An Etymological Dictionary of Astronomy and Astrophysics

A type of → brown dwarf with an → effective temperature ranging from about 2200 K to about 1300 K, corresponding to luminosities about 4 x 10^-4 to 3 x 10^-5 times that of the Sun. L dwarfs are intermediate in temperature between M and → T dwarfs. Their spectra in the optical show weak titanium oxide (TiO) and vanadium oxide (VO) absorption lines and strong metallic hydrides CrH (8611 and 9969 Å) and FeH (8692 and 9896 Å). Also are present strong neutral atomic lines of alkali metals Na I (8183, 8195 Å), K I (7665, 7699 Å), Rb I (7800, 7948 Å), Cs I (8521, 8943 Å), and sometimes Li I (6708 Å). The prototype of the L-dwarf class is → GD 165B. The spectral classification was first defined by Kirkpatrick et al. 1999, ApJ 519, 802 and Martin et al. 1999, AJ 118, 2466.

See also: For the reasoning behind the choice of the letter L, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.

A → semiregular variable with a period of 141 days. It has a → spectral type of M5 III corresponding to an → effective temperature of ~ 3500 K. It has a radial velocity relative to the → Local Standard of Rest of 33.3 km s^-1. At a distance of 64 → parsecs,
L₂ Pup is the second nearest → asymptotic giant branch (AGB) star next to R Doradus; it is ~ 30% closer than → Mira. Recent → near-infrared JHKL band observations by Kervella et al. (2014, 2015) show a compact dust disk around the central star, with an inner rim of 6 → astronomical units and an outer edge of 13 au, at an inclination of approximately 82°.

The → circumstellar environment of L₂ Pup was observed with → ALMA. The → molecular emission shows a → differentially rotating disk, inclined to a nearly edge-on position (Homan et al., 2017, A&A 601, A5 and references therein).

See also: Star name in the → Bayer designation scheme; → Puppis.

La Niña. A condition in which a significant decrease (more than 0.5 °C from average water temperatures) occurs in sea surface temperature (cold event) in the central and eastern equatorial Pacific. La Niña has a natural 3-6 year cycle and can persist for 1-3 years. It is the counterpart to the → El Nino (warm event), and its spatial and temporal evolution in the equatorial Pacific is, to a considerable extent, the mirror image of El Niño, although La Niña events tend to be somewhat less regular in their behavior and duration.

See also: American Sp. La Niña “the girl,” to distinguish it from
→ El Nino.

The site of the → European Southern Observatory’s first observatory in Chile, inaugurated in 1969. It is located 160 km north of the town of La Serena and 600 km north of Santiago at an altitude of 2,400 m bordering the southern extremity of the Atacama Desert. La Silla is equipped with several optical telescopes with mirror diameters of up to 3.6 m. The 3.5 m New Technology Telescope was the first in the world to have a computer-controlled main mirror, a technology developed at ESO. The ESO 3.6 m telescope is now home to the world’s largest extrasolar planet hunter: HARPS (High Accuracy Radial velocity Planet Searcher), a spectrograph with unrivalled precision.

See also: From Sp. la silla “the saddle,” after the apparent shape of the mountain on which the observatory is situated. Originally known as Cinchado.

A building or place equipped for carrying out scientific research,
experimentation, investigation, observation, etc.

Etymology (EN): M.L. laboratorium “a place for labor or work,” from L. laboratus, p.p. of laborare “to work.”

Etymology (PE): Âzmâyešgâh, from âzmâyeš, → experiment,

• -gâh “place, time,” → university.

The Lizard. A small constellation in the northern hemisphere, at about 22h right ascension, 45° north declination. Its brightest star is only of magnitude +3.8, and the constellation contains no other star above fourth magnitude. Its most famous object is BL Lacerta, the prototype → BL Lac objects. Abbreviation: Lac; genitive: Lacertae.

Etymology (EN): From L. lacertus (fem. lacerta) “lizard,” of unknown origin.

Etymology (PE): Calpâsé “lizard,” variants karpâsa, karisa, kelpasa; cf. Skt. krakacapad- “saw-footed, a lizard, chameleon,” from krakaca- “saw” + pad “foot” (Pers. pâ).

1a) Deficiency or absence of something needed, desirable, or customary.

1b) Something missing or needed. See also → default, → deficiency, → shortage.

2a) (v.tr.) To be without or deficient in; to fall short in respect of.

2b) (v.intr.) to be absent or missing, as something needed or desirable (Dictionary.com).

Etymology (EN): M.E. lak; cognate with M.L.G. lak, M.Du. lac “deficiency;” akin to O.Norse lakr “deficient.”

Etymology (PE): Nast, from negation prefix na- “in-, non,” → not, + ast, hast “is,” from astan, hastan “to be,” → exist.

Absent; wanting; deficient.

See also: → lack; → -ing.

1. A piece of equipment consisting of a series of bars or steps between two upright lengths of wood, metal, or rope, used for climbing up or down something.
   

2. A series of ascending stages by which someone or something may progress (OxfordDictionaries.com).

Etymology (EN): M.E. laddre, O.E. hlæder “ladder, steps” (cognates: M.Du. ledere, O.H.G. leitara, Ger. Leiter), from PIE root *klei- “to lean,” → incline.

Etymology (PE): Nardebân “ladder.”

1a) A lagging or falling behind; retardation.

1b) Mechanics: The amount of retardation of some motion.

1c) Electricity: The retardation of one alternating quantity, as current, with respect to another related alternating quantity, as voltage.

2. To fail to maintain a desired pace or to keep up; fall or stay behind (Dictionary.com).

Etymology (EN): Possibly from Scandinavian; cf. Norwegian lagga “to go slowly.”

Etymology (PE): Lek, from lek lek kardan “to walk slowly, to lag behind.”

1. A body of seawater that is almost completely cut off from the ocean by a barrier beach.
   
2. The body of seawater that is enclosed by an atoll.

Etymology (EN): Lagoon, from Fr. lagune, from It. laguna “pond, lake,” from L. lacuna “pond, hole,” from lacus “pond;” → nebula.

Etymology (PE): Mordâb “lagoon,” literally “dead water,” from mord, mordé “dead”

• âb “water.”
  The first element
  from mordan, mir- “to die,” marg “death,” mard “man;” Mid.Pers. murdan “to die;” O.Pers. marta- “dead,” martiya- “man;” Av. mərəta- “died, dead,” amərətāt- “immortality;” cf. Skt. mar- “to die,” mriyáe “dies;” Gk. emorten “to die,” ambrotos “immortal;” L. morior “to die” (Fr. mourir), mors, mortis “death” (Fr. mort), immortalis “immortal;” Lith. mirtis “mortal;” O.C.S. mrutvu “dead;” O.Ir. marb; Welsh marw “died;” O.E. morþ “murder;” PIE base *mor-/*mr- “to die.”
  The second element âb “water,” from
  Mid.Pers. âb “water;” O. Pers. ap- “water;” Av. ap- “water;” cf. Skt. áp- “water;”
  Hitt. happa- “water;” PIE āp-, ab- “water, river;”
  cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis “stream, river” (from *abnis); O.Ir. ab “river,” O.Prus. ape “stream,” Lith. upé “stream;” Latv. upe “brook.”

A giant → H II region lying in the direction of
→ Sagittarius about 5,000 → light-years away. It represents a giant cloud of interstellar matter which is currently undergoing star formation, and has already formed a considerable cluster of young stars (NGC 6530).

See also: → lagoon; → nebula.

A set of second order → differential equations for a system of particles which relate the kinetic energy of the system to the → generalized coordinates, the generalized forces, and the time. If the motion of a → holonomic system is described by the generalized coordinates q₁, q₂, …, q_n and the → generalized velocities 

q^.₁, q^.₂, …, q^._n, the equations of the motion are of the form:

d/dt (∂T/∂q^._i) - ∂T/∂q^._i = Q_i (i = 1, 2, …, n), where T is the kinetic energy of the system and Q_i the generalized force.

See also: → Lagrangian; → equation.

1. Of or relating to Joseph-Louis Lagrange (1736-1813), see below.
   

2. Same as → Lagrangian function. The Lagrangian of a → dynamical system describes its → dynamics and when subjected to an → action gives rise to → field equations and a → conservation law for the theory. Lagrangians are the keys for the mathematical formulation of field theories ( → field theory).
   

See also:
→ inner Lagrangian point, → Lagrangian density, → Lagrangian dynamics, → Lagrangian formalism, → Lagrangian function, → Lagrangian method, → Lagrangian multiplier, → Lagrangian particle, → Lagrangian point.

See also: After the French/Italian mathematician Joseph-Louis Lagrange (1736-1813), who was the creator of the → calculus of variations (at the age of nineteen). He made also great advances in the treatment of → differential equations and applied his mathematical techniques to problems of → mechanics, especially those arising in astronomy.

A quantity, denoted L_d, describing a continuous system in the → Lagrangian formalism, and defined as the → Lagrangian per unit volume. It is related to the Lagrangian L by:
L = ∫∫∫L_d d³V.

Lagrangian density is often called Lagrangian when there is no ambiguity.

See also: → Lagrangian; → density.

A reformulation of → Newtonian mechanics in which dynamical properties of the system are described in terms of generalized variables.
In this approach the → generalized coordinates and → generalized velocities are treated as independent variables. Indeed applying Newton’s laws to complicated problems can become a difficult task, especially if a description of the motion is needed for systems that either move in a complicated manner, or other
coordinates than → Cartesian coordinates are used, or even for systems that involve several objects. Lagrangian dynamics encompasses Newton dynamics, and moreover leads to the concept of the → Hamiltonian of the system
and a process by means of which it can be calculated. The Hamiltonian is a cornerstone in the field of → quantum mechanics.

