An Etymological Dictionary of Astronomy and Astrophysics

→ lithium I line.

See also: → lithium; I for → neutral atom; → line.

A → giant star whose observed → lithium abundance is much higher (A(Li) ~ 2.95) than that predicted by stellar → evolutionary models.
Standard evolutionary models predict severe → depletion of surface Li → abundance, which is as low as 1.4 → dex in K giants, a factor of about 80 lower than the maximum value of about 3.3 dex observed in → main sequence stars. Observations confirm model predictions showing much less Li compared to model predictions in most → red giant branch (RGB) stars (Kumar et al., 2018, J. Astrophys. Astr. 39, 25 and references therein).

See also: → lithium; → rich; → giant.

The Scales. An inconspicuous constellation in the southern hemisphere and a sign of the → Zodiac, at 15h 30m right ascension, 15° south declination.
Abbreviation: Lib; genitive: Librae.

Etymology (EN): L. libra “balance,” of obscure origin.

Etymology (PE): Tarâzu “balance, scales,” Mid.Pers. tarâzên-, taraênidan “to weigh,” Proto-Iranian *tarāz-, from *tarā- “balance, scale” (cf. Skt. tulā- “scales, balance, weight,” from tul- “to weigh, make equal in weight, equal,” tolayati “weighs, balances,” L. tollere “to raise,”
Gk. talanton “balance, weight,” Atlas “the Bearer” of Heaven," Lith. tiltas “bridge;” PIE base telə- “to lift, weigh”) + Av. az- “to convey, conduct, drive,” azaiti drives" (cf. Skt. aj- “to dive, sling,” ájati “drives,” ajirá- “agile, quick,”
Gk. agein “to lead, guide, drive, carry off,” L. agere “to do, set in motion, drive,” from PIE root *ag- “to drive, move,” → act).

Small oscillations of a → celestial body about its mean position. The term is used mainly to mean the Moon’s libration caused by the apparent wobble of the Moon as it orbits the
Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon’s surface can be seen over a period of time. This results from three kinds of libration working in combination: → libration in longitude, → libration in latitude, and → diurnal libration. See also: → geometrical libration, → physical libration.

Etymology (EN): L. libration- “a balancing.”

Etymology (PE): Halâzân “to and fro motion, oscillation,” literally “a swing: a seat suspended by ropes on which a person may sit for swinging,”
from Gilaki halâcin “a swing,” Ilâmi harazân “a swing,” variants (Dehxodâ) holucin, holu “a swing,” probably from Proto-Ir. *harz- “to send, to set.”
Roxgard, literally “turning the face,” from rox,
variant ru(y) “face, surface; aspect; appearance” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”)

• gard “turning, changing,” from gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;” O.H.G. werden “to become;” PIE base *wer- “to turn, bend”).

A tiny oscillating motion of the Moon arising from the fact that the Moon’s axis is slightly inclined relative to the Earth’s. More specifically, the Moon’s polar axis is tilted nearly 7° with respect to the plane of its orbit around Earth. Hence for half of each orbit we see slightly more of the north pole when its tipped toward us, and for the other half we see slightly more of its south pole. Libration in latitude displaces the mean center of the Moon north-south by between 6°.5 and 6°.9.

See also: → libration; → latitude.

A tiny oscillating motion of the → Moon arising from the fact that the Moon’s orbit is not a precise circle but rather an → ellipse. Therefore, Moon is sometimes a little closer to the Earth than at other times, and as a result its → orbital velocity varies a bit. Since the Moon’s rotation on its own axis is more regular, the difference appears as a slight east-west oscillation. Libration in longitude is the most significant kind of libration. It varies between about 4°.5 and 8°.1 because of gravitational perturbations in the Moon’s orbit caused by the Sun.

See also: → libration; → longitude.

The condition that distinguishes living organisms from inorganic objects, i.e. non-life, and dead organisms. It is manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally.

Etymology (EN): O.E. life, from P.Gmc. *liba- (cf. O.N. lif “life, body,” Du. lijf “body,” O.H.G. lib “life,” Ger. Leib “body”), properly “continuance, perseverance,” from PIE *lip- “to remain, persevere, continue, live;” cf. Gk. liparein “to persist, persevere.”

Etymology (PE): Zist “life, existence,” from zistan “to live;” Mid.Pers. zivastan “to live,” zivižn “life,” zivik, zivandag “alive, living;” O.Pers./Av. gay- “to live,” Av. gaya- “life,” gaeθâ- “being, world, mankind,” jivya-, jva- “aliving, alive;” cf. Skt. jiva- “alive, living;” Gk. bios “life;” L. vivus “living, alive,” vita “life;” O.E. cwic “alive;” E. quick; Lith. gyvas “living, alive;” PIE base *gweie- “to live.”

In fluid mechanics, the component of aerodynamical force which is perpendicular to the oncoming flow direction. In aeronautics, the perpendicular component of the force of the air against an airplane; the component that is effective in supporting the plane’s weight. → drag; → thrust.

Etymology (EN): M.E. liften, from O.N. lypta “to raise;” cf. M.L.G. lüchten, Du. lichten, Ger. lüften “to lift;” O.E. lyft “heaven, air.”

Etymology (PE): Bâlâbar “lift,” from bâlâ “up, above, high, elevated, height” (variants boland “high,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high”) + bar present stem of bordan “to carry, transport” (Mid.Pers. burdan;
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive);” Skt. bharati “he carries;” Gk. pherein “to carry;” PIE base *bher- “to carry”).

