G

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

An acceleration equal to the acceleration of gravity, 980.665 centimeter-second-squared, approximately 32.2 feet per second per second at sea level; used as a unit of stress measurement for bodies undergoing acceleration. See acceleration of gravity; gravity.

gage pressure

In engineering literature, a term used to indicate the difference between atmospheric pressure and absolute pressure, as read from a differential manometer.

gain

1. A general term used to denote an increase in signal power or signal strength in transmission from one point to another. Gain is usually expressed in decibels and is widely used to denote transducer gain.

2. An increase or amplification. In radar there are two general usages of the term: (a) antenna gain, or gain factor, is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; (b) receiver gain, or video gain, is the amplification given a signal by the receiver. See height gain.

gain factor

See gain, sense 2(a).

gal

(From Galileo). A unit of acceleration equal to 1 centimeter per second per second, or 1000 milligals.

The gal and milligal are used in measuring the acceleration of gravity.

galactic

1. Pertaining to our galaxy, the Milky Way.

2. Pertaining to the galactic system of coordinates, as galactic latitude.

galactic circle = galactic equator.

See galactic system of coordinates.

galactic equator

See galactic system of coordinates.

galactic pole

See galactic system of coordinates.

galactic radio waves

Radio waves emanating from our galaxy. See cosmic radio waves.

galactic system of coordinates

An astronomical coordinate system using latitude measured north and south from the galactic equator and longitude measured in the sense of increasing right ascension from 0 to 360 degrees. See coordinate, table.

Galactic latitude is designated b, galactic longitude l. The reference points for galactic coordinates were changed by action of the International Astronomical Union in 1958. The new values are: the north galactic pole lies in the direction right ascension = 12 hours 49 minutes, declination = 27.4 degrees N (equinox 1950); the new zero of longitude is the great semicircle originating at the new north galactic pole at the position angle 0 = 123 degrees with respect to the equatorial pole for 1950.

galaxy

A vast assemblage of stars, nebulae, etc., composing an island universe separated from other such assemblages by great distances.

The sun and its family of planets is part of a galaxy commonly called the Milky Way. The nearest galaxy to the Milky Way is the spiral galaxy Andromeda at a distance of approximately 800,000 light years.

gamma photon = gamma ray.

gamma radiation = gamma ray.

gamma ray

A quantum of electromagnetic radiation emitted by a nucleus, each such photon being emitted as the result of a quantum transition between two energy levels of the nucleus. Gamma rays have energies usually between 10 thousand electron volts and 10 million electron volts with correspondingly short wavelengths and high frequencies. Also called gamma radiation.

X-rays occur in the same energy range as gamma rays but are of nonnuclear origin. In atmospheric electricity, gamma rays are of some importance in contributing to atmospheric ionization, along with alpha particles and beta particles. Gamma ray photons have much greater penetration ranges than do alpha and beta particles, often amounting to distances of the order of a hundred meters in air at sea level. These high-energy photons may initiate their ionizing action by ejecting photoelectrons from neutral atoms or molecules of the air, by ejecting electrons by the Compton effect, or (for gamma photons with energies above a few million electron volts) by pair production in which an electron and a positron are created.

gantry

A frame structure that spans over something, as an elevated platform that runs astride a work area, supported by wheels on each side; short for gantry crane or gantry scaffold.

gantry crane

A large crane mounted on a platform that usually runs back and forth on parallel tracks astride the work area. Often shortened to gantry.

gantry scaffold

A massive scaffolding structure mounted on a bridge or platform supported by a pair of towers or trestles that normally run back and forth on parallel tracks, used to assemble and service a large rocket as the rocket rests on its launching pad. Often shortened to gantry.

This structure is a latticed arrangement of girders, tubing, platforms, cranes, elevators, instruments, wiring, floodlights, cables, and ladders - all used to attend the rocket.

Ganymede

A satellite of Jupiter orbiting at a mean distance of 1,071,000 kilometers. Also called Jupiter III.

garbage

Miscellaneous objects in orbit, usually material ejected or broken away from a launch vehicle or satellite.

gas

The state of matter in which the molecules are practically unrestricted by intermolecular forces so that the molecules are free to occupy any space within an enclosure.

In vacuum technology the word gas has been loosely applied to the noncondensable gas and vapor within a vacuum system.

gas cap

The gas immediately in front of a body as it travels through the atmosphere.