See also: → Lagrangian; → dynamics.

A reformulation of classical mechanics that describes the evolution of a physical system using → variational principle The formalism does not require the concept of force, which is replaced by the → Lagrangian function. The formalism makes the description of systems more simpler. Moreover, the passage from classical description to quantum description becomes natural.
Same as → Lagrangian dynamics.

See also: → Lagrangian; → formalism.

A physical quantity (denoted L), defined as the difference between the → kinetic energy (T) and the → potential energy (V) of a system: L = T - V. It is a function of → generalized coordinates, → generalized velocities, and time. Same as
→ Lagrangian, → kinetic potential.

See also: → Lagrangian; → function.

Fluid mechanics: An approach in which a single fluid particle (→ Lagrangian particle) is followed during its motion. The physical properties of the particle, such as velocity, acceleration, and density are described at each point and at each instant. Compare with → Eulerian method.

See also: → Lagrangian; → method.

Math.: A constant that appears in the process for obtaining extrema of functions of several variables. Suppose that the function f(x,y) has to be maximized by choice of x and y subject to the constraint that g(x,y)≤ k. The solution can be found by constructing the → Lagrangian function  L(x,y,λ) = f(x,y) + λ[k - g(x,y)], where λ is the Lagrangian multiplier.

See also: → Lagrangian point; → multiplier.

Fluid mechanics: In the → Lagrangian method, a particle that moves as though it is an element of fluid. The particle concept is an approach to solving complicated fluid dynamics problems by tracking a large number of particles representing the fluid. The particle may be thought of as the location of the center of mass of the fluid element with one or more property values.

See also: → Lagrangian; → particle.

On of the five locations in space where the → centrifugal force and the → gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body,
located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m’s orbit approximately 60° ahead of it,
while L5 lies in the m’s orbit approximately 60° behind m. See also → Trojan asteroid; → Roche lobe; → equipotential surface; → horseshoe orbit.

See also: → Lagrangian; → point.

A body of fresh or salt water entirely surrounded by land.

Etymology (EN): From O.Fr. lack, from L. lacus “pond, lake,” related to lacuna “hole, pit,” from PIE *lak- (cf. Gk. lakkos “pit, tank, pond,” O.C.S. loky “pool, cistern,” O.Ir. loch “lake, pond”).

Etymology (PE): Daryâcé, from daryâ “sea” Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- “sea;” cf. Skt. jráyas- “expanse, space, flat surface”

• -cé diminutive suffix, from Mid.Pers. -cak, variants -êžak (as in kanicak “little girl,” sangcak “small stone,” xôkcak “small pig”), also Mod.Pers. -ak.

A young sheep; the meat of a young sheep.

Etymology (EN): M.E., O.E.; cognate with Du. lam, Ger. Lamm, Goth. lamb; akin to Gk. elaphos “deer.”

Etymology (PE): Mid.Pers. warrag “lamb; sheep;” warân “ram;” Av. varən-; cf. Skt. uaran-; L. vervex (Fr. brebis); Arm. garn; Baluci garând “ram;” Lori, Laki veran “ram;” PIE *wrhen- “lamb.”

A tiny change in the → energy levels of the → hydrogen atom between the states 2S_1/2 and 2P_1/2, which creates a shift in the corresponding → spectral lines. The 2P_1/2 state is slightly lower than the 2S_1/2 state, contrarily to the Schrodinger’s solution. The difference is explained by the interaction between → vacuum energy fluctuations and the hydrogen electron in different orbitals.

See also: Named after Willis Eugene Lamb, Jr. (1913-2008), an American physicist who discovered this effect in 1951, and won the Nobel Prize in physics in 1955 “for his discoveries concerning the fine structure of the hydrogen spectrum;” → shift.

The eleventh letter of the Greek alphabet. In lower case, λ, it denotes → wavelength. It is also used in the → Bayer designation system to identify a specific star in a → constellation. See also → lambda point.

In upper case, Λ, it represents the → cosmological constant or → dark energy.

See also: From Phoenician lamedh.

The prototype of a small class of stars (A-F types) which have weak metallic lines (indicating that they are depleted in metals heavier than Si, but with solar abundances of C, N, O, and S). Moreover, they have moderately large rotational velocities and small space velocities. Lambda Boo stars may be pre-main-sequence objects, or they may be main sequence stars that formed from gas whose metal atoms had been absorbed by interstellar dust.

See also: Named after the prototype, the star → Lambda (λ) of constellation → Bootes; → star.

The → standard model of → Big Bang that incorporates both → dark matter and → dark energy. See also → cold dark matter (CDM).

See also: → lambda, → cosmological constant; → cold; → dark; → matter; → model.

Same as → Meissa.

See also: Lambda (λ), a Greek letter used in the → Bayer designation of star names.

The temperature (roughly 2.17 K) at which → liquid helium (→ helium I) becomes → superfluid (→ helium II).

See also: The name was given by the Dutch physicist Willem Hendrik Keesom (1876-1956), who discovered the behavior of helium near this transition point and successfully solidified helium in 1926 (under an external pressure of 25 atmospheres). The name was originally suggested by Paul Ehrenfest (1880-1933), who
was inspired by the shape of the → specific heat curve, which
resembles the Gk. letter → lambda; → point.

A centimeter-gram-second (cgs) unit of luminance (or brightness) equal to 1/π candle per square centimeter. Physically, the lambert is the luminance of a perfectly diffusing white surface receiving an illuminance of 1 lumen per square centimeter.

See also: Johann Heinrich Lambert (1728-1777), German scientist and mathematician; → law.

The intensity of the light emanating in any given direction from a perfectly diffusing surface is proportional to the cosine of the angle between the direction and the normal to the surface. Also called → Lambert’s law.

See also: → lambert; → cosine; → law.

Same as → Lambert’s cosine law.

See also: → lambert; → law.

A → planetary or → satellite disk with → Lambertian surface. Such a disk has the same → surface brightness at all angles.

See also: → lambert; → disk.

A surface whose → luminous intensity obeys → Lambert’s cosine law. Such a source has a → reflectance that is uniform across its surface and uniformly emits in all directions from all its points. It appears equally bright from all viewing directions. Lambertian surface is a very useful concept for the approximation of radiant power transfer.

See also: → lambert; → surface.

1a) A real or imagined cause for → complaint, especially unfair treatment.

1b) A feeling of resentment over something believed to be wrong or unfair.

2. To feel or express sorrow or regret for; to mourn for or over (OxfordDictionaries.com).

Etymology (EN): M.E., from M.Fr. lament and directly from L. lamentum “a wailing, moaning, weeping” from lamentum “a wailing,” from PIE root *la- “to shout, cry.”

Etymology (PE): Šekvé, Pers. construction from Ar. šakvâ “complaint.”

A thin plate, layer, or flake.

Etymology (EN): From L. lamina “thin plate or layer, leaf.”

Etymology (PE): Varaqé “sheet, plate,” from varaq “a leaf of tree or of paper,” from Ar. waraq, from Pers. barg “leaf” (Tabari, Gilaki valg, balg; Kurd. belg, balk, Semnâni valg); Mid.Pers. warg “leaf;” Av. varəka- “leaf;” cf. Skt. valká- “bark, bast, rind;” Russ. volokno “fibre, fine combed flax.”

Composed of, or arranged in, laminae, sheets.

See also: → lamina + → -ar.

In a fluid flow, layer next to a fixed boundary. The fluid velocity is zero at the boundary but the molecular viscous stress is large because the velocity gradient normal to the wall is large. → turbulent boundary layer.

See also: → laminar; → boundary; → layer.

A flow in which the particles of fluid are moving orderly, and in which adjacent layers or laminas glide smoothly over another
with little mixing between them. A laminar flow may rapidly transform into a → turbulent flow for large → Reynolds numbers.

See also: → laminar; → flow.

A layer in which the fluid undergoes smooth, nonturbulent flow. It is found between any surface and a turbulent layer above.

See also: → laminar; sublayer, from → sub- + → layer.

Any of various devices producing artificial light, as by electricity, gas, or oil.

Etymology (EN): From O.Fr. lampe, from L. lampas, from Gk. lampas “torch, lamp, beacon, meteor, light,” from lampein “to shine,” from PIE base *lap- “to shine” (cf. Lith. lope “light,” O.Ir. lassar “flame”).

Etymology (PE): Lâmp, loanword from Fr., as above.
Cerâq “lamp” (variants Kurd. cira, cerâh, Laki cerâx, Zâzâ cərâ, cərâx “lamp, candle,” Shughni cirow, cirâw, cərêγ “candle, lamp”); Mid.Pers. cirâq “lamp,” related to foruq “light,”
afruxtan “to light, kindle,” rowšan “bright, clear,” rowzan “window, aperture;” ruz “day;”
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; E. light; O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- “light, brightness.”

Any part of the earth’s surface not covered by a body of water.

Etymology (EN): M.E., from O.E. land, lond, “ground, soil, territory;” PIE base *lendh- “land, heath” (cf. O.N., O.Fris. Du., Ger., Goth. land; O.Ir. land; Welsh llan “enclosure, church,” Breton lann “heath,” source of Fr. lande; O.C.S. ledina “waste land, heath,” Czech lada “fallow land”).