1. That portion of → electromagnetic radiation visible to the human → eye. However, other bands of the → electromagnetic spectrum are also often referred to as different forms of light.
   

2. Of little weight; not heavy.

Etymology (EN): 1) O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- “light, brightness,” cognate with Pers. rowšan “bright, clear,” ruz “day,” rowzan “window, aperture;” foruq “light,” and afruxtan “to light, kindle;” Mid.Pers. rôšn “light; bright, luminous,” rôc “day;” O.Pers. raucah-rocânak “window;” O.Pers. raocah- “light, luminous; daylight;”
Av. raocana- “bright, shining, radiant;”
akin to Skt. rocaná- “bright, shining,” roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light” (also lumen, luna); Fr. lumière.
2) From O.E. leoht, from P.Gmc. *lingkhtaz (cf. O.N. lettr, Swed. lätt, O.Fris., M.Du. licht, Ger. leicht, Goth. leihts), from PIE base *le(n)gwh- “easy, agile, nimble.”

Etymology (PE): 1) Nur, from Ar.
Luž “light,” in Sangesari (luženg variant of rowzan “vent, window”), variant of Pers. ruz “→ day,” cognate with light, as above.

2. Sabok, from Mid.Pers. sapuk “light, brisk;” from O.Pers. *θapu-ka-, from Proto-Iranian *θrapu-ka-, from PIE *trep- “to shake, tremble;” cf. Gk. trepein “to turn;” L. trepidus “agitated, anxious;” Skt. trepa- “hasty.”

The reflex adaptation of the eye to bright light, consisting of an increase in the number of functioning cones, accompanied by a decrease in the number of functioning rods; opposed to dark adaptation.

See also: → light; → adaptation.

A bright, confusing, and excessive grouping of light sources. Light clutter is a type of → light pollution. It is a general term relating to lights put up everywhere, without regard to what their purpose really is.

See also: → light; → clutter.

The set of all directions in which a light signal can travel toward an event (past light cone) or from an event (future light cone).

See also: → light; → cone.

1. A curve showing the behavior of the light from a → variable star over a period of time.
   

2. → supernova light curve.

See also: → light; → curve.

A cylinder of radius cP/(2π) around a → pulsar’s spin axis, where P is the pulsar period and c the → speed of light. At this surface,
the velocity of a hypothetical object that corotates with the → neutron star would reach the speed of light.

See also: → light; → cylinder.

The deviation of a light ray by the gravitational field of a massive body. For example, stellar light passing near the Sun will be deviated by 1’’.75 at the Sun’s limb.

See also: → light; → deflection.

Reflection of light from a stellar outburst by successively more distant clouds of dust surrounding the star. For example, the light echoes from two shells of dust near supernova 1987A, or those of star V838 Mon.

See also: → light; → echo.

In astrophysics, a chemical element that has an atomic number of one, two, or three, such as hydrogen, helium, and lithium; sometimes also beryllium and boron.

See also: → light; → element.

An atmospheric optical phenomenon appearing as a vertical shaft of light extending from the Sun or other bright light source during very cold weather.

Light pillars or → sun pillars occur when artificial light or sunlight near the horizon is reflected from falling ice crystals associated with thin, high-level clouds.

The ice crystals have a hexagonal plate shape and fall with a horizontal orientation, gently rocking from side to side as they fall.

See also: → light; → pillar.

The inappropriate or excessive use of artificial light.
It brightens the sky and has a particularly damaging effect on astronomical observations. More generally, light pollution can have serious environmental consequences for humans, wildlife, and our climate. Types of light pollution include: → glare, → skyglow, → light trespass, and → light clutter.

See also: → light; → pollution.

An imaginary line directed along the path that the light follows. In other words, light pictured simply in terms of straight lines.

See also: → light; → ray.

A type of → light pollution which is light falling where it is not wanted or needed. Light trespass occurs when poorly shielded or poorly aimed fixtures cast light into unwanted areas, such as buildings, neighboring property, and homes. This light is a main contributor to → skyglow which interferes with astronomical instruments.

See also: → light; → trespass.

→ light-year

See also: → light; → year.

The most important function of an astronomical telescope, which is
directly related to the area (or to the square of the diameter) of the main mirror or lens.

Etymology (EN): → light; gathering, from O.E. gadrian, gædrian “to gather, collect;” → power.

Etymology (PE): Tavân, → power; gerdâvari, verbal noun of gerd âvardan, from gerd “round; around” (Mid.Pers. girt “round, all around,” O.Iranian *gart- “to twist, to wreathe,” cf. Skt. krt “to twist threads, spin, to wind, to surround,” kata- “a twist of straw;” Pali kata- “ring, bracelet;” Gk. kartalos “a kind of basket,” kyrtos “curved”) + âvardan “to bring,” Mid.Pers. âwurtan, âvaritan;
Av. ābar- “to bring, to possess,” from prefix ā- +
Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother;” Mod.Pers. bordan “to carry;” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry;” nur,
→ light.

The distance travelled by light in free space in one second. It is equivalent to 2.997924580 × 10⁸ m or 2.998 × 10⁵ km. This unit of length is mainly used in astronomy, telecommunications, and relativistic physics. Some quantities expressed in this unit are as follows.
The mean diameter of the Earth: about 0.0425 light-seconds.
The average distance from the Earth to the Moon: about 1.282 light-seconds. The diameter of the Sun: about 4.643 light-seconds. The average distance from the Earth to the Sun: 499.0 light-seconds.