This gas is compressed and heated. If the speed is sufficiently high, the gas becomes incandescent; it is to this condition that the term is usually applied, as in the gas cap of a meteor.

gas constant (symbol R, R*)

The constant factor in the equation of state for perfect gases. The universal gas constant is R = 8.3143 joules/degrees K-mol The gas constant for a particular gas, specific gas constant, r = R/ m where m is the molecular weight of the gas. See Boltzmann constant.

gas constant per molecule = Boltzmann constant.

gaseous discharge = electric discharge.

gaseous electric discharge = electric discharge.

gaseous electronics

The study of the conduction of electricity through gases, involving study of the Townsend, glow, and arc discharges, and all the collision phenomena on an atomic scale. Formerly called gaseous discharges.

gas laws

The thermodynamic laws applying to perfect gases: Boyle-Mariotte law, Charles-Gay-Lussac law, Dalton law, equation of state. Also called perfect-gas laws, ideal-gas laws.

gas scrubbing

The contacting of a gaseous mixture with a liquid for the purpose of removing gaseous contaminants or entrained liquids or solids.

gas turbine

1. A turbine rotated by expanding gases, as in a turbojet engine or in a turbosupercharger.

2. A gas-turbine engine.

gas-turbine engine

An engine incorporating as its chief element a turbine rotated by expanding gases. In its most usual form, it consists essentially of a rotary air compressor with an air intake, one or more combustion chambers, a turbine, and an exhaust outlet.

gate

1. To control passage of a signal as in the circuits of a computer.

2. A circuit having an output and inputs so designed that the output is energized only when a definite set of input conditions are met. In computers, called AND-gate.

gating

The process of selecting those portions of a wave which exist during one or more selected time intervals or which have magnitudes between selected limits.

gauss

A unit of magnetic induction (or magnetic flux density) equal to 1 dyne per unit cgs magnetic pole.

Prior to 1932, the gauss was used both as a unit of magnetic induction and as a unit of magnetic field intensity, but the latter quantity is now measured in oersteds.

Gaussian constant (symbol k )

Originally used in astronomical calculations as k = G^1/2 , where G is the constant of gravitation. k is now defined as exactly 0.01720209895 radians. Also called Gaussian gravitation constant. See astronomical unit.

Gaussian distribution = normal distribution.

Gaussian gravitation constant = Gaussian constant.

Gaussian noise = random noise.

Gauss theorem = divergence theorem.

Gay-Lussac law = Charles-Gay-Lussac law.

GCA (abbr) = ground-controlled approach.

GCI (abbr) = ground-controlled intercept.

G-display

In radar, a rectangular display in which a target appears as a laterally centralized blip when the radar antenna is aimed at it in azimuth and wings appear to grow on the blip as the distance to the target is diminished. Horizontal and vertical aiming errors are respectively indicated by horizontal and vertical displacement of the blip. Also called G-scan, G-scope, G-indicator.

gee

A suffix meaning earth, as in perigee , apogee. See perigee, note.

gegenschein

A round or elongated spot of light in the sky at a point 180 degrees from the sun. Also called counterglow, zodiacal counterglow. Compare zodiacal light.

Geiger counter

An instrument for detecting and measuring radioactivity. In full, Geiger-Muller counter.

This counter, essentially a thin-walled gas-filled metallic tube with a needle electrode projected within, detects the radiating particle indirectly. The particle penetrates the thin wall and ionizes the gas; a current is momentarily set up, which is detectable and measurable. Compare scintillation counter.

Geiger-Muller counter

Full term for Geiger counter.

Gem, Gemi

International Astronomical Union abbreviations for Gemini. See constellation.

Gemini (abbr Gem, Gemi)

See constellation.

general circulation = planetary circulation.

generalized coordinates

Any set of coordinates specifying the state of the system under consideration. Usually employed in problems involving a finite number of degrees of freedom, the generalized coordinates are chosen so as to take advantage of the constraints of the system in reducing the total number of coordinates. Also called Lagrangian coordinates.

generalized transmission function

In atmospheric-radiation theory, a set of values, variable with wavelength, each one of which represents an average transmission coefficient for a small wavelength interval and for a specified optical path through the absorbing gas in question. See universal transmission function.

general perturbations

In orbital determinations, a method of calculating perturbative effects by expanding and integrating in series. See perturbation.

general precession

The resultant motion of the components causing precession of the equinoxes. The general precession is westward along the ecliptic at the rate of about 50.3 seconds of arc per year.

The effect of the sun and moon, called lunisolar precession, is to produce a westward motion of the equinoxes along the ecliptic. The effect of other planets, called planetary precession, tends to produce a much smaller motion eastward along the ecliptic. The component of general precession along the celestial equator, called precession in right ascension, is about 46.1 seconds of arc per year; and the component along a celestial meridian, called precession in declination, is about 20.0 seconds of arc per year. See astronomical constants.

generation

In any technical or technological development, as of a missile, jet engine, or the like, a stage or period that is marked by features or performances not marked, or existent, in a previous period of development or production, as in the first generation of rockets using liquid propellants.

genetic effect of radiation

Inheritable changes, chiefly mutations, produced in living organisms by the absorption of ionizing radiations. On the basis of present knowledge these effects are purely additive, and there is no recovery.

geo

A prefix meaning earth, as in geology, geophysics.

Some writers use the established terms such as geology to refer to the same concept on other bodies of the solar system, as the geology of Mars, rather than areology or marsology, geology of the Moon, rather than selenology.

geocentric

Relative to the earth as a center; measured from the center of the earth.

geocentric diameter

The diameter of a celestial body measured in seconds of arc as viewed from the earth's center.

geocentric latitude

Of a position in the earth's surface; the angle between a line to the center of the earth and the plane of the equator.