Etymology (PE): Xoški, from xošk, → dry, + noun suffix -i; zamin “land, → earth.”

A coastal breeze blowing from land to sea after sunset, caused by the temperature difference when the sea surface is warmer than the adjacent land. The warmer air above the water continues to rise, and cooler air from over the land replaces it, creating a breeze.

Etymology (EN): Land, → lander; → breeze.

Etymology (PE): Xoški “land,” from xošk “dry;” Mid.Pers. xušk “dry;” O.Pers. uška- “mainland;” Av. huška- “dry;” cf. Skt. śuska- “dry, dried out;” Gk. auos “dry, dried up;” O.E. sēar “dried up, withered;” Lith. sausas “dry, barren.”

The process wherein a → plasma gains energy at the expense of the → Langmuir wave. In the presence of the → Landau resonance, the particles in resonance moving slightly faster than the wave lose energy, while those moving slightly slower will gain energy. Since the Maxwellian distribution is decreasing with velocity, in a Maxwellian plasma, near the Landau resonance, there are more particles at lower velocities than at higher velocities. Also called collisionless damping.

See also: Lev Landau (1908-1968), a prominent Soviet physicist, 1962 Nobel Prize in Physics for his development of a mathematical theory of → superfluidity; → damping.

The → energy level which can be occupied by
a → free electron in a → magnetic field.

See also: → Landau damping; → level.

For parallel propagating → electrostatic waves in a → plasma, the → resonance which occurs when the particle velocity equals the parallel phase velocity of the wave.

See also: → Landau damping; → damping.

The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1.

See also: After Alfred Landé (1888-1976), a German-American physicist, known for his contributions to quantum theory; → facteur.

A → space probe designed to land on a → planet or other solid → celestial body.

See also: → land; → -er.

A second-order nonlinear → differential equation that gives the structure of a → polytrope of index n.

See also: Named after the American astrophysicist Jonathan Homer Lane (1819-1880) and the Swiss astrophysicist Robert Emden (1862-1940); → equation

Equation of motion for a weakly ionized cold plasma.

See also: Paul Langevin (1872-1946), French physicist, who developed the theory of magnetic susceptibility of a paramagnetic gas; → equation.

A disturbance of a → plasma in the form of a
longitudinal, → electrostatic wave that propagates in the plasma due to variations in the plasma’s electron density. More specifically,
Langmuir waves are collective oscillations of inhomogeneous bunches of electrons displaced from their natural equilibrium, in which the inertia of the relatively massive ions serves to establish an electrostatic restoring force that tries to bring the electrons back to their equilibrium positions. → Landau damping causes dissipation of Langmuir waves as the electrons are either accelerated or decelerated so as to be in resonance with the phase velocity of the waves themselves.

See also: Irving Langmuir (1881-1957), American chemist and physicist, Nobel Prize in Chemistry 1932; → wave.

See also: Suggested by Irving Langmuir (1881-1957) in 1921, who was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry. And further developed by Cyril Hinshelwood (1897-1967) in 1926, who received the Nobel Prize in Chemistry in 1956 for his researches into the mechanism of chemical reactions.

Any means of conveying or communicating ideas; specifically, human speech.

Etymology (EN): M.E., from O.Fr. langage, from L. lingua “tongue; speech, language.”

Etymology (PE): Zabân “tongue; language,” from Mid.Pers. uzwân “tongue; language;” O.Pers. hzanm, hizânam “tongue,” Av. hizuua-, hizū- “tongue;” cf. Skt. jivhā- “tongue;” L. lingua “tongue, speech, language;” O.Ir. tenge; Welsh tafod; Lith. liezuvis; O.C.S. jezyku; M.Du. tonghe; Du. tong; O.H.G. zunga; Ger. Zunge; Goth. tuggo; PIE base *dnghwa-.

An approach in which terms reconstructed in the → proto-language are used to make inferences about its speakers’ culture and environment.

See also: → language;→ paleontology.

A → supercluster of galaxies that includes our → Local Group and about 300 to 500 known → galaxy clusters and groups. Also called → Local Supercluster. If approximated as round, it has a diameter of 12,000 km s^-1 in units of the → cosmic expansion or 160 megaparsecs, and encompasses about 10¹⁷ → solar masses.
Our Local Group lies toward the outer regions of Laniakea. Its main components are the four previously known superclusters:

→ Virgo supercluster (the part where the → Milky Way resides),
Hydra-Centaurus Supercluster (including the → Great Attractor, Antlia Wall, known as Hydra Supercluster, → Centaurus supercluster), Pavo-Indus Supercluster, and Southern Supercluster (including Fornax Cluster, Dorado and Eridanus clouds).

The most massive galaxy clusters of Laniakea are Virgo, Hydra, Centaurus, Abell 3565, Abell 3574, Abell 3521, Fornax, Eridanus, and Norma. The Laniakea supercluster was discovered by Tully et al. (2014, Nature 513, 71).

See also: From the Hawaiian words lani “heaven,” and akea “spacious, immeasurable;” → supercluster.

Any of the series of 15 consecutive → chemical elements in the → periodic table from → lanthanum to lutetium (→ atomic numbers 57 to 71 inclusive). The atoms of these metals have similar configurations and similar physical and chemical properties. They are grouped apart from the rest of the elements in the → Periodic Table because they all behave in a similar way in chemical reactions. Also called
→ rare-earth element. International Union of Pure and Applied Chemistry currently recommends the name lanthanoid rather than lanthanide.

See also: From the chemical element → lanthanum.

A soft, malleable, ductile, silver-white metallic → chemical element; symbol La. → atomic number 57; → atomic weight 138.9055; → melting point about 920°C; → boiling point about 3,460°C; → specific gravity 6.19 at 25°C; → valence +3. Lanthanum is a member of the → lanthanide group, also called → rare-earth elements. Two naturally occurring → isotopes of lanthanum are known,
¹³⁹La (more than 99%) and ¹³⁸La (less than 0.1%). The → half-life of ¹³⁸La is 1.1 x 10¹¹
years.

See also: From lanthan- + suffix -um, variant of → -ium.
The first component from Gk. lanthanein for “to lie hidden, to escape notice” because it hid in cerium ore and was difficult to separate from that rare-earth mineral. It was discovered in the form lanthanium oxide, called lanthana, by the Swedish surgeon and chemist Carl-Gustav Mosander (1797-1858) in 1839. Subsequently, in 1842, Mosander separated his lanthanium sample into two oxides; for one of these he retained the name lanthanum and for the other he gave the name didymium (or twin).

The French great mathematician, physicist, and astronomer Pierre-Simon Marquis de Laplace (1749-1827).

→ Laplace operator; → Laplace plane; → Laplace resonance; → Laplace transform; → Laplace’s demon ; → Laplace’s equation ; → Kant-Laplace hypothesis

Same as → Laplacian.

See also: → Laplace; → operator.

The plane normal to the axis about which the pole of a satellite’s orbit → precesses. In his study of Jupiter’s satellites, Laplace (1805) recognized that the combined effects of the solar tide and the planet’s oblateness induced a “proper” inclination in satellite orbits with respect to Jupiter’s equator. He remarked that this proper inclination increases with the distance to the planet, and defined an orbital plane (currently called Laplace plane) for circular orbits that lies between the orbital plane of the planet’s motion around the Sun and its equator plane (Tremaine et al., 2009, AJ, 137, 3706).

See also: → Laplace; → plane.

An → orbital resonance that makes a 4:2:1 period ratio among three bodies in orbit. The → Galilean satellites → Io, → Europa, → Ganymede are in the Laplace resonance that keeps their orbits elliptical. This interaction prevents the orbits of the satellites from becoming perfectly circular (due to tidal interactions with Jupiter), and therefore permits → tidal heating of Io and Europa.

For every four orbits of Io, Europa orbits twice and Ganymede orbits once. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted over short time periods.

See also: This commensurability was first pointed out by Pierre-Simon Laplace, → Laplace; → resonance.

An integral transform of a function obtained by multiplying the given function f(t) by e^-pt, where p is a new variable, and integrating with respect to t from t = 0 to t = ∞.

See also: → Laplace; → transform.

An imaginary super-intelligent being who knows all the laws of nature and all the parameters describing the state of the Universe at a given moment can predict all subsequent events by virtue of using physical laws. In the introduction to his 1814 Essai philosophique sur les probabilités, Pierre-Simon Laplace puts forward this concept to uphold → determinism, namely the belief that the past completely determines the future. The relevance of this statement, however, has been called into question by quantum physics laws and the discovery of → chaotic systems.

See also: → Laplace; → demon.

A → linear differential equation of the second order the solutions of which are important in many fields of science, mainly in electromagnetism, fluid dynamics, and is often used in astronomy. It is expressed by:

∂²V/ ∂x² + ∂²V/ ∂y² + ∂²V/ ∂z² = 0. Laplace’s equation can more concisely expressed by: ∇²V = 0.
The function V may, for example, be the potential at any point in the electric field where there is no free charge. The general theory of solutions to Laplace’s equation is known as potential theory.

See also: → Laplace; → equation.

A differential → operator, denoted ∇² = ∇.∇,
which is the sum of all second partial derivatives of a dependant variable:

∇²≡ ∂²/∂x² + ∂²/∂y² + ∂²/∂z², in Cartesian coordinates.

It has numerous applications in several fields of physics and mathematics. Also called Laplace operator.

See also: Named after → Laplace.

