
- 1. General name for instruments which measure, at more than one angle, the
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.
- 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
- 3. Chemical analysis by use of the nephelometer.
- 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
- 3. = nephoscope.
- 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.
- 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.
- See planet, table.
- 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.
- 1. A combination of electrical
- 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.
- The study of the anatomy, physiology, and pathology of the nervous system.
- Pertaining jointly to nerves and muscles, as neuromuscular junction.
- 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
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
- 2. In aircraft, that location of the center of gravity at which the
aircraft would exhibit neutral aerodynamic stability.
- 3. = Lagrangian
- 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
- A subatomic
particle with no electric charge, and with a mass of 1.67482 X 10E-24
- 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.
- 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
- 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
- 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
of the moon.
- 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 a2 = p / ρ where p is pressure and ρ is density. Compare Laplacian
speed of sound. See acoustic
- Newton derived this expression by assuming the propagation of sound to
be an isothermal process. It leads to values about 16 percent below those
- 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
- Newtonian universal constant of gravitation =
- 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) / r2,
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
- 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
- nocturnal radiation = effective terrestrial radiation.
- nodal point = 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
- The appropriate modifier should be used before the word node to signify
the type that is intended; e.g., displacement node, velocity node, pressure
- 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 ,
- nodical month = dracontic month.
- nodical period
- The interval between successive passages of a satellite
through the ascending
- 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 level
- See level.
- nonadiabatic process
- See diabatic
- noncoherent echo
- See coherent
- 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
- nondimensional parameter
- Any parameter of a problem which has the dimensions of a pure number,
usually rendered so deliberately. See nondimensional
- 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
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.
- 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.
- 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.
- 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
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 equation
where μ is the arithmetic mean (or first moment) and σ is the standard deviation. Also called
- 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
- normal gravity
- See acceleration
- 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
- 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
- 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
- normal shock = normal
- 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
- northern lights = aurora
- north polar sequence
- A list of stars near the north celestial pole arranged in order of
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
- 2. The north-seeking end of a magnet.
- north-upward plan position indicator
- See plan
- 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
- 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
- nose gear
- That part of a landing
gear which is located at the forward end of the vehicle.
- A manner of representing quantities. See positional
- NOT circuit
- In computers, a device or circuit which
inverts the polarity of a
- 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
nozzle, jet 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 εc). Ratio of the cross-sectional area for gas flow
at the nozzle inlet to that at the throat.
- 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
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
- 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 CF). 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
- 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
material of reasonable long life, used or usable in producing energy in a nuclear
- nuclear magneton (symbol MN)
- A unit of magnetic moment of the proton equal to 5.0505 X 10 -24 erg per
- 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.
- 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.
- 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
- 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
- An individual atom of given atomic
number Z and mass number
A, for example, .
- 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
- In direction-finding systems wherein the output amplitude is a function of
the direction of arrival of the signal, the minimum output amplitude (ideally
- The null is frequently employed as a means of determining bearing. The
term minimum is often used to indicate an imperfect null.
- 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
- Composed wholly or partly of digits. See alphanumeric,
- 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.
- 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)
- 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
- Nusselt number (symbol NNu)
- (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
- 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,
- 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
- An involuntary oscillation of the eyeballs, especially occurring as a
result of eye fixations and stimulations of the inner ear during rotation of