Because the earth is approximately an oblate spheroid, rather than a true sphere, this differs from geographic latitude, the maximum difference being 11.6 minutes of arc at latitude 45 degrees.

geocentric parallax

The difference in the apparent direction or position of a celestial body measured in seconds of arc, as observed from the center of the earth and a point on its surface. See parallax.

geocorona

1. The shell of hydrogen surrounding the earth at the limit of the atmosphere.

In Shlovsky's system of nomenclature, the geocorona includes the metasphere, the outer, fully ionized zone, and the protosphere, the inner zone.

2. = Van Allen radiation belts.

The use of geocorona in sense 2 should be discouraged, since it conflicts with the relatively well-established usage in sense 1.

geodesic line

The shortest line on a mathematically derived surface, between two points on the surface. Also called geodesic.

A geodesic line on the spheroidal earth is called a geodetic line.

geodesy

The science which deals mathematically with the size and shape of the earth, and the earth's external gravity field, and with surveys of such precision that overall size and shape of the earth must be taken into consideration.

geodetic

Of or pertaining to geodesy; geodesic.

geodetic coordinates

Quantities which define the position of a point on the spheroid of reference with respect to the planes of the geodetic equator and of a reference meridian. Compare geographic coordinates.

geodetic datum

A datum consisting of five quantities, the latitude and longitude and elevation above the reference spheroid of an initial point, a line from this point, and two constants which define the reference spheroid. Azimuth or orientation of the line, given the longitude, is determined by astronomic observations. Alternatively, the datum may be considered as three rectangular coordinates fixing the origin of a coordinate system whose orientation is determined by the fixed stars, and the reference spheroid is an arbitrary coordinate surface of an orbiting ellipsoidal coordinate system.

A geodetic datum forms the basis for the computation of horizontal control surveys in which the curvature of the earth is considered.

geodetic equator

The great circle midway between the poles of revolution of the earth, connecting points of 0 degrees geodetic latitude. See astronomical equator.

geodetic latitude

Angular distance between the plane of the equator and a normal to the spheroid. It is the astronomical latitude corrected for the meridional component of the deflection of the vertical. Also called geographic latitude, topographical latitude.

This is the latitude used for charts.

geodetic line

A geodesic line on the spheroidal earth. Also called geodesic. Compare geodesic line.

geodetic longitude

The angle between the plane of the reference meridian and the plane through the polar axis and the normal to the spheroid. It is the astronomical longitude corrected for the prime vertical component of the deflection of the vertical divided by the cosine of the latitude. Also called geographic longitude.

This is the longitude used for charts.

geodetic meridian

A line connecting points of equal geodetic longitude. Also called geographic meridian. See astronomical meridian.

geodetic parallel

A line connecting points of equal geodetic latitude. Also called geographic parallel. See astronomical parallel.

geodetic position

A position of a point on the surface of the earth expressed in terms of geodetic latitude and geodetic longitude.

A geodetic position implies an adopted geodetic datum, which must be stated for a complete record of the position.

geodetic survey

1. A survey which takes into account the size and shape of the earth.

2. An organization engaged in making geodetic surveys, sense 1.

geodynamic height = dynamic height.

geodynamic meter = dynamic meter. See dynamic height.

geographical coordinates = geographic coordinates.

geographical mile

The length of 1 minute of arc of the equator, or 6087.08 feet.

geographical pole

Either of the two points of intersection of the surface of the earth with its axis or rotation where all meridians meet, labeled N or S to indicate whether the north geographical pole of the south geographical pole.

geographical position

1. That point on the earth at which a given celestial body is in the zenith of a specified time.

The geographical position of the sun is also called the subsolar point, of the moon the sublunar point, and of a star the substellar or subastral point.

2. Any position on the earth defined by means of its geographic coordinates, either astronomical or geodetic.

geographic coordinates

Coordinates defining a point on the surface of the earth, usually latitude and longitude. Also called terrestrial coordinates, geographical coordinates. See coordinate, table, for relationship between geographic coordinates and celestial coordinates.

Geographic coordinates can refer to either astronomical or geodetic coordinates.

geographic latitude = geodetic latitude.

geographic longitude = geodetic longitude.

geoid

The figure of the earth as defined by the geopotential surface which most nearly coincides with mean sea level over the entire surface of the earth.

Because of variations in the direction of gravity, to which it is everywhere perpendicular, the geoid is not quite an ellipsoid of revolution, the sea-level surface being higher under mountainous areas. Compare equilibrium spheroid, geosphere.

geoidal horizon

That circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through a point on the geoid perpendicular to the zenith-nadir line. See horizon.

geomagnetic

Of or pertaining to geomagnetism.

geomagnetic coordinates

A system of spherical coordinates based on the best fit of a centered dipole to the actual magnetic field of the earth.

geomagnetic equator

The terrestrial great circle everywhere 90 degrees from the geomagnetic poles.

Geomagnetic equator should not be confused with magnetic equator, the line connecting all points of zero magnetic dip. Compare aclinic line.

geomagnetic latitude

Angular distance from the geomagnetic equator, measured northward or southward through 90 degrees and labeled N or S to indicate the direction of measurement. Also erroneously called magnetic latitude.