Of more than average size, quantity, degree, etc.; of great scope or range.

Etymology (EN): From O.Fr. large “broad, wide,” from L. largus “abundant, copious, plentiful,” of unknown origin.

Etymology (PE): Bozorg “great, large, immense, grand, magnificient;” Mid.Pers. vazurg “great, big, high, lofty;” O.Pers. vazarka- “great;” Av. vazra- “club, mace” (Mod.Pers. gorz “mace”); cf. Skt. vájra- “(Indra’s) thunderbolt,” vaja- “strength, speed;” L. vigere “be lively, thrive,” velox “fast, lively,” vegere “to enliven,” vigil “watchful, awake;”
P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen “to be awake;” E. wake); PIE base *weg- “to be strong, be lively.”

The larger of the two Magellanic Cloud galaxies visible in the southern hemisphere at
about 22 degrees from the South Celestial Pole. It is approximately on the border between the constellations → Dorado and → Mensa in a region of faint stars. The center of the LMC is approximately RA: 5h 23m 35s, dec: -69° 45’ 22’’. The LMC shines with a total → apparent visual magnitude of approximately zero.

It spans an area of the sky about 9 by 11 degrees, corresponding to about 30,000 → light-years across in the longest dimension, for a distance of some 162,000 light-years.

It has a visible mass of about one-tenth that of our own Galaxy (10¹⁰ Msun). The LMC and its twin, the → Small Magellanic Cloud, are two of our most prominent Galactic neighbors.

The LMC is classified as a disrupted → barred spiral galaxy of type SBm, the prototype of a class of → Magellanic spirals. The galaxy is characterized by a prominent offset → stellar bar located near its center with the dominant → spiral arm to the north with two “embryonic” arms situated to the south.

The → metallicity in the LMC is known to be lower than in the solar neighborhood by a factor 2 or more.

Based on 20 → eclipsing binary systems, the distance to the LMC is measured to one percent precision to be 49.59±0.09 (statistical) ±0.54 (systematic) kpc (Pietrzynski et al., 2019, Nature 567, 200).

See also: → large; → Magellanic; → cloud.

A → dimensionless number representing the ratio of various → physical constants. For example:

1. The ratio of the → Coulomb force to the → gravitational force for a proton-electron pair
   (e²/Gm_pm_e), which is of the order of 10⁴⁰.
   

2. The age of Universe (T = 10-20 x 10⁹ years) in units of the → elementary time: T/t_e≅ 10⁴⁰.
   

3. The square root of the total number of particles in the Universe ≅ 10⁴⁰.

See also: → large; → number.

The idea whereby the coincidence of various → large numbers would bear a profound sense as to the nature of physical laws and the Universe. Dirac suggested that the coincidence seen among various large numbers of different nature is not accidental but must point to a hitherto unknown theory linking the quantum mechanical origin of the Universe to the various cosmological parameters. As a consequence, some of the → fundamental constants cannot remain unchanged for ever. According to Dirac’s hypothesis, atomic parameters cannot change with time and hence the → gravitational constant should vary inversely with time (G∝ 1/t). Dirac, P. A. M., 1937, Nature 139, 323; 1938, Proc. R. Soc. A165, 199.

See also: → large; → number; → hypothesis.

A turbulent flow in which viscous forces are negligible compared to nonlinear advection terms, which characterize the variation of fluid quantities. The dynamics becomes generally turbulent when the Reynolds number is high enough. However, the critical Reynolds number for that is not universal, and depends in particular on boundary conditions.

See also: → large; → Reynolds number; → flow.

1. A scale representing measures that significantly override the usual ones of the same kind.
   

2. In meteorology, a scale in which the curvature of the earth is not negligible.

See also: → large; → scale.

Initial name given to → Vera C. Rubin Observatory.

See also: → large; → synoptic; → survey; → telescope.

The distribution of galaxies and other forms of mass on large distance scales, covering hundreds of millions of → light-years.

See also: → large; → scale; → structure.

The fifth of Neptune’s known satellites. It orbits 73,600 km from Neptune and is a non spherical object about 208 × 178 km in size. It was discovered using NASA’s Voyager 2 mission in 1989.

See also: In Gk. mythology, Larissa is a princess of Argos (in central Greece) who, according to some, bore Poseidon three sons: Akhaios, Pelasgos and Pythios (though others gave these eponymous heroes different parents).

The frequency of precession of a charged particle describing a circular motion in a plane perpendicular to the magnetic induction in a uniform magnetic field.

See also: Named after Joseph Larmor (1857-1942), an Irish physicist, the first to calculate the rate at which energy is radiated by an accelerated electron, and the first to explain the splitting of spectrum lines by a magnetic field; → frequency.

The radius of the circular motion of a → charged particle moving in a → uniform magnetic field. Same as → gyroradius, → radius of gyration, → cyclotron radius. The Larmor radius (r_L) is obtained by equating the → Lorentz force with the → centripetal force: qvB = mv²/r_L, which leads to r_L = p/(ZeB), where p is → momentum, Z is → atomic number, e is the → electron charge, and B is → magnetic induction. The frequency of this circular motion is known as the → gyrofrequency.

See also: → Larmor frequency; → radius.

If a system of → charged particles, all having the same ratio of charge to mass (q/m), acted on by their mutual forces, and by a central force toward a common center, is subject in addition to a weak uniform magnetic field (B), its possible motions will be the same as the motions it could perform without the magnetic field, superposed upon a slow → precession of the entire system about the center of force with angular velocity ω = -(q/2mc)B.

See also: → Larmor frequency; → theorem.

An → empirical relationship between the internal → velocity dispersion of → molecular clouds and their size. The velocity dispersions are derived from molecular → linewidths, in particular those of → carbon monoxide. It was first established on star forming regions and found to be:

σ (km s^-1) = 1.10 L (pc)^0.38,

where σ is the velocity dispersion and L the size. The relation holds for 0.1 ≤ L ≤ 100 pc. More recent set of cloud data yield:

σ (km s^-1) = L (pc)^0.5.

This relation indicates that larger molecular clouds have larger internal velocity dispersions. It is usually interpreted as evidence for → turbulence in molecular clouds. Possible sources of interstellar turbulence include the following processes operating at various scales: galactic-scale (→ differential rotation, → infall
of extragalactic gas on the galaxy), intermediate-scale (expansion of → supernova remnants, → shocks, → stellar winds from → massive stars), and smaller-scale processes
(→ outflows from → young stellar objects).

See also: First derived by Richard B. Larson, American astrophysicist working at Yale University (Larson, 1981, MNRAS 194, 809). See Falgarone et al. (2009, A&A 507, 355) for a recent study; → relation.

The analytical solution to the → hydrodynamic equations describing the → collapse of an → isothermal sphere. The Larson-Penston solution is → self-similar for a purely dynamical isothermal collapse with spherical symmetry. It corresponds to the collapse prior to the formation of a → protostar, and thus is suitable for the study of → pre-stellar cores. The Larson-Penston solution was extended by Shu (1977) to obtain a whole family of solutions for this problem.

See also: Named after R. B. Larson (1969, MNRAS 145, 271) and M. V. Penston (1969, MNRAS 144, 425), who simultaneously, but independently, did this study.

1. Of, pertaining to, or located in the larynx.
   

2. Phonetics: articulated in the larynx (Dictionary.com).

See also: → larynx; → -al.

A consonant generated in the → larynx with the → vocal cords partly closed and partly vibrating. It is hypothesized that the → Proto-Indo-European language contained some laryngeal consonants (denoted by H).

See also: → laryngeal; → consonant.

A muscular and cartilaginous structure lined with mucous membrane at the upper part of the → trachea in humans, in which the → vocal cords are located (Dictionary.com).

Etymology (EN): From M.Fr. larynx, from M.L. from Gk. larynx (genitive laryngos) “the upper windpipe,” probably from laimos “throat,” influenced by pharynx “throat, windpipe.”

Etymology (PE): Hanjaré, from Ar. Hanjarah.

1. A device that generates an intense directional beam of → monochromatic and → coherent light by exciting atoms to a higher energy level and causing them to radiate their energy in phase. The high degree of collimation arises from the fact that excited atoms are are situated in a cavity bounded by two parallel front and back mirrors. A first photon stimulates
   an atom which emits a second photon, and so on thanks to the mirrors. The resulting photons are all identical. They have the same energy which gives them the same color and a unique direction. The first working laser, a pulsed ruby device, was developed by T. Maiman in 1959.

See also → gas laser, → stimulated emission; → maser.

2. The light produced in this way.

See also: Acronym for light amplification by stimulated emission of radiation,
on pattern of → maser.

A technique that uses a suitable arrangement of → laser beams and magnetic fields to capture → cesium (¹³³Cs) atoms from a thermal vapor and slow the motion of the atoms, cooling them to just a few micro-kelvins above the → absolute zero. The technique allows trapping some 10⁷ cesium atoms in a cloud a few millimeters in diameter in a few tenths of a second. At a temperature of 2 μK, the average thermal velocity of the cesium atoms is of the order of 1 cm s^-1, so they stay together for a relatively long time. The laser cooling technique is the key tool which enabled the operation of an → atomic fountain clock.

See also: → laser; → cooling; → technique.

An optical instrument using laser → beams to form → interference pattern.

There are two types of laser interferometers: → homodyne and → heterodyne. A homodyne interferometer, like → Michelson interferometer, uses a single-frequency laser source.