See also: → light; → second.

The time it takes for light, travelling at about 300 000 km per second, to travel a certain distance.

See also: → light; → time.

The distance traversed by a photon between the time it is emitted and the time it reaches the observer. It is
also referred to as the → look-back time.

See also: → light; → travel; → distance.

The distance that light travels in one year at about 300,000 km per second, i.e. 9.5 x 10¹² km. It is equal to about 63,000 → astronomical units. See also → parsec.

See also: → light; → year.

A bundle of optical fibers arranged randomly for the purpose of transmitting energy, not an image.

See also: → light; → guide.

Of, pertaining to, or describing an → event on the → light cone.

See also: → light; → -like.

The space-time interval between two events if it is zero, i.e.
ds² = 0.

See also: → lightlike; → interval.

A → flash of light produced by an → electric discharge in response to the buildup of an → electric potential between → cloud and → Earth’s surface, or between different portions of the same cloud.

Etymology (EN): Lightning, pr.p. of lightnen “make bright,” extended form of O.E. lihting, from leht, → light.

Etymology (PE): Âzaraxš, from âzar “fire,” variants âtaš, taš (Mid.Pers. âtaxš, âtur “fire;” Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire”) +
raxš “lightning, reflection of light,” raxšidan “to shine, flash,” variant deraxš, deraxšidan “to shine, radiate”
(O.Pers. raucah-, Av. raocah- “light” (cf. Skt. roka- “brightness, light,” Gk. leukos “white, clear,” L. lux “light” (also lumen, luna), E. light, Ger. Licht, Fr. lumière;
PIE base *leuk- “light, brightness”); cognate with Mod.Pers. words
ruz “day,” rowšan “bright, clear,” foruq “light,” and afruxtan “to light, kindle”).

1. The state of being likely or probable; a probability or chance of something.
   
2. In technical language likelihood is not synonymous for probability. Same as → likelihood function.

Etymology (EN): From → likely + -hood a suffix denoting state, condition, character, nature, etc., from M.E. -hode, -hod, O.E. -hād (cf. Ger. -heit).

Etymology (PE): Šodvâri, noun of šodvâr, → likely.

A function that allows one to estimate unknown parameters based on known outcomes. Opposed to → probability, which allows one to predict unknown outcomes based on known parameters. More specifically, a probability refers to the occurrence of future events, while a likelihood refers to past events with known outcomes.

Etymology (EN): → likelihood; → function.

Probably or apparently destined; having a high probability of occurring or being true.

Etymology (EN): Perhaps from O.N. likligr “likely,” from likr “like” (adj.).

Etymology (PE): Šodvâr, from šod past stem of šodan “to become, to be, to be doing, to go, to pass” + -vâr a suffix with several meanings “resembling, like, in the manner of; having, endowed with.” The first element from Mid.Pers. šudan, šaw- “to go;” Av. šiyav-, š(ii)auu- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro.”

1. The outer edge or border of the apparent disk of a celestial body. → limb brightening, → limb darkening.
   

2. The raised edge of the mater of a → planispheric astrolabe, bearing a scale divided into 360°. The limb is the reference against which the rete is rotated in the computation process so that the planispheric astrolabe will simulate the sky’s appearance on a given day at a specific hour. Vice versa, the limb can be configured from the observation of the altitude of an celestial body on the horizon. In that case, the limb will indicate an angle that, thanks to the hour scale on the back, can be converted into the hour of day or night (online museo galileo, VirtualMuseum).

Etymology (EN): From L. limbus “border, hem, fringe, edge,” cognate with Skt. lambate “hangs down.”

Etymology (PE): Labé “limb, edge,” from lab “lip;” Mid.Pers. lap; cognate with
L. labium, E. lip; Ger. Lefze.

An observed increase in the intensity of radio, extreme ultraviolet, or X-radiation from the Sun from its center to its limb.

See also: → limb; → brightening.

An apparent decrease in brightness of the Sun near its edge as compared to its brightness toward the center. Limb darkening is readily apparent in photographs of the Sun. The reason is that when we look toward the disk’s center we look into deeper and hence hotter layers along the line of sight. Toward the limb, we get radiation from higher and hence cooler and less bright layers of the → photosphere. Limb darkening has been detected in the case of several other stars. A similar phenomenon occurs in → eclipsing binaries where the effect of limb darkening on one or both components manifests itself in the shape of the system’s → light curve.

See also: → limb; → darkening.

A white or grayish-white substance obtained by burning → limestone, used in mortars, plasters, cements, and in the manufacture of steel, paper, glass, and various chemicals of calcium.

Etymology (EN): M.E., O.E. lim; akin to Du. lijm, Ger. Leim,
O.N. lim “glue;” L. limus “slime.”

Etymology (PE): Âhak, probably a variant of xâk, → soil.

A → sedimentary rock composed principally of calcium carbonate. Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

See also: → lime; → stone.

1. General: The final, utmost, or furthest → boundary or → point as to extent, amount, continuance, procedure, etc.
   

2a) Math.: Of a → sequence, a → number which is approached ever more closely, but never reached, by the successive terms of a convergent infinite sequence.

2b) Of a → variable, a constant C which has the property with respect to some variable V that, as the variable approaches C in value (according to some formula), the numerical difference (C - V) between the constant and the variable diminishes toward 0 but is always greater than 0.

Etymology (EN): From O.Fr. limite “a boundary,” from L. limitem (nom. limes) “a boundary, embankment between fields, border,” related to limen “threshold.”