Geomagnetic latitude should not be confused with magnetic latitude, the magnetic dip. Phenomena closely related to the earth's magnetic field are often plotted according to geomagnetic latitude rather than geographic latitude.

geomagnetic meridian

1. The meridional lines of a geomagnetic coordinate system.

2. = magnetic meridian.

geomagnetic pole

Either of two antipodal points marking the intersection of the earth's surface with the extended axis of a dipole assumed to be located at the center of the earth and approximating the source of the actual magnetic field of the earth.

That pole in the Northern Hemisphere (latitude, 78 1/2 degrees N; longitude, 69 degrees W) is designated north geomagnetic pole, and that pole in the Southern Hemisphere (latitude, 78 1/2 degrees S, longitude, 111 degrees E) is designated south geomagnetic pole. The great circle midway between these poles is called geomagnetic equator. The expression geomagnetic pole should not be confused with magnetic pole, which relates to the actual magnetic field of the earth. See geomagnetic latitude.

geomagnetic storm = magnetic storm.

geomagnetism

1. The magnetic phenomena, collectively considered, exhibited by the earth and its atmosphere and by extension the magnetic phenomena in interplanetary space.

2. The study of the magnetic field of the earth. Also called terrestrial magnetism.

geometrical horizon

See horizon.

geometric chord

See chord, note.

geometric dilution of precision

(abbr GDOP). As applied to a trajectory measuring system, the increase in error of calculated space position which is caused by the location of the measured position relative to the system.

This is an increase which is caused solely by the geometry of the problem and assumes that all basic measurements maintain a constant accuracy.

geometric mean

A measure of central position. The geometric mean of n quantities equals the n th root of the product of the quantities.

geometric position = true position.

geophysics

The physics of the earth and its environment, i.e., earth, air, and (by extension) space.

Classically, geophysics is concerned with the nature of and physical occurrences at and below the surface of the earth including, therefore, geology, oceanography, geodesy, seismology, hydrology, etc. The trend is to extend the scope of geophysics to include meteorology, geomagnetism, astrophysics, and other sciences concerned with the physical nature of the universe.

geopotential

The potential energy of a unit mass relative to sea level, numerically equal to the work that would be done in lifting the unit mass from sea level to the height at which the mass is located; commonly expressed in terms of dynamic height or geopotential height. Compare gravitational potential.

The geopotential Φ at height Z is given mathematically by the expression, where g is the acceleration of gravity.

geopotential height

The height of a given point in the atmosphere in units proportional to the potential energy of unit mass ( geopotential) at this height, relative to sea level.

The relation, in the cgs system, between the geopotential height H and the geometric height Z is where g is the acceleration of gravity, so that the two heights are numerically interchangeable for most meteorological purposes. Also, 1 geopotential meter is equal to 0.98 dynamic meter. See dynamic height. At the present time, the geopotential height unit is used for all aerological reports, by convention of the World Meteorological Organization.

geopotential meter

A unit of length used in measuring geopotential height; 1 geopotential meter is equal to 0.98 dynamic meter. See dynamic height.

geopotential surface

A surface of constant geopotential, i.e., a surface along which a particle of matter could move without undergoing any changes in its potential energy. Also called equigeopotential surface, level surface.

Geopotential surfaces almost coincide with surfaces of constant geometric height. Because of the poleward increase of the acceleration of gravity along a constant geometric-height surface, a given geopotential surface has a small geometric-height over the poles than over the equator. See potential, geopotential height, dynamic height.

georef (abbr) = World Geographic Reference System. Pronounced as a word.

geosphere

The solid and liquid portions of the earth; the lithosphere plus the hydrosphere. Compare geoid, equilibrium spheroid.

Above the geosphere lies the atmosphere and at the interface between these two regions is found almost all of the biosphere, or zone of life.

geostrophic wind

That horizontal wind velocity for which the coriolis acceleration exactly balances the horizontal pressure force. Compare gradient wind.

geostrophic wind level

The lowest level at which the wind becomes geostrophic in the theory of the Ekman spiral, proportional to where v is the kinematic eddy viscosity and the latitude. Also called gradient wind level.

In practice it is observed that the geostrophic wind level is between 1.2 and 1.6 kilometers, and it is assumed that this marks the upper limit of frictional influence of the earth's surface. The geostrophic wind level may be considered to be the top of the Ekman layer and planetary boundary layer, i.e., the base of the free atmosphere.

get

To remove gas from a vacuum system by sorption.

getter

1. A material which is included in a vacuum system or device for removing gas by sorption.

2. To remove gas by sorption. Also called get.

G-force

An inertial force usually expressed in multiples of terrestrial gravity.

GHA (abbr) = Greenwich hour angle.

giant planets

The planets Jupiter, Saturn, Uranus, and Neptune. Also called Jovian planets.

gibbous

Bounded by convex curves.

The term is used particularly in reference to the moon when it is between first quarter and full or between full and last quarter, or to other celestial bodies when they present a similar appearance. See phases of the moon.

Gibbs free energy = Gibbs function.

Gibbs function

A mathematically defined thermodynamic function of state, which is constant during a reversible isobaric-isothermal process. Also called Gibbs free energy, thermodynamic potential. Compare Helmholtz function.