A → heterodyne interferometer uses a laser source with two close frequencies.

See also: → laser; → interferometer.

A facility dedicated to the detection and measurement of cosmic → gravitational waves. It consists of two widely separated installations, or detectors, within the United States, operated in unison as a single observatory. One installation is located in Hanford (Washington) and the other in Livingston (Louisiana), 3,000 km apart.

Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001.

Each LIGO detector beams laser light down arms 4 km long, which are arranged in the shape of an “L.” If a gravitational wave passes through the detector system, the distance traveled by the laser beam changes by a minuscule amount – less than one-thousandth of the size of an atomic nucleus (10^-18 m). Still, LIGO should be able to pick this difference up.

LIGO directly detected gravitational waves for the first time from a binary → black hole merger (GW150914) on September 14, 2015 (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102).

The Nobel Prize in physics 2017 was awarded to three physicists (Rainer Weiss, Barry C. Barish, and Kip S. Thorne) for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO had a prominent role in the detection of → GW170817, the first event with an → electromagnetic counterpart.

See also: → laser; → interferometer; → gravitational; → wave; → observatory.

A collaborative project between → NASA and → ESA to develop and operate a space-based gravitational wave detector sensitive at frequencies between 0.03 mHz and 0.1 Hz. LISA detects gravitational-wave induced strains in → space-time by measuring changes of the separation between fiducial masses in three spacecraft 5 million km apart. Ultimately, NASA and ESA decided in 2011 not to proceed with the mission. LISA was not the highest ranked mission in the 2010 Decadal Survey and funding constraints prevented NASA from proceeding with multiple large missions (http://lisa.nasa.gov). → LISA pathfinder.

See also: → laser; → interferometer; → space; → antenna.

Occurring or coming after all others, as in time, order, or place. → last contact, → last quarter

Etymology (EN): Last, from O.E. latost (adj.) and lætest (adv.), superlative of læt (adj.) and late (adv.);
cognate with O.Fris. lest, Du. laatst, O.H.G. laggost, Ger. letzt.

Etymology (PE): Vâpasin, from vâ-, as intensive prefix, → de-,

• pasin, from pas “after, afterward, behind; finally, at last” (Mid.Pers. pas “behind, before, after;” O.Pers. pasā “after;” Av. pasca “behind (of space); then, afterward (of time);” cf. Skt. pazca “behind, after, later;”
  L. post “behind, in the rear; after, afterward;” O.C.S. po “behind, after;” Lith. pas “at, by;”
  PIE base *pos-, *posko-) + -in superlative suffix.

To continue in time; go on; endure.

Etymology (EN): M.E. lasten, from O.E. læstan “to continue, endure;” cf. Goth. laistjan “to follow after,” Ger. leisten “to perform, achieve,”), from PIE root *lois- “furrow, track.”

Etymology (PE): Pâyidan “to watch, observe; remain or continue in existence, last,” variants pâsidan, pâhidan; Mid.Pers. pây- “to protect, guard;” Sogdian p’y “to observe, protect, watch over;” O.Pers. pā- “to protect,” pāta- “protected;” Av. pā- “to protect,” pāti “guards,” nipā(y)- (with ni-) “to watch, observe, guard,” nipātar- “protector, watcher,” nipāθri- “protectress;”
cf. Skt. pā- “to protect, keep,” tanû.pā- “protecting the body,” paś.pā- “shepherd;” Gk. poma “lid, cover,” poimen “shepherd;” L. pascere “to put out to graze,” pastor “shepherd;” Lith. piemuo “shepherd;” PIE base *pā- “to protect, feed.”

Same as → fourth contact at an eclipse.

See also: → last; → contact.

One of the phases of the Moon that appears when it is 90 degrees west of the Sun. Approximately one week after a full moon, when half of the Moon’s disk is illuminated by the Sun. → first quarter.

See also: → last; → quarter.

The epoch in the early evolution of the Universe when matter and photons decoupled. Once atoms formed, light and matter stopped constantly interacting with one another, and photons were able to travel freely. As a result, the Universe became transparent. Light from this period is observed today as the → cosmic microwave background radiation. Same as → decoupling era and → recombination era.

See also: → last; → scattering.

The set of locations in space corresponding to the → last scattering epoch in the early Universe. It is a spherical surface around the present-day observer from which the → cosmic microwave background radiation appears to emanate.

See also: → last; → scattering; → surface.

Last in order of birth; youngest.

Etymology (EN): → last; → born.

Etymology (PE): Demâzâ, from Lori, Laki demâzâ, from demâ “last, end, back,” related to dom, → tail, + zâ contraction of zâd, → born.

Continuing or remaining for a long time; enduring.

See also: → last; → -ing.

1. Happening or arriving after an expected or arranged time; not on time, beyond usual time.
   

2. Belonging to an advanced stage or period in the development of something. → Late Heavy Bombardment, → late helium flash, → late thermal pulse, → late-type galaxy, → late-type star.

Etymology (EN): M.E., from O.E. læt “slow, late;” cf. Ger. lass “slothful;” O.N. latr, Goth. lats “slow, lazy;” L. lassus “tired, faint;” Gk. ledein “to be weary.”

Etymology (PE): Dir “late; tardily, slowly; a long while; old, antique,” from Mid.Pers. dêr, variants dagr, drâz “long”
(Mod.Pers. derâz “long,” variant Laki, Kurdi derež);
O.Pers. darga- “long;” Av. darəga-, darəγa- “long,” drājištəm “longest;” cf. Skt. dirghá- “long (in space and time);” L. longus “long;” Gk. dolikhos “elongated;” O.H.G., Ger. lang; Goth. laggs “long;” PIE base *dlonghos- “long.”
Dirân, adjective, variant of dir, as above.

A cataclysmic event in the history of the → solar system, estimated to have occurred 3.9 billion years ago (about 600 million years after the formation of the → terrestrial planets) during which → asteroid and → comet impacts with Earth were some 20,000 times more frequent than today. It is estimated that during this period the terrestrial planets were bombarded with an object 1 km in size every 20 years. This hypothetical event lasted 50 to 150 million years. Several explanations have been put forward, among which the occurrence of an instability in the outer solar system which caused → orbital migration of small bodies from the → Kuiper belt inward.

See also: → late, with respect to the formation time of the planets; → heavy; bombardment, noun from bombard, from Fr. bombarder, from bombarde “mortar, catapult” from bombe, from It. bomba, probably from L. bombus “a booming sound,” from Gk. bombos “deep and hollow sound.”

A → helium flash event that occurs during the → post-AGB phase.

Some of the central stars of planetary nebulae (→ CSPN) experience a final → thermal pulse after having achieved a → white dwarf configuration and begun their descent along a → white dwarf cooling track of nearly constant radius. During such a pulse, most of the hydrogen remaining in the star at pulse onset is incorporated into the helium-burning convective shell and completely burned. Following the pulse, the star swells briefly to → red giant dimensions
(Iben et al. 1983; ApJ 264, 605).

See also: → late; → helium; → flash.

In evolutionary models of → low-mass and → intermediate-mass stars, the occurrence of a → helium shell flash on the → horizontal branch of the → post-AGB track, while → hydrogen shell burning is still going on.

See also: → late; → thermal; → pulse.

In the → Hubble classification, a galaxy on the left part of the → Hubble sequence. See also → early-type galaxy.

See also: → late; → type; → galaxy.

A star of → spectral type K, M, S, or C, with a surface temperature lower than that of the Sun. → early-type star. See also → spectral classification.

See also: → late; → type;
→ star.

Present but not visible, apparent, or actualized; existing as potential (Dictionary.com). → latent heat.

Etymology (EN): From L. latentem (nominative latens), pr.p. of latere “to lie hidden.”

Etymology (PE): Nahân “concealed, hid; clandestine;” Mid.Pers. nihân “secrecy, a secret place, a hiding place,” nihânik “concealed;” Av. niδāti- “deposing, deposit.”

The amount of → thermal energy that is absorbed or released by a unit amount of a substance in the process of a phase change under conditions of constant pressure and temperature.

See also: → latent; → heat.

Of or relating to the → side; situated at, proceeding from, or directed to a side (Dictionary.com).

Etymology (EN): M.E., from O.Fr. latéral and directly from L. lateralis “belonging to the side,” from latus “the side, flank; lateral surface.”

Etymology (PE): Kenâri, relating to kenâr, → side.

The angle between a perpendicular at a location, and the → equatorial plane of the Earth. → longitude. See also:
→ astronomical latitude, → celestial latitude, → circle of latitude, → colatitude, → ecliptic latitude, → Galactic latitude, → geocentric latitude, → geodetic latitude, → geographic latitude, → high latitudes, → horse latitudes, → middle latitudes, → spherical latitude, → supergalactic latitude.

Etymology (EN): L. latitudo “breadth, width, size,” from latus “wide,” from PIE base *stela- “to spread” (cf. O.C.S. steljo “to spread out,” Arm. lain “broad”).

Etymology (PE): Varunâ, from var “breadth, side, breast,” variant bar, Tabari vari “width,” Mid.Pers. var “breast,” Av. varah- “breast”
(Sk. vara- “width, breadth”) + -u a suffix forming adjectives;
Av. vouru- “wide;” + -nâ a suffix of dimension.

1. A regular geometric arrangement of points in a plane or in space.
   

2. → Crystal lattice.
   

3. A structure in a nuclear reactor containing nuclear fuel and other materials arranged in a regular geometrical pattern.
   