Etymology (PE): Loan from Ar. Hadd “limit, term.”

Confined within limits; restricted or circumscribed.

See also: Adj. of → limit.

The faintest magnitude reachable by an instrument.

See also: → limit; → magnitude.

A widely occurring ore of yellowish-brown to black color
that consists of amorphous oxides of iron.

See also: Gk. leimon “meadow” in reference to its occurrence as “bog ore” in meadows and marshes + → -ite.

A kinematic resonance hypothesized to explain the existence of galactic → spiral arms. It occurs
when the frequency at which a star encounters the spiral → density wave is a multiple of its → epicyclic frequency.
Orbital resonances occur at the location in the disk where Ω_p = Ω ± κ/m, where Ω_p is → pattern speed, κ → epicyclic frequency, and m an integer representing the number of spiral arms. The minus sign corresponds to the inner Lindblad resonance (ILR) and the plus sign to the outer Lindblad resonance (OLR). The corotation resonance corresponds to Ω_p = Ω. In general, the Lindblad resonances are defined for two spiral arms (m = 2), and low order. There are other less important resonances corresponding to higher m values. These resonances tend to increase the object’s orbital eccentricity and to cause its longitude of periapse to line up in phase with the perturbing force. Lindblad resonances drive spiral density waves both in galaxies (where stars are subject to forcing by the spiral arms themselves) and in Saturn’s rings (where ring particles are subject to forcing by Saturn’s moons).

See also: After the originator of the model, Bertil Lindblad (1895-1965), a Swedish astronomer, who made important contributions to the study of the rotation of the Galaxy; → resonance.

1. A mark or stroke long in proportion to its breadth, made with a pen, pencil, or other tools.
   

2. Math.: A continuous extent of length, straight or curved, without breadth or thickness; the trace of a moving point.

Etymology (EN): M.E. li(g)ne “cord, rope, stroke, series,” from O.E. line “rope, row of letters,” partly from O.Fr. ligne, from L. linea “linen thread, string, line,” from phrase linea restis “linen cord,” from fem. of lineus (adj.) “of linen,” from linum “flax, linen.”

Etymology (PE): Xatt, xat, used also in Ar., but it has no Hebrew counterpart. Xat is probably of Iranian origin, from
*kerš-/*xrah- “to draw, plow;” cf. Av. karš- “to draw; to plow,” karša- “furrow;” Mid/Mod.Pers. kešidan, kašidan “to draw, protract,
trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
cf. Skt. kars-, kársati “to pull, drag, plow;”
Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow.”

In stellar atmosphere models, the effect of metallic lines on the atmospheric structure of stars. The additional opacities of thousands of metallic lines alter the radiative transfer, leading to changes in the temperature. The emergent spectrum is consequently modified.

See also: → line-blanketed model.

Mixing of two or more spectral lines of adjacent wavelengths into a broad, single line due to insufficient dispersion of the spectrograph.

See also: → line; → blend.

Reduction of the radiative flux in a model atmosphere due to absorption by a large number of lines. Line blocking affects the radiative transfer, ionisation and temperature structures, and the atmosphere’s hydrodynamics.

Etymology (EN): → line; block, from M.E. blok “log, stump,” from O.Fr. bloc “log, block,” via M.Du. bloc “trunk of a tree.”

Etymology (PE): Bandâri “blocking,” from band âvardan “to block,” from band “dam, tie, chain,” from bastan “to bind, shut, close up” (Mid.Pers. bastan, band; Av./O.Pers. band- “to bind, fetter,” banda- “band, tie;” cf. Skt. bandh- “to bind, tie, fasten,” bandhah “a tying, bandage;” Goth bandi “that which binds;” O.Fr. bande, bende; O.E. bend; M.E. bende; E. band; PIE base *bendh- “to bind”) + âvari verbal noun of âvardan “to cause, produce; to bring” (Mid.Pers. âwurtan, âvaritan; Av. ābar- “to bring; to possess,” from prefix ā- + Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother;” Mod.Pers. bordan “to carry;” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry”).

A widening of → spectral lines due to any of several factors, including the → Doppler broadening, → instrumental broadening, → microturbulence, → pressure broadening, → rotational broadening, the → Stark effect, and the → Zeeman effect.

See also: → line; → broadening.

In stellar atmosphere models, the decrease of temperature in the outer layers of atmosphere due to the escape of photons through optically thin metallic lines. Energy is transferred from the thermal pool to photons and is lost for the atmosphere, leading to a temperature decrease.

See also: → line; → cooling.

The process of recognizing the spectral lines in a spectrum.

See also: → line; → identification.

A measure of the total effect of an absorption or emission line. The line intensity is equal to the integration of the absorption coefficient over the entire shape of the absorption line.

See also: → line; → intensity.

Of a force, the straight line along which the force → vector is directed. The action of a force on a → rigid body does not change when its point of application is displaced along the line of action. Hence, forces applied to a rigid body can be regarded as non-localized, or sliding, vectors.

See also: → line; → action.

Of an elliptical orbit, the straight line drawn from the → perigee to the → apogee.

See also: → line, → apsides.

The intersection of the planes of ecliptic and celestial equator.

See also: → line; → equinox.

Same as → streamline.

See also: → line; → flow.

One of many → imaginary lines whose direction at all → points along its length is that of the electric or → magnetic field at those points. In → electric fields the lines of force are directed toward → negative charges and point away from → positive charges. In magnetic fields the lines of force are directed from the → north pole to the → south pole.