In symbols the specific Gibbs function g is g = h - Ts where h is specific enthalpy; T is Kelvin temperature; and s is specific entropy. By use of the first law of thermodynamics for reversible processes, dg = -s dT + dp.

giga (abbr G)

A prefix meaning multiplied by 10⁹.

gimbal

1. A device with two mutually perpendicular and intersecting axes of rotation, thus giving free angular movement in two directions, on which an engine or other object may be mounted.

2. In a gyro, a support which provides the spin axis with a degree of freedom.

3. To move a reaction engine about on a gimbal so as to obtain pitching and yawing correction moments.

4. To mount something on a gimbal.

gimbaled motor

A rocket engine mounted on a gimbal.

gimbal freedom

In a gyro, the maximum angular displacement about the output axis of a gimbal.

It is expressed in degrees of output angle or in equivalent angular input.

gimbal lock

A condition of a two-degree-of-freedom gyro wherein the alignment of the spin axis with an axis of freedom deprives the gyro of a degree of freedom, and therefore of its useful properties.

G-indicator

1. G-display.

2. A display that shows the amount of inertial force acting on body.

Giorgi system = MKSA system.

G-layer

A layer of free electrons in the ionosphere which is occasionally observed above the F2-layer. See ionosphere.

glaze icing

Glaze aircraft icing (also known as clear icing) is hard and transparent. It is formed by the relatively slow freezing of supercooled water droplets as they spread over the surface. It tends to acccumulate more rapidly than rime icing, and is often very hard and therefore more difficult to remove. Since it does not freeze instantly, the ice can form into shapes that cause significant aerdynamic penalties, and it therefore the most hazardous form of icing. However, it is the least frequent type of ice encountered, reponsible for about 10% of icing reports.

glide

1. A controlled descent by a heavier-than-air aeronautical vehicle under little or no engine thrust in which forward motion is maintained by gravity and vertical descent is controlled by lift forces.

2. A descending flight path of a glide, sense 1, as, a shallow glide. 3. To descend in a glide, sense 1.

glide angle = gliding angle.

glide path

1. The flight path of an aeronautical vehicle in a glide, seen from the side.

2. The path used by an aircraft or spacecraft in approach procedure and which is generated by an instrument-landing facility.

glider

1. A fixed-wing aircraft specially designed to glide, or to glide and soar. This kind of aircraft ordinarily has no powerplant.

2. See hypersonic glider.

glide ratio

The ratio of the horizontal distance traveled to the vertical distance descended in a glide. Also called gliding ratio.

glide scope

1. An inclined surface which includes a glide path and which is generated by an instrument-landing facility.

2. = slope angle.

3. = gliding angle.

gliding angle

The angle between the horizontal and the glide path of an aircraft. Also called glide angle or glide slope.

gliding ratio = glide ratio.

global radiation

The total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface.

Global radiation is measured by pyranometers.

global velocities

The range of velocities, slightly less than circular velocity, that permit sustained flight once around the earth in equilibrium glide. Compare orbital velocity.

globe lightning = ball lightning.

glow discharge

Any electrical discharge which produces luminosity.

Thus corona discharge is a glow discharge, but point discharge is not. Relatively high electric field strengths are required for glow discharges, for the density of radiatively recombining gas atoms and molecules must be high. See gaseous electronics.

G-meter

A meter that indicates acceleration.

GMT (abbr) = Greenwich mean time.

gnotobiotics

The study of germ-free animals.

goniometer

An instrument for measuring angles.

gox

Gaseous oxygen.

gradient

1. The space rate of decrease of a function. Of a function in three space dimensions, the vector normal to surfaces of constant value of the function and directed toward decreasing values, with magnitude equal to the rate of decrease of the function in this direction. The gradient of a function f is denoted by -f (without the minus sign in the older literature), and is itself a function of both space and time. The ascendant is the negative of the gradient. See del-operator.

2. Often loosely used to denote the magnitude of the gradient or ascendant. 3. Either the rate of change of a quantity (as temperature, pressure, etc.) or a diagram or curve representing this.

gradient wind

Any horizontal wind velocity tangent to the contour line of a constant pressure surface or an isobar of a geopotential surface. At such points the coriolis acceleration and the centripetal acceleration together exactly balance the horizontal pressure force. Compare geostrophic wind.

grain

1. An elongated molding or extrusion of solid propellant for a rocket, regardless of size.

2. In photography, a small particle of metallic silver remaining in a photographic emulsion after development and fixing. In the agglomerate, these grains form the dark area of a photographic image.

3. An individual crystal in a polycrystalline metal or alloy.

gram

The standard of mass in the metric system.

gram-calorie

See calorie.

gram-centimeter

The CGS gravitation unit of work.

gram-molecule

The mass in grams of a substance numerically equal to its molecular weight.

granules

Small bright features of the photosphere of the sun, covering 50 to 60 percent of the surface. They have been likened in appearance to rice grains.

graph

A diagram indicating the relationship between two or more variables.