4. Math.: A partially ordered set in which each two-element subset has both a greatest lower bound and a least upper bound.

Etymology (EN): From O.Fr. latiz “lattice,” from late “lath, board, plank, batten” (Fr. latte); cf. O.H.G. latta “lath.”

Etymology (PE): Jâré, from jarra “net; snare,” Afghan jâli “reticulated garment,” Tabari jarazin “grilled apparatus used in a watercourse to gather thatch and trash;” cf. Skt. jāla- “net, snare, lattice.”

The energy required to separate an ion from a → crystal to an infinite distance. In other words, the energy released when one → mole of a crystal is formed from gaseous ions.

See also: → lattice; → energy.

The chord through a focus and perpendicular to then major axis of a conic section.

Etymology (EN): L. latus “side;” rectum “straight,” → right.

Etymology (PE): Târ “thread, warp, string”
(related to tur “net, fishing net, snare,”
tâl “thread” (Borujerdi dialect), tân “thread, warp of a web,” from tanidan, tan-
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- “to stretch, extend;” cf. Skt. tan- “to stretch, extend;” tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch.”

1. To throw or propel with force; hurl. → jet launching.
   

   2. To set (a missile, spacecraft, etc) into motion

Etymology (EN): From M.E. launchen “to throw as a lance,” O.Fr. lanchier, lancier “to hurl, throw, cast,” from L.L. lanceare “wield a lance,” from L. lancea “light spear, lance.”

Etymology (PE): From partâb “a throw, an arrow that flies far,” partâbidan “to throw,” → ballistics.

Molten → magma released from a volcanic vent or fissure.

Etymology (EN): Lava, from It. lava “torrent, stream,” from L. lavare “to wash;” PIE base *lou- “to wash;” cf. Persian Lori, Kurdi, Malâyeri laf “flood,” variants Tabari lé, [Mo’in, Dehxodâ] lur, lây “flood;” Gk. louein “to wash.”

Etymology (PE): Godâzé noun from godâxtan “to melt,” from Mid.Pers. vitâxtan, vitâcitan “to melt,” from Av. vi-taxti- “flowing away, melting,” from vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + tak- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
tacar- “course,” tacan “current, streaming;” Mod.Pers. tâz-, tâxtan “to run; to hasten; to assault,” tâzi “swift (greyhound),” tak “running, rush;”
Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
Khotanese ttajs- “to flow, to walk;” cf. Skt. tak- “to rush, to hurry,” takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.”

1. A rule of conduct or procedure established by custom, agreement, or authority.
   
2. A code of principles based on morality, conscience, or nature.
   
3. Physics: A statement of a scientific fact or phenomenon that is invariable under given conditions; e.g. → Newton’s law of gravitation, → second law of thermodynamics.
   
4. Math.: A general principle deduced from particular facts expressed by the statement that a particular phenomenon always occurs if certain conditions are present.

Etymology (EN): M.E., O.E. lagu, from O.N. *lagu, variant of lag “that which is laid down;” cf. Ger. liegen, E. lay, lie; PIE *legh- “To lie, lay;” compare with Hittite laggari “falls, lies,” Gk. lekhesthai “to lie down,” L. lectus “bed,” O.Ir. lige “bed, tomb,” Tokharian lake, leke “bed.”

Etymology (PE): Qânun, from Ar., ultimately from Gk. kanon “rule.”
Arté, from O.Pers. arta- “law, justice;” Av. arəta-, ərəta- “law, order,” variant aša- “truth, cosmic order,” aipi-ərəta- “firmly assigned,” from root ar- “to fix;” cf. Skt. rtá- “truth, world order; oath;” Ossetci ard “oath;” Gk. arthon “limb, articulation,” artus “a joint;”
L. artus “a joint;” PIE base *ar- “to join, to fit together.”

An expression that for any triangle relates the length of a side to the cosine of the opposite angle and the lengths of the two other sides. If a, b, and c are the sides and A, B, and C are the corresponding opposites angles:

a² = b² + c²

• 2bc cos A; b² = c² + a²
• 2ca cos B; c² = a² + b²
• 2ab cos C.

See also: → law; → cosine.

Same as → principle of excluded middle.

See also: → law; → exclude; → middle.

Same as → principle of identity.

See also: → law; → identity.

Same as → Newton’s first law. The → reference frames to which the law applies are called → inertial frames.

See also: → law; → inertia.

Same as → principle of non-contradiction.

See also: → law; → non-; → contradiction.

One of the two laws governing reflection of light from a surface: a) The → incident ray, normal to surface, and reflected ray lie in the same plane. b) The → angle of incidence (with the normal to the surface) is equal to the → angle of reflection.

See also: → law; → reflection.

One of the two laws governing → refraction of light when it enters another transparent medium: a) The → incident ray, normal to the surface, and refracted ray, all lie in the same plane. b) → Snell’s law is satisfied.

See also: → law; → refraction.

In any triangle the sides are proportional to the sines of the opposite angles: a/sin A = b/sin B = c/sin C, where A, B, and C are the three vertices and a, b, and c are the corresponding sides.

See also: → law; → sine.

An artificially produced → radioactive→ chemical element; symbol Lr (formerly Lw). → Atomic number 103; → atomic weight of most stable isotope 262; → melting point about 1,627°C; → boiling point and → specific gravity unknown; → valence +3. The longest half-life associated with this unstable element is 3.6 hour ²⁶²Lr. Credit for the first synthesis of this element in 1971 is given jointly to American chemists from the University of California laboratory in Berkeley
under Albert Ghiorso and the Russian team at the Joint Institute for Nuclear Reactions lab in Dubna, under Georgi N. Flerov.

See also: Named the American physicist Ernest 0. Lawrence (1901-1958), who developed the → cyclotron, + → -ium.

The three basic laws of → dynamics which were first formulated by Isaac Newton in his classical work “Mathematical Principles of Natural Philosophy” published in 1687. → Newton’s first law of motion; → Newton’s second law of motion; → Newton’s third law of motion.

See also: → law; → dynamics.

A thickness of some material laid on or spread over a surface.

Etymology (EN): From M.E. leyer, legger + -er. The first element from layen, leggen “to lay,” from O.E. lecgan;
cf. Du. leggen; Ger. legen; O.N. legja; Goth. lagjan

Etymology (PE): Lâyé “layer,” from lâ, lây “fold” + -é nuance suffix of nouns.

The ordinary Newtonian energy conservation equation when expressed in expanding cosmological coordinates. More specifically, it is the relation between the → kinetic energy per unit mass associated with the motion of matter relative to the general → expansion of the Universe and the → gravitational potential energy per unit mass associated with the departure from a homogeneous mass distribution. In other words, it deals with how the energy of the → Universe is partitioned between kinetic and potential energy.
Also known as → cosmic energy equation.
In its original form, the Layzer-Irvine equation accounts for the evolution of the energy of a system of → non-relativistic particles, interacting only through gravity, until → virial equilibrium

is reached. But it has recently been generalized to account for interaction between → dark matter and a homogeneous → dark energy component. Thus,
it describes the dynamics of local dark matter perturbations in an otherwise homogeneous and → isotropic Universe (P. P. Avelino and C. F. V. Gomes, 2013, arXiv:1305.6064).

See also: W. M. Irvine, 1961, Ph.D. thesis, Harvard University; D. Layzer, 1963, Astrophys. J. 138, 174; → equation.

A change in one of the variables (such as temperature, pressure, and concentration of various species) that describe a system at equilibrium produces a shift in the position of the equilibrium that counteracts the effect of this change.

See also: Named after the French chemist and engineer Henry Louis Le Chatelier (1850-1936); → principle.

A metallic chemical element; symbol Pb (L. plumbum, of unknown origin). Atomic number 82; atomic weight 207.2; melting point 327.502°C; boiling point about 1,740°C. One of the oldest metals used by humanity, because of its relatively low melting point.

Etymology (EN): O.E. lead, from W.Gmc. *loudhom (cf. O.Fris. lad, M.Du. loot “lead,” Ger. Lot “weight, plummet”).

Etymology (PE): Sorb, from Mid.Pers. srub “lead;” Av. sru- “lead.”

In a → sunspot group, the first spot to form in the direction of rotation and the last to disappear. It is the largest, the strongest in magnetic intensity, and the closest to the solar equator among the group sunspots. See also → follower spot.

Etymology (EN): From M.E. leder(e), O.E. lædan “cause to go with one, lead,” from W.Gmc. *laithjan (cf. O.S. lithan, O.N. liða “to go,” O.H.G. ga-lidan “to travel,” Goth. ga-leiþan “to go”); → spot.

Etymology (PE): Lakké, → spot; pišrow “leader, forerunner,” from piš “in front, forward, before”
(Mid.Pers. pêš “before, earlier;” O.Pers. paišiya “before; in the presence of”)

• row “going,” present stem of raftan “to go, elapse, glide by, depart” (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

One of the organs, usually green and flat, growing from the side of a stem or branch or direct from the root of a tree or plant. A leaf constitutes a unit of the foliage, and functions primarily in food manufacture by → photosynthesis.

Etymology (EN): M.E. leef, lef; O.E. leaf; cf. O.S. lof, O.N. lauf, O.Fris. laf, Du. loof, O.H.G. loub, Ger. Laub.

Etymology (PE): Barg “leaf” (Tabari, Gilaki valg, balg; Kurd. belg, balk, Semnâni valg); Mid.Pers. warg “leaf;” Av. varəka- “leaf;” cf. Skt. valká- “bark, bast, rind;” Russ. volokno “fibre, fine combed flax.”