See also: → line; → force.

Same as → line of force in a magnetic field.

See also: → line; → induction.

The line created by the intersection of the equatorial plane and the orbital plane.

See also: → line, → node.

The imaginary straight line connecting the object and the objective lens of the viewing device.

See also: → line; → sight.

The representation of a spectral line as produced by an observing instrument.

See also: → line, → profile.

The change in the shape of a → spectral line over time. For instance, in → Wolf-Rayet stars the line profile varying on time-scales of minutes to hours is attributed to → microturbulence.

See also: → line; → profile; → variability.

Spectrum consisting of discrete lines (emission or absorption), each corresponding to a particular wavelength, as opposed to a continuous spectrum.

See also: → line, → spectrum.

Same as → line intensity.

See also: → line, → strength.

Part of the line profile between the continuum level and the half value of the emission or absorption peak. The wings are due to matter traveling at much greater speeds than that providing the main peak. → red wing; → blue wing.

See also: → line; → wing.

A stellar atmosphere model which includes metals or uses
methods to reproduce their effects, → line blanketing.

See also: → line; → blanket; → model.

Same as → radiation-driven wind.

See also: → line; → drive; → wind.

Any of a countless number of dark streaks visible on → Europa’s surface that crisscross the whole → Galilean satellite. They are up to 1,000 km long, 20 km wide, and 1 km deep, but only hundred of meters high. In many cases, the ridges are double, often with dark outer edges and a central band. Images show that on each side of the lines, the edges have moved relative to each other. According to the most likely hypothesis, lineae result from eruptions of warm water, in a scenario similar to the present day mid- oceanic ridges on Earth.

Etymology (EN): From L. linea, → line.

Etymology (PE): Xaš, → streak.

Confined to first-degree algebraic terms in the relevant variables.

See also: Adj. of → line.

The rate of change of the → linear velocity with time. It is defined by the expression Δv/Δt and is equal to the → first derivative of the → linear velocity.

See also: → linear; → acceleration.

Taking the first term in the Taylor series as an approximation to a mathematical function at a given point. → first approximation.

See also: → linear; → approximation.

A version of → planispheric astrolabe in which the → celestial sphere and the various circles of altitude and declination are projected on to a line represented by a staff. The staff is equivalent to the meridian line and contains markings to indicate the centers of these circles and their intersections with the meridian. By attaching three ropes to the appropriate points on the staff to act as radii, the circles and their intersections can be reconstructed. One of the ropes was attached to a plumb line. A scale giving chord lengths in the meridian circle extended the linear astrolabe’s range of applications. It was invented by the Iranian mathematician and astronomer Sharafeddin Tusi (c1135-1213), but no early example has survived. Same as → Sharafeddin’s staff and Tusi’s staff.

See also: → linear; → astrolabe.

A measure of how well data points fit a straight line. When all the points fall on the line it is called a perfect correlation. When the points are scattered all over the graph there is no correlation.

See also: → linear; → correlation.

The real physical diameter, as opposed to angular diameter.

See also: → linear; → diameter.

An equation in which the → dependent variable y and all its differential coefficients occur only in the first degree. A linear differential equation of → order  order n has the form:
f_n(x)y⁽ⁿ⁾ + f_n-1(x)y^(n-1) + … + f₁(x)y^’ + f₀(x)y = Q(x),

where f₀(x), f₁(x), …, f_n(x) and Q(x) are each continuous functions of x defined on a common interval I and f_n(x)≠ 0 in I. A linear differential equation cannot have, for example, terms such as y² or (y^’)^1/2. See also:
→ homogeneous linear differential equation; → nonhomogeneous linear differential equation.

See also: → linear; → differential; → equation.

A system of three charges +q, -2q, and +q, arranged along a line to form an axial quadrupole. The → electric potential  V due to a linear quadrupole varies as 1/r³, whereas the → electric intensity  E varies as 1/r⁴.

See also: → linear; → electric; → quadrupole.

An equation composed of first degree variables and representing a straight line.

See also: → linear; → equation.

A function expressed by a → first degree equation
that can be graphically represented in the → Cartesian coordinate plane by a → straight line.

See also: → linear; → function.

An instability that can be described (to first-order accuracy) by linear (or tangent linear) equations.

See also: → linear; → instability.

The product of an object’s → mass and → velocity. It is a → vector and points in the same direction as the velocity vector. Linear momentum is distinguished from → angular momentum. When there is no opportunity for confusion, usually the term momentum is used instead of linear momentum.

See also: → linear; → momentum.

Assumption that the variations in the plasma parameters, due to the presence of waves, are small (to the first order) as compared to the undisturbed parameters. This makes it possible to linearize equations by dropping out second order (and higher) nonlinear terms.

See also: → linear; → perturbation; → theory.

Of an electromagnetic radiation, a → polarization in which the electric vibrations are confined to one plane along the direction of propagation. Also called → plane polarization. See also → circular polarization.

See also: → linear; → polarization.

A procedure for finding the maximum or minimum of a → linear function where the → arguments are subject to linear → constraints. For problems involving more than two variables or problems involving a large number of constraints, solution methods used are those that are adaptable to computers. A well-known such
→ algorithm is the → simplex method.

See also: → linear; → programmings.

In statistics, a regression method that establishes a linear relationship between two random variables.

See also: → linear; → regression.

The real, physical size, as opposed to angular size.

See also: → linear; → size.