Grashof number (symbol N_Gr, G_r)

A nondimensional parameter used in the theory of heat transfer, defined by N_Gr = l³g [(T₁ - T₀) / v²T₀] where l is a representative length; T₁ and T₀ are representative temperatures; g is the acceleration of gravity; and v is the kinematic viscosity.

The Grashof number is associated with the Reynolds number and the Prandtl number in the study of convection.

grass

1. Sharp, closely space discontinuities in the trace of a cathode-ray tube, produced by random interference; so named because of their resemblance to blades of lawn grass.

2. In radar, a descriptive colloquialism used to refer to the indication of noise on an 'A' or similar type of display. See noise.

graticule

1. The network of lines representing parallels and meridians on a map, chart, or plotting sheet. See grid.

2. A scale at the focal plane of an optical instrument to aid in the measurement of objects. See reticle.

graviceptor = gravireceptor.

gravireceptors

Highly specialized nerve endings and receptor organs located in skeletal muscles, tendons, joints, and in the inner ear which furnish information to the brain with respect to body position, equilibrium, and the direction of gravitational forces. See gravitation.

gravitation

The acceleration produced by the mutual attraction of two masses, directed along the line joining their centers of masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.

This acceleration on a unit mass has the magnitude G(m/r2), where m is the mass of the attracting body, r is the distance between the centers of mass, and G is the gravitational constant equal to 6.670 +/- 0.005 X 10^-8 cm²/gram sec². In the case of masses in the earth's gravitational field, m is the mass of the earth, equal to 5.975 X 10²⁷ grams. However, the rotation of the earth and atmosphere modifies this field to produce the field of gravity.

gravitational

Pertaining to gravitation.

gravitational constant (symbol G)

The coefficient of proportionality in Newton law of gravitation. G = 6.670 +/- 0.005 X 10^-8 dyne-centimeter squared per gram squared. Also called constant of gravitation, Newtonian universal constant of gravitation.

In celestial mechanics, G may be used as a symbol with units unspecified or in a particular problem may be made equal to 1 or 4 pi squared by the choice of units for other parameters in the particular problem.

gravitational potential

1. The potential associated with the force of gravitation arising from the attraction between mass points, e.g., the earth's center and a particle in space.

The gravitational potential, associated with the force of gravitation, should not be confused with the geopotential associated with the force of gravity. The latter is equal to the former plus the centrifugal force due to the earth's rotation. The potentials of the three forces are related in the same manner.

2. At any point, the work needed to remove an object from that point to infinity.

gravitational red shift

See red shift, note.

gravitational tide

An atmospheric tide due to gravitational attraction of the sun or moon. See thermal tide.

The simidiurnal solar atmospheric tide is partly gravitational; the semidiurnal lunar atmospheric tide is totally gravitational.

gravitational wave = gravity wave.

gravitation constant

See Newtonian universal constant of gravitation, Gaussian constant.

graviton

The hypothetical elementary unit of gravitation which is equivalent to the electron in electromagnetic theory.

gravity

1. Viewed from a frame of reference fixed in the earth, force imparted by the earth to a mass which is at rest relative to the earth. Since the earth is rotating, the force observed as gravity is the resultant of the force of gravitation and the centrifugal force arising from this rotation and the use of an earthbound rotating frame of reference. It is directed normal to sea level and to its geopotential surfaces. See virtual gravity, geopotential height, standard gravity.

The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is a maximum but directed opposite to the force of gravitation. This difference is accentuated by the shape of the earth, which is nearly that of an oblate spheroid of revolution slightly depressed at the poles. Also, because of the asymmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's center. The magnitude of the force of gravity is usually called either gravity, acceleration of gravity, or apparent gravity.

2. = acceleration of gravity.

3. By extension, the attraction of any heavenly body for any mass; as Martian gravity.

gravity wave

A wave disturbance in a fluid in which buoyancy (or reduced gravity) acts as the restoring force on fluid parcels displaced from hydrostatic equilibrium. Also called gravitational wave.

gravity well

An analogy in which the gravitational field is considered as a deep pit out of which a space vehicle has to climb to escape from a planetary body.

gray body

A hypothetical body which absorbs some constant fraction, between zero and one, of all electromagnetic radiation incident upon it, which fraction is the absorptivity and is independent of wavelength. As such, a gray body represent a surface of absorptive characteristics intermediate between those of white body and a black body. No such substance are known in nature. Also called grey body.

Gray code

A binary code in which adjacent quantities differ in one place or column only. Often used in mechanical devices.

grayout

A temporary condition in which vision is hazy, restricted, or otherwise impaired, owing to insufficient oxygen. Compare blackout.

great circle

The intersection of a sphere and a plane through its center. Also called orthodrome.

The intersection of a sphere and a plane which does not pass through its center is called a small circle.

greatest elongation

The maximum angular distance of a body of the solar system from the sun, as observed from the earth. The direction of the body east or west of the sun is usually specified, as greatest elongation west.

Great Red Spot

An oval feature in the visible cloud surface of Jupiter, at latitude 20 to 25 degrees S. It is about 25,000 miles long in the planet's east-west direction, and about 7000 miles wide in the north-south direction. It is often reddish in color, but may be white or grey, or nearly invisible compared to its surroundings. See South Tropical Disturbance.