In a calendar, having an extra day or month inserted.

Etymology (EN): O.E. hleapan “to jump, run, leap” (cf. O.S. hlopan, O.N. hlaupa, O.Fris. hlapa, Du. lopen, Ger. laufen “to run,” Goth. us-hlaupan “to jump up”), of uncertain origin, with no known cognates beyond Germanic. The noun is O.E. hlyp (Anglian *hlep). Noun in leap year, so called from its causing fixed festival days to “leap” ahead one day in the week.

Etymology (PE): Andarheli, from andarhel, verbal noun of andarhelidan, andarheštan “to insert,” from andar-→ inter- + helidan, heštan “to place, put” from Mid.Pers. hištan, hilidan “to let, set, leave, abandon;” Parthian Mid.Pers. hyrz; O.Pers. hard- “to send forth;” ava.hard- “to abandon;”
Av. harəz- “to discharge, send out; to filter,” hərəzaiti “releases, shoots;” cf. Skt. srj- “to let go or fly, throw, cast, emit, put forth;” Pali sajati “to let loose, send forth.”

The extra day added to a solar calendar (e.g. Gregorian, Iranian) in a leap year.

See also: → leap; → day.

An intercalary month employed in some calendars to preserve a seasonal relationship between the Lunar and Solar cycles. → embolismic month.

See also: → leap; → month.

A one-second added between 60s and 0s at announced times to keep the Coordinated Universal Time (UTC), counted by atomic clocks, within 0s.90 of mean solar time (UT1). Generally, leap seconds are added at the end of June or December.

See also: → leap; → second.

In solar calendars the year that contains 366 days, instead of 365, in order to keep the calendar in pace with the real solar time.

See also: → leap; → year.

The three criteria that identify → leap years in the → Gregorian calendar: 1)
The year must be evenly divisible by 4;

2. If the year can be evenly divided by 100, it is not a leap year, unless;

3. The year is also evenly divisible by 400.

This means that in the Gregorian calendar, the years 1600, 2000, and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years.

See also: → leap; → year; → rule.

Smallest in size, amount, degree, etc.

Etymology (EN): From M.E. leest(e), O.E. læst, læsest “smallest” (superlative of læs “smaller, less”).

Etymology (PE): Kucaktarin, kamtarin, superlatives of kucak and kam, → small.

Of two or more → integers, the smallest positive number that is divisible by those integers without a remainder.

See also: → least; → common; → multiplier.

Any statistical procedure that involves minimizing the sum of squared differences.

See also: → least; → square.

A → cross correlation technique for computing average profiles from thousands of → spectral lines simultaneously. The technique, first introduced by Donati et al. (1997, MNRAS 291,658), is based on several assumptions: additive → line profiles, wavelength independent
→ limb darkening, self-similar local profile shape, and weak → magnetic fields. Thus, unpolarized/polarized stellar spectra can indeed be seen as a line pattern → convolved with an average line profile. In this context, extracting this average line profile amounts to a linear → deconvolution problem. The method treats it as a matrix problem and look for the → least squares solution. In practice, LSD is very similar to most other cross-correlation techniques, though slightly more sophisticated in the sense that it cleans the cross-correlation profile from the autocorrelation profile of the line pattern. The technique is used to investigate the physical processes that take place in stellar atmospheres and that affect all spectral line profiles in a similar way. This includes the study of line profile variations (LPV) caused by orbital motion of the star and/or stellar surface inhomogeneities, for example. However, its widest application nowadays is the detection of weak magnetic fields in stars over the entire → H-R diagram based on → Stokes parameter V (→ circular polarization) observations (see also Tkachenko et al., 2013, A&A 560, A37 and references therein).

See also: → least; → square; → deconvolution.

A fit through data points using least squares.

See also: → least squares; → fit.

1a) Go away from.

1b) To let remain or have remaining behind after going, disappearing, ceasing, etc.

2a) Permission to be absent, as from work or military duty.

2b) The time this permission lasts (Dictionary.com).

Etymology (EN): M.E. leven, from O.E. laefan “to allow to remain in the same state or condition” (cf. O.Saxon farlebid “left over;” Ger. bleiben “to remain”) ultimately from PIE *leip- “to stick, adhere;” also “fat,” from which the cognates: Gk. lipos “fat;” O.E. lifer “liver,” → life.

Etymology (PE): Parižidan, on the model of
Sariqoli barēzj “leavings;” Yaghnobi piraxs- “to stay behind, remain;” ultimately from Proto-Ir. *apa-raic-, from *raic- “to abandon, leave;” cf. Av. raēc- “to leave, let” (Cheung 2006), → heritage.

Same as the → period-luminosity relation.

See also: Named after Henrietta Swan Leavitt (1868-1921), American woman astronomer, who discovered the relation between the luminosity and the period of → Cepheid variables (1912); → law.

A → primary cell in which the anode is a rod of carbon and the cathode a zinc rod both immersed in an electrolyte of ammonia plus a depolarizer.

See also: Named after the inventor Georges Leclanché (1839-1882), a French chemist, → cell.

1. The ninth of Jupiter’s known satellites and the smallest. It is 16 km in diameter and has its orbit at 11 million km from its planet. Also called Jupiter XIII, it was discovered by Charles Kowal (1940-), an American astronomer, in 1974.
   
2. An asteroid, 38 Leda, discovered by J. Chacornac in 1856.

See also: In Gk. mythology, Leda was queen of Sparta and the mother, by Zeus in the form of a swan, of Pollux and Helen of Troy.

An improvement of → Schwarzschild’s criterion for convective instability, which includes effects of chemical composition of the gas. In the Ledoux criterion the gradient due to different molecular weights is added to the adiabatic temperature gradient.

See also: After the Belgian astrophysicist Paul Ledoux (1914-1988), who studied problems of stellar stability and variable stars. He was awarded the Eddington Medal of the Royal Astronomical Society in 1972 (Ledoux et al. 1961 ApJ 133, 184); → criterion.

Of, pertaining to, or located on or toward the west when somebody or something is facing north. Opposite of → right.

Etymology (EN): M.E. left, lift, luft, O.E. left, lyft- “weak, idle,” cf. Ger. link, Du. linker “left,” from O.H.G. slinc, M.Du. slink “left,” Swed. linka “limp,” slinka “dangle.”

Etymology (PE): Cap “left,” from unknown origin.

See → Fleming’s rules.

See also: → left; → hand; → rule.

Using the left hand with greater ease than the right.

Etymology (EN): → left; → hand + -ed.

Etymology (PE): Capâl, from cap, → left, + -al,
→ -al. Capdast, with dast, → hand.

1. The part of the body from the top of the → thigh down to the → foot.
   

   2. Anatomy: The lower limb of a human being between the → knee and the → ankle.

Etymology (EN): M.E., from O.Norse leggr; cognate with Dan. læg, Swed. läg “the calf of the leg.”

Etymology (PE): Leng, related to Mid.Pers. zang “shank, ankle;” Av. zanga-, zənga- “bone of the leg; ankle bone; ankle;” Skt. jánghā- “lower leg;” maybe somehow related to E. → shank.

1. Permitted by law; lawful.
   

2. Of or relating to law; connected with the law or its administration (Dictionary.com).

Etymology (EN): From M.Fr. légal or directly from L. legalis “legal, pertaining to the law,” from lex (genitive legis) “law.”

Etymology (PE): Qânuni, of or relating to qânun, → law.

1. A non-historical or unverifiable story handed down by tradition from earlier times and popularly accepted as historical.
   

2. The body of stories of this kind, especially as they relate to a particular people, group, or clan (Dictionary.com).

Etymology (EN): M.E. legende “written account of a saint’s life,”
from O.Fr. legende and directly from M.L. legenda literally, “(things) to be read,” noun use of feminine of L. legendus, gerund of legere “to read” (on certain days in church).

Etymology (PE): Cirok, from Kurd. cirok “story, fable,” related to Kurd. cir-, cirin “to sing, [to recite?];” Av. kar- “to celebrate, praise;” Proto-Ir. *karH- “to praise, celebrate;” cf. Skt. kar- “to celebrate, praise;” O.Norse herma “report;” O.Prussian kirdit “to hear;” PIE *kerH₂- “to celebrate” (Cheung 2007).

Of, relating to, or of the nature of a legend.

See also: → legend; → -ary.

The → differential equation of the form: d/dx(1 - x²)dy/dx) + n(n + 1)y = 0. The general solution of the Legendre equation is given by y = c₁P_n(x) + c₂Q_n(x), where P_n(x) are Legendre polynomials and Q_n(x) are called Legendre functions of the second kind.

See also: Named after Adrien-Marie Legendre (1752-1833),
a French mathematician who made important contributions to statistics, number theory, abstract algebra, and mathematical analysis; → equation.

A mathematical operation that transforms one function into another. Two differentiable functions f and g are said to be Legendre transforms of each other if their first derivatives are inverse functions of each other: df(x)/dx = (dg(x)/dx)^-1. The functions f and g are said to be related by a Legendre transformation.

See also: → Legendre equation; → transformation.

1. The act of making or enacting laws.
   

2. A law or a body of laws enacted (Dictionary.com).

Etymology (EN): From Fr. législation, from L.L. legislationem, from legis latio, “proposing (literally ‘bearing’) of a law,” → legislator.

Etymology (PE): Qânungoz&acric;ri “act or process followed by the qânungoz&acric;r”, → legislator.