Physics: A → dynamical system whose evolution is a linear process. If a change in any variable at some initial time produces a change in the same or some other variable at some later time, twice as large a change at the same initial time will produce twice as large a change at the same later time.

See also: → linear; → system.

The rate of change of the position of an object that is traveling along a straight path.
In other words, the velocity of an object when its moving direction is not changing. For a given → angular velocity (ω), the linear velocity v of the particle is directly proportional to the distance of the particle from the center of the circular path: v = ω ×r.

See also: → linear; → velocity.

The property, condition, or state of being linear.
Math.: A relationship between two variables so that when plotted on a graph they yield a straight line.

See also: → linear; → -ity.

A process of reduction to linear form by appropriate change of variables or by approximation.

See also: Verbal noun of → linearize.

To make linear; give linear form to.

See also: → linear + → -ize.

A differential equation that has been derived from an original nonlinear equation.

See also: Linearized, p.p. of → linearize;
→ differential; → equation.

In a manner characterized by first-degree algebraic terms.

See also: Adverb of → linear.

A set of objects x₁, x₂, …, x_n (→ vectors, → matrices, → polynomials, etc.) on a given set if there is a linear combination of them:

a₁x₁ + a₂x₂ + … + a_nx_n,

which is zero, but at least one of the coefficients is non-zero. For example the binomials (2x + y) and (6x + 3y) are linearly dependent, since 3(2x + y) - (6x + 3y) = 0.

See also: → linearly; → dependent.

1. A set of objects x₁, x₂, …, x_n (→ vectors, → matrices, → polynomials, etc.) if it si not → linearly dependent.
   

2. Of two particular solutions (y₁, y₂) of a → homogeneous linear differential equation of the second order (y^’’ + a₁ y^’ + a₂y = 0) on an interval [a, b], if their ratio in this interval is not a constant: y₁/y₂≠ constant.

See also: → linearly; → independent.

Light exhibiting → linear polarization.

See also: → linearly; → polarized; → light.

A type of galactic nucleus that is defined by its spectral line emission. The lines are very weak, the most prominent ones being from low ionization states (such as [O II], [N II], [S II] and [OI]). There is so far no generally accepted interpretation of the spectra of
liners. It is likely that galaxies of different histories may have their nuclei with liner-type spectra. → retired galaxy.

See also: Short for → Low-Ionization Nuclear Emission-line Region. The term liner was first introduced by T. M. Heckman (1980, A&A 87, 152).

1. The range of frequencies or wavelengths over which radiations are absorbed or emitted in a transition between a specific pair of atomic energy levels. The full width is determined between half-power points of the line.
   
2. In a laser, the range of frequencies over which most of the beam energy is distributed.

See also: → line; → width.

One of several → input or → output
variables of a → fuzzy logic system whose values are → words or → sentences from a natural language, instead of numerical values.

See also: → linguistic; → variable.

The study of the structure and evolution of → language, including → phonetics, → morphology, syntax, semantics.

Etymology (EN): From linguistic, from linguistic, from linguist, from L. lingua, → language, + → ics.

Etymology (PE): Zabânik, from zabân, → language,

• -ik, → -ics. Zabânšenâsi, from zabân + šenâsi, → -logy.

1. General: Anything serving to connect one part or thing with another; a bond or tie.
   

2. A direct connection in a hypertext document to the Internet address (URL) of another document.

Etymology (EN): From M.E. link(e), of Scandinavian origin; akin to O.Dan. lænkia “chain;” Old Norse hlekkr “chain;” Ger. Gelenk “joint.”

Etymology (PE): Peyvand “join, union,” from peyvandidan, peyvastan “to join, connect;” Mid.Pers. peywand, peywastan “connection, offspring; to join, connect, attach,” from *pati-basta-, from suffix pati-
(Mid.Pers. pât-,from O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of,” Av. paiti, akin to Skt. práti “toward, against, again, back, in return, opposite,” Pali pati-, Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti) + basta- “tied, shut” (Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind), cf. Skt. prati-bandh- “to tie.”

1. General: An act or mode of linking; the fact of being linked.
   

2. Physics: The amount of magnetic flux passing through a coil of wire or other electric circuit.

Etymology (EN): From → link + -age a suffix of abstract nouns from O.Fr.

Etymology (PE): From ham-, → com-, + peyvand, → link, + noun suffix -y.

A large, usually tawny-yellow cat, Panthera leo, native to Africa and southern Asia, having a tufted tail and, in the male, a large mane (dictionary.com).

Etymology (EN): M.E., from O.Fr., variant of leon, from L. leon- (stem of leo), from Gk. leon.

Etymology (PE): Mid.Pers. šagr “lion;” cf. Parth. šarg; Sogd. šarγu “lion;” its O.Pers. and Av. forms are not extant. Šir may be cognate with Skt. kēsarin- “lion; literally maned,” from kēsar- “mane;” PIE *kaisar- “mane; hairs.” If so, šir could be related to PIE *kaisaraka-, provided that the the initial *kai- is dropped and *saraka- has transformed in *sarg, šarg, šir.

A key theorem in statistical mechanics of classical systems which states that the motion of phase-space points defined by Hamilton’s equations conserves phase-space volume.

See also: After Joseph Liouville (1809-1882), a French mathematician; → theorem.

1. The act or process of liquefying or making liquid.
   

2. The state of being liquefied.

See also: → liquefy; → -tion.

1. To reduce to a liquid state.
   

2. To become liquid.

See also: M.E. lyquefyen, from O.Fr. liquefier, from L. liquefacere “make → liquid, melt,” from liquere “be fluid”

• facere “to make,” → -fy.