The neighboring cloud matter seems to pass around it on the northern side, producing the so-called Red Spot Hollow, by which it may be detected even when the spot itself is invisible. Its rotation period averages 9 hours, 55 minutes, 38 seconds (very nearly the same as the rest of the planet), but varies enough so that through the years since its discovery in 1878 it has made more than one complete revolution with respect to the underlying planet.

great year

The period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years. Also called platonic year. See precession of the equinoxes.

green flash

A brilliant green coloring of the upper edge of the sun as it appears at sunrise or disappears at sunset when there is a clear, distinct horizon.

The green flash is due to refraction by the atmosphere, which disperses the first (or last) spot of light into a spectrum. The green is bent more than red or yellow and hence is visible sooner at sunrise and later at sunset.

greenhouse effect

The heating effect exerted by the atmosphere upon the earth by virtue of the fact that the atmosphere (mainly, its water vapor) absorbs and remits infrared radiation. In detail: the shorter wavelengths of insolation are transmitted rather freely through the atmosphere to be absorbed at the earth's surface. The earth then reemits this as long-wave (infrared) terrestrial radiation, a portion of which is absorbed by the atmosphere and again emitted (see atmospheric radiation). Some of this is emitted downward back to the earth's surface ( counter-radiation).

The mean surface temperature for the entire world, 14 degrees C, is almost 40 degrees C higher than the mean temperature required for radiative equilibrium of a black body at the earth's mean distance from the sun. It is essential, in understanding the concept of the greenhouse effect, to note that the important additional warming is due to the counterradiation from the atmosphere. The glass panes of a greenhouse function in this manner, hence the name.

Green theorem

See divergence theorem.

Greenwich apparent time

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

Greenwich civil time (abbr GCT) = Greenwich mean time.

(United States terminology from 1925 through 1952).

Greenwich hour angle (abbr GHA)

Angular distance west of the Greenwich celestial meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial meridian and the hour circle of a point on the celestial sphere, measured westward from the Greenwich celestial meridian through 360 degrees; local hour angle at the Greenwich meridian.

Greenwich mean time (abbr GMT)

Local mean time at the Greenwich meridian; the arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the Greenwich celestial meridian and the hour circle of the mean sun, measured westward from the lower branch of the Greenwich celestial meridian through 24 hours; Greenwich hour angle of the mean sun, expressed in time units, plus 12 hours. Called Greenwich civil time in U.S. terminology from 1925 through 1952. Also called universal time, Z-time.

Mean time reckoned from the upper branch of the Greenwich meridian is called Greenwich astronomical time.

Greenwich meridian

The meridian through Greenwich, England, serving as the reference for Greenwich time.

The Greenwich meridian is accepted almost universally as the prime meridian, or the origin of measurement of longitude.

Greenwich sidereal time

(abbr GST) Local sidereal time at the Greenwich meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial meridian and the hour circle of the vernal equinox, measured westward from the upper branch of the Greenwich celestial meridian through 24 hours; Greenwich hour angle of the vernal equinox, expressed in time units.

G-region

See ionosphere.

Gregorian calendar

The calendar now in common use, in which each year has 365 days except leap years. See calendar year.

grey body = gray body.

grid

1. A series of lines, usually straight and parallel, superimposed on a chart or plotting sheet to serve as a directional reference for navigation. See graticule.

2. Two sets of mutually perpendicular lines dividing a map or chart into squares or rectangles to permit location of any point by a system of rectangular coordinates. See military grid, world geographic reference system.

3. An electrode with one or more openings to permit passage of electrons or ions. It usually consists of a wire mesh electrode placed between the anode and cathode of an electron tube to serve as a control of the current flowing between them.

4. Pertaining to or measured from a reference grid, as grid azimuth, grid latitude, grid meridian.

grid variation

See variation, note.

grivation = grid variation.

gross thrust

The total thrust of a jet engine without deduction of the drag due to the momentum of the incoming air (ram drag).

The gross thrust is equal to the product of the mass rate of fluid flow and the velocity of the fluid relative to the nozzle, plus the product of the nozzle exit area and the difference between the exhaust pressure and ambient pressure.

gross weight

The total weight of an aircraft, rocket, etc., as loaded; specifically, the total weight with full crew, full tanks, payload, etc. Also called take-off weight. See design gross weight.

ground

1. The earth's surface, especially the earth's land surface. Used in combination to form adjectives, as in ground-to-air, ground-to-ground , and air-to-ground. See surface.

2. The domain of nonflight operations that normally take place on the earth's surface or in a vehicle or on a platform that rests upon the surface, as in ground support.

3. = electrical ground.

ground clutter = ground return.

ground-controlled approach

(abbr GCA) A ground radar system providing information by which aircraft approaches may be directed via radio communications.

ground-controlled intercept

(abbr GCI) A radar system by means of which a controller may direct an aircraft to make an interception of another aircraft.

ground-effect machine

A machine that hovers or moves just above the ground by creating a cushion of supporting air between it and the ground surface and varying the thrust vector and magnitude to regulate direction and rate of motion.

ground environment

1. The environment that surrounds and affects a system of piece of equipment while it operates on the ground.

2. That system or part of a system, as of a guidance system, that functions on the ground; the aggregate of equipment, conditions, facilities, and personnel that go to make up a system, or part of a system, functioning on the ground. See environment.

ground-handling equipment

Equipment on the ground used to move, lift, or transport a space vehicle, a rocket, or component parts.

Such equipment includes the gantry, the transporter, and the forklift.

ground layer = surface boundary layer.

ground return

Radar echoes reflected from the terrain. Also called ground clutter, land return.

Echoes from the sea are called sea clutter or sea return.

ground start

An ignition sequence of a rocket's main stage, initiated and cycled through on the ground. Compare air start, in-flight start.

In large rockets, the ground start may be fueled from pressurized tanks external to the rocket, permitting takeoff with the rocket's own internal propellant load intact.

ground support equipment

(abbr GSE) That equipment on the ground, including all implements, tools, and devices (mobile or fixed), required to inspect, test, adjust, calibrate, appraise, gage, measure, repair, overhaul, assemble, disassemble, transport, safeguard, record, store, or otherwise function in support of a rocket, space vehicle, or the like, either in the research and development phase or in an operational phase, or in support of the guidance system used with the missile, vehicle, or the like.

The GSE is not considered to include land or buildings; nor does it include the guidance-station equipment itself, but it does include the test and checkout equipment required for operation of the guidance-station equipment.

ground wave

A radio wave that is propagated over the earth and is ordinarily affected by the presence of the earth's surface and the troposphere. The ground wave includes all components of a radio wave over the earth except ionospheric and tropospheric waves. Compare sky wave.

The ground wave is refracted because of variations in the dielectric constant of the troposphere including the condition known as a surface duct.

group velocity

The velocity of a wave disturbance as a whole, i.e., of an entire group of component simple harmonic waves. The group velocity U is related to the phase speed c of the individual harmonic waves of length l by the frequency equation U = c - l ( dc / dl ). The phase speed c is thus equal to the group velocity only in the case of nondispersive waves, i.e., when dc/dl = 0. See velocity of propagation.

Gru, Grus

International Astronomical Union abbreviations for Grus. See constellation.

Grus (abbr Gru, Grus)

See constellation.

G-scan = G-display.

G-scope = G-display.

GSE (abbr) =ground-support equipment.

g-suit or G-suit

A suit that exerts pressure on the abdomen and lower parts of the body to prevent or retard the collection of blood below the chest under positive acceleration. Compare pressure suit.

g-tolerance

A tolerance in a person or other animal, or in a piece of equipment, to an acceleration of a particular value.

guidance

The process of directing the movements of an aeronautical vehicle or space vehicle, with particular reference to the selection of a flight path. See control.

In preset guidance a predetermined path is set into the guidance mechanism and not altered, in inertial guidance accelerations are measured and integrated within the craft, in command guidance the craft responds to information received from an outside source. Beam-rider guidance utilizes a beam; terrestrial-reference guidance, some influence of the earth; celestial guidance, the celestial bodies and particularly the stars; and homing guidance the information is in response to transmissions from the craft, in semiactive homing guidance the transmissions are from a source other than the craft, and in passive homing guidance natural radiations from the destination are utilized. Midcourse guidance extends from the end of the launching phase to an arbitrary point enroute and terminal guidance extends from this point to the destination.

guided missile

1. Broadly, any missile that is subject to, or capable of, some degree of guidance or direction after having been launched, fired, or otherwise set in motion.

2. Specifically, an unmanned, self-propelled flying vehicle (such as a pilotless aircraft or rocket) carrying a destructive load and capable of being directed or of directing itself after launching or take-off, responding either to external direction or to direction originating from devices within the missile itself.

3. Loosely, by extension, any steerable projectile. See ballistic missile.

guiding center

The center point of the Larmor orbit of a charged particle gyrating in a magnetic field.

It is often convenient to separate the total motion of a particle into the Larmor orbit plus the motion of the guiding center. In addition to the unimpeded motion of the guiding center of a particle along the magnetic field, the presence of electric fields will displace the guiding center perpendicular to the magnetic field.

gust tunnel

A wind tunnel in which gusts are simulated. Specifically, a tunnel in which models are passed over a vertical jet or jets simulating gusts.

gyro

1. A device which utilizes the angular momentum of a spinning mass (rotor) to sense angular motion of its base about one or two axes orthogonal to the spin axis. Also called gyroscope.

This definition does not include more complex systems such as stable platforms using gyros as components.

2. Short for directional gyro, gyrocompass , etc.

gyrofrequency

The natural period of revolution of a free electron in the earth's magnetic field. See magnetoionic theory.

gyro horizon

An artificial horizon or an attitude gyro.

gyro pickoff

A device which produces a signal, generally a voltage, as a function of the angle between two gyro gimbals or between a gimbal and the base.

gyroscope = gyro.

gyroscopic inertia

The property of a rotor of resisting any force which tends to change its axis of rotation. See gyro.