The state of matter in which a substance exhibits a characteristic readiness to flow, little or no tendency to disperse, and relatively high incompressibility.

Etymology (EN): O.Fr. liquide, from L. liquidus “fluid, liquid, moist,” from liquere
“be fluid,” related to liqui “to melt, flow.”

Etymology (PE): Âvé, from âv, variant of âb “→ water” + nuance suffix -e.
Âbgun literally “resembling water,” from âb “water,” +
gun “resembling; manner, fashion; color”
(Mid.Pers. gônak “kind, species;” Av. gaona- “color”).

A type of material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature, electric field, and other quantities.

See also: → liquid; → crystal.

The state of helium (⁴He) below its boiling point of 4.2 K. Its normal form is called → helium I, but converts into superfluid → helium II below 2.17 K (→ lambda point).
Liquid helium is colorless and transparent so that it is impossible to see the surface of the liquid with the naked eye. Helium was first liquefied in 1911 by the Dutch physicist Heike Kamerlingh Onnes (1853-1936), Physics Nobel Prize 1913.

See also: → liquid; → helium.

A mirror composed of liquid, taking advantage of the parabolic shape of a spinning liquid and the fact that the mirror’s focal length can be adjusted by altering the velocity at which the liquid’s container spins.

See also: → liquid; → mirror.

Water in a state that is neither ice nor vapor.

See also: → liquid; → water.

In the → phase diagram of a → mixture (such as an → alloy) at constant pressure, the → curve that separates the all liquid phase from the liquid+solid phase. Below the liquidus the mixture will be partly or entirely → solid. See also → solidus.

See also: From L. liquidus, → liquid.

An → ESA spacecraft that was launched on December 3, 2015 to test technologies needed for the Evolved Laser Interferometer Space Antenna (eLISA), an ESA → gravitational wave observatory planned to be launched in 2034. LISA Pathfinder paves the way for future missions by testing in flight the very concept of gravitational wave detection.

See also: LISA, short for → Laser Interferometer Space Antenna; → pathfinder.

A curve traced out by a point which is oscillating simultaneously in two mutually perpendicular directions. In general, the amplitude and frequency may be different in the two directions, and the two oscillations may have an arbitrary initial phase difference. The simplest pattern is a straight line, being obtained from two oscillations of equal frequency in phase with each other. The patterns can become very involved if the ratio of the frequencies is not a simple one.

Etymology (EN): After the French physicist Jules Antoine Lissajous (1822-1880), who first demonstrated such curves (Comptes-Rendus, 1857, 44, 727); → figure.

Etymology (PE): Xam, → curve.

A quasi-periodic path resembling a → Lissajous figure around the L1 or L2 → Lagrangian points of a two-body system. Lissajous orbits, resulting from a combination of planar and vertical components, are used by certain space telescopes (such as → WMAP, → Planck Satellite, and
→ Herschel Satellite) that are required to be in a stable position relative to the Earth and Sun while making long-term observations.

See also: → Lissajous figure; → orbit.

1a) A series of names or other items written or printed together in a meaningful grouping or sequence so as to constitute a record.

1b) Computers: A series of records in a file.

2. To set down together in a list; make a list of (Dictionary.com).

Etymology (EN): From M.E. liste “border, edging, stripe,” from O.Fr. liste “border, band, row,” also “strip of paper,” or from O.It. lista “border, strip of paper, list,” both from Germanic sources (compare O.H.G. lista “strip, border, list,” O.Norse lista “border.”

Etymology (PE): List, loan from Fr. liste, as above.

1. A list; the act of compiling a list; something listed.
   

   2. Computers: A display or printed list of lines in a program or digital data.

See also: → list; → -ing.

A metric unit of volume, formerly defined as the volume of one kilogram of pure water under standard conditions; now equal to 1 cubic decimeter (dm³); hence 1 liter = 0.001 m³ and 1000 liter = 1 m³.

See also: From Fr. litre, from litron, obsolete Fr. measure of capacity for grain, from M.L. litra, from Gk. litra “pound.”

1. Writings in which expression and form, in connection with ideas of permanent and universal interest, are characteristic or essential features, as poetry, novels, history, biography, and essays.
   

2. The entire body of writings of a specific language, period, people, etc.
   

3. The writings dealing with a particular subject (Dictionary.com).

Etymology (EN): From L. literatura/litteratura “writing, grammar, learning,” from litera/littera “letter.”

Etymology (PE): Neveštârgân, from neveštâr, literally “written; writing,” verbal noun from neveštan, nevis- “to write;” Mid.Pers. nibištan, nibes- “to write;” Av./O.Pers. nī- “down; in, into,” → ni- (PIE), + paēs- “to paint; to adorn,” paēsa- “adornment” (Mid.Pers. pēsīdan “to adorn”); O.Pers. pais- “to adorn, cut, engrave” (Mod.Pers. pisé “variegated”); cf. Skt. piśáti “adorns; cuts;” Gk. poikilos “multicolored;” L. pingit “embroiders, paints;” O.C.S. pisati “to write;” O.H.G. fēh “multicolored;” Lith. piēšti “to draw, adorn;” PIE base *peik- “colored, speckled,” + -gân suffix of suffix forming plural entities, from Mid.Pers. -gânag.

A metallic → chemical element; symbol Li. → Atomic number 3; → atomic weight 6.941; → melting point about 180.54°C; → boiling point about 1,342°C. Lithium is a soft, silver-white metal. It is the lightest metal and one of the alkali metals in Group 1 of the → periodic table. Lithium does not occur on Earth in its free form. It is a minor part of almost all igneous rocks and is found in many natural brines, in total 0.0007% of the Earth’s crust. It has two stable → isotopes, ⁷Li (92.5%) and ⁶Li (7.5%). The element was discovered in the mineral petalite, LiAl(Si2O5)2, by the Swedish mineralogist Johan August Arfwedson in 1817. It was isolated by W.T. Brande and Sir Humphrey Davy. Many uses have been found for lithium and its compounds. Lithium has the highest → specific heat (3.6 J/gK) of any solid element and is used in heat transfer applications. It is used in rechargeable lithium ion batteries. It is also used as an alloy with → aluminum, → copper, and → manganese to make high performance aircraft parts. It is used to make special glasses and ceramics, including the Mount Palomar telescope’s 5 m mirror. Lithium also has various nuclear applications, for example as a coolant in nuclear breeder reactors and a source of → tritium, which is formed by bombarding lithium with neutrons. In medicine it is used to treat bipolar disorder (manic depression), a serious mental illness that causes extreme shifts in mood, energy, and functioning.

In astrophysics, → Spite plateau.

See also: Lithium, from L. lithos “stone” because lithium was thought to exist only in minerals.

A → resonance line of → lithium at 6707.81 Å doublet 6707.76 and 6707.91 Å.

See also: → lithium; I for → neutral atom; → line.

A peculiar evolved star of spectral type G or M whose spectrum displays a high abundance of lithium.

See also: → lithium; → star.

The presence or not of the lithium absorption line at 6708 Å, which is considered to be a sufficient condition for → substellarity in → L dwarfs.
It has been shown that any object with lithium absorption and → effective temperature less than 2670 K is a → brown dwarf. For a discussion of potential problems with the lithium test see Kirkpatrick et al. (1993, ApJ 406, 701).

See also: → lithium; → test.

A prefix meaning “stone,” used in the formation of compound words.

Etymology (EN): From Gk. lithos “stone.”

Etymology (PE): Lito-, loan from Gk., as above.
Sang-, from sang, → stone.

→ lithophile element.

See also: → litho-; → -phile.

In the → Goldschmidt classification, a → chemical element that shows an → affinity for → silicate phases and is concentrated in the silicate portion of the Earth (→ crust and → mantle). This group includes → lithium (Li), → beryllium (Be), → sodium (Na), → magnesium (Mg), → potassium (K), → calcium (Ca), → barium (Ba), → titanium (Ti), → chromium (Cr), → aluminium (Al), → silicon (Si), → phosphorus (P), → chlorine (Cl), etc.

See also: → lithophile; → element.

The solid portion of the → Earth, as compared to the → atmosphere and the → hydrosphere. The lithosphere consists of semi-rigid plates that move relative to each other on the underlying → asthenosphere. The process is known as → plate tectonics and helps explain → continental drift.

See also: → litho-; → sphere.

An asterism in the constellation → Ursa Minor. This group of stars ends at → Polaris, the pole star of the Northern Hemisphere.

Etymology (EN): Little, from M.E., O.E. lytel, from W.Gmc. *lutila- (cf. Du. luttel, O.H.G. luzzil, Ger. lützel, Goth. leitils), from PIE *leud- “small;” dipper, from dip, O.E. dyppan “immerse,” from P.Gmc. *dupjanan.

Etymology (PE): Haft xâharân “the seven sisters,” from haft “seven” (Mid.Pers. haft; Av. hapta; cf. Skt. sapta; Gk. hepta; L. septem; P.Gmc. *sebun; Du. zeven; O.H.G. sibun; Ger. sieben; E. seven; PIE *septm)

• xâharân plural of xâhar “sister;” Mid.Pers. xwâhar “sister;” Av. x^vanhar- “sister;” cf. Skt. svásar- “sister;” Sogdian xwār; Gk. eor; L. soror (Fr. soeur); Arm. k’oyr;
  O.C.S., Rus. sestra; Lith. sesuo; O.Ir. siur; Welsh chwaer; M.Du. suster; Du. zuster; O.H.G. swester; Goth. swistar; Ger. Schwester; Swed. sister; Dan. søster; O.E. sweostor, swuster; E. sister.
  Camcé “a spoon, ladle; a wooden bowl or cup;” kucak “small” (Mid.Pers. kucak “small”), related to kutâh “short, small, little,” kudak “child, infant,” kutulé, → dwarf, Mid.Pers. kôtâh “low,” kôtak “small, young; baby;” Av. kutaka- “little, small”).

A roughly 400-year period from the mid-16th through the mid-19th centuries when temperatures over much of Europe were unusually cold. Glaciers in the Alps advanced and European rivers froze much more often than during the past century. Harvests failed, livestock perished, and poor people suffered from famine and disease. The Little Ice Age coincided with two successive low → solar activity periods, the
→ Sporer minimum and the → Maunder minimum.

See also: → little; → ice; → age.

A prism having angles of 30, 60, and 90°, which uses the same face for input and dispersed radiation. The beam is reflected at the face opposite to the 60° angle because it is coated to be highly reflecting. A beam entering at the → Brewster angle undergoes minimum deviation and hence maximum dispersion. Littrow prisms are used in laser cavities to fine tune lasers by selection of wavelength.

See also: Joseph Johann Littrow (1781-1840), Austrian astronomer; → prism.