1. A person who gives or makes laws.
   

2. A member of a legislative body (Dictionary.com).

Etymology (EN): From L. legis lator “proposer of a law,” from legis, genitive of lex, → law, + lator “proposer,” agent noun of latus “borne, brought, carried.”

Etymology (PE): Qânungozâr, literally “he who places the law,” from qânun, → law, + gozâr, present stem and agent noun of gozâštan “to place, put; perform; allow, permit,” related to gozaštan “to pass, to cross,” → trans-

A cosmological hypothesis, based on Einstein’s relativity, in which the expanding Universe began from an exploding “primeval atom.” In the Lemaître Universe the rate of expansion steadily decreases.

See also: Named after Monsignor Georges Edouard Lemaître (1894-1966), a Belgian Roman Catholic priest, honorary prelate, professor of physics and astronomer; → universe.

1. A subsidiary proposition, proved for use in the proof of another proposition.
   

2. Linguistics: A word considered as its citation form together with all the → inflected forms. For example, the lemma go
   consists of go together with goes, going, went, and gone.
   

3. An argument or theme, especially when used as the subject or title of a composition (Dictionary.com).

Etymology (EN): From L. lemma, from Gk. lemma “something received or taken; an argument; something taken for granted,” from root of lambanein “to take,” → analemma.

Etymology (PE): Nehak, from neh present stem of nehâdan “to place, put; to set,” → position, + -ak a diminutive suffix of nouns.

A closed curve with two loops resembling a figure 8. It is represented by the Cartesian equation (x² + y²)² = a²(x² - y²), where a is the greatest distance from the origin (pole) to the curve. Its polar equation is r² = a² cos 2θ.

See also: From L. Latin lemniscatus “adorned with ribbons,” from lemniscus “a pendent ribbon,” from Gk. lemniskos “ribbon;” First described by Jacques Bernoulli (1654-1705) in 1694.

A distance determined by the extent of something specified. → Jeans length

Etymology (EN): M.E. length(e), O.E. lengthu “length,” from P.Gmc. *langitho, noun of quality from *langgaz (root of O.E. lang “long,” cognate with Pers. derâz, as below) + -itho, abstract noun suffix. Cognate with O.N. lengd, O.Fris. lengethe, Du. lengte.

Etymology (PE): Derâzâ quality noun of derâz “long,” variants Laki, Kurdi derež;
Mid.Pers. drâz “long;” O.Pers. dargam “long;” Av. darəga-, darəγa- “long,” drājištəm “longest;” cf. Skt. dirghá- “lon (in space and time);” L. longus “long;” Gk. dolikhos “elongated;” O.H.G., Ger. lang; Goth. laggs “long;” PIE base *dlonghos- “long;” tul loan from Ar. ţaul, used in → wavelength.

Same as → Lorentz contraction.

See also: → length; → contraction.

1. Having or being of great length; very long.
   

2. Tediously verbose; very long; too long (Dictionary.com).

Etymology (EN): From → length + -y.

Etymology (PE): Kešnâk “lengthy” (Bardsiri, Kermâni), from kešidan, kašidan “to draw, protract, trail, drag, carry,” → tide. Bardesir, Kermân

A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.

Etymology (EN): From L. lens (gen. lentis) “lentil,” cognate with Gk. lathyros, on analogy of the double-convex shape.

Etymology (PE): Adasi, related to adas “lentil,” from Ar. ‘adas.
The original Pers. terms for lentil are: mažu, mažâné, (Gilaki, Tabari) marju, marji, murje, marjum, (Laki) noži, (Aftari) marju, Mid.Pers. mijûg “lentil;” cf. Skt. masura- “lentil.”

→ optical system.

See also: → lens; → system.

An effect predicted by → general relativity whereby a rotating body alters the → space-time around it. This effect can be thought of as a kind of “dragging of inertial frames,” as first named by Einstein himself. A massive spinning object pulls nearby objects out of position compared to predictions for a non-rotating object.
The effect is important for rapidly rotating → neutron stars and → black holes, but that near Earth is extraordinarily small: 39 milli-arc second per year, about the width of a human hair seen from 400 meters away.

See also: Named after Austrian physicists Joseph Lense (1890-1985) and Hans Thirring (1888-1976), who first discovered this phenomenon in 1918; → effect.

1. The act or effect produced by a lens, in particular a → gravitational lens.
   
2. Pertaining to the object that produces the lensing effect. → lensing galaxy.

Etymology (EN): From → lens + → -ing.

Etymology (PE): 1) Lenzeš, verbal noun of lenzidan, verb formed from E. lens + -idan infinitive suffix.
2) Lenzandé, verbal adj. from lenzidan, as above.

Effect created by a → gravitational lens.

See also: → lensing; → effect.

A galaxy that acts as a → gravitational lens. The effect can also be due to a cluster of galaxies.

See also: → lensing; → galaxy.

An astronomical object that creates → gravitational lensing.
See also → lensing galaxy.

See also: → lensing; → object.

An important quantity in the characterization of → gravitational lensing. The lensing potential is obtained by projecting the three-dimensional Newtonian potential on the lens plane and by properly re-scaling it. It is a two-dimensional analog to the → gravitational potential.

See also: → lensing; → potential.

Circular and elliptical features on the surface of → Europa with diameters ranging from 10 to 100 km. Many are domes that seem to have been pushed up from below. These domes might have been formed by warm water rising between the cold ices of the outer crust, in a scenario recalling the → magma chambers on Earth.

Etymology (EN): From L. lenticula “freckle,” diminutive of lens (genitive lentis) “lentil,” → lens.

Etymology (PE): Kakmak “freckle.”

A lens-shaped galaxy, which is an enormous grouping of old stars with very little internal structure.

See also: Lenticular “resembling a lentil in form,” → lens;
→ galaxy.

The direction of an induced current is such as to oppose the cause producing it. The cause of the current may be the motion of a conductor in a magnetic field, or it may be the change of flux through a stationary circuit.

See also: Named after Heinrich Friedrich Emil Lenz (1804-1865), German physicist, who published the law in 1834; → law.

The Lion. One of the most conspicuous → constellations in the northern hemisphere, at 10h 30m → right ascension, 15° north → declination. Leo is a constellation of the
→ Zodiac crossed by the Sun from August 10th to September 16th. Leo is visible from February through June. Its brightest star, α Leonis or → Regulus, is of the first magnitude.
Abbreviation: Leo; Genitive: Leonis.

The neighboring constellations are → Cancer, → Coma Berenices, → Crater, → Hydra, → Leo Minor, → Lynx, → Sextans, → Ursa Major, and → Virgo.

Leo contains many bright stars, many of which were individually identified by the ancients. There are four stars of first or second magnitude, which render this constellation especially prominent.

Apart from Regulus, the constellation is home to the bright stars → Denebola, the nearby star Wolf 359, and to a number of famous deep sky objects, among them galaxies Messier 65, Messier 66, Messier 95, Messier 96, Messier 105, and NGC 3628.

There are two → meteor showers associated with the constellation. The → Leonids usually peak on November 17-18 every year and have a → radiant near the bright star Gamma Leonis. The January Leonids are a minor shower that peaks between January 1 and 7. Leo has 11 stars with known planets.

See also: → lion.

The Lesser Lion. A faint constellation in the northern hemisphere, at 10h 20m right ascension, 35° north declination.
Abbreviation: LMi; genitive: Leonis Minoris.

Etymology (EN): → Leo; Minor, from L. minor “lesser, smaller, junior,” from PIE base *min- “small.”

Etymology (PE): Šir, → Leo; 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 → dwarf galaxy recently discovered near the Milky Way in
→ neutral hydrogen → 21-centimeter line observations. Optical imaging observations indicate that
it is located between 1.5 and 2.0 Mpc from the Milky Way. This places Leo P in the → Local Volume but beyond the → Local Group. The dwarf galaxy has extreme properties: it is the lowest-mass system known that contains significant amounts of gas and is
currently forming stars (Katherine L. Rhode et al. 2013, AJ 145, 149).

See also: → Leo.

A → meteor shower emanating from an apparent point in the constellation → Leo that occurs from November 14-20, with a maximum on November 17-18. It is due to the annual passage of the Earth through the orbit of the comet → Tempel-Tuttle and encounter with the dust debris from the comet.

See also: From L. Leon, → Leo, + → -ids.

A frequency distribution with a positive → kurtosis, that has a more elevated peak around the mean than the corresponding normal distribution.
→ platykurtic.

Etymology (EN): From Gk. lepto-, combining form of leptos “thin, slight”

• → kurtosis.

Etymology (PE): Boland-afrâšté “highly-elevated,” from boland “high, tall, elevated, sublime” (variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Laki 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. and 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”) + afrâšté “raised, elevated, erect,” p.p. of afrâštan,
→ kurtosis.

An → elementary particle that does not participate in the → strong interaction. The Lepton family
includes → electrons, → muons, tau leptons, → neutrinos and their → antiparticles. The lepton is a
→ fermion.

See also: From Gk, lepto-, combining form of leptos “small, slight” + -on a suffix used in the names of subatomic particles (gluon; meson; neutron), quanta (photon; graviton), and other minimal entities or components.

Postulate that the magnitude of the lepton number density is comparable to or larger than the thermal radiation photon number density, so relaxation to equilibrium produces a degenerate sea of neutrinos. Degenerate neutrinos would suppress the number of neutrons relative to protons in the very early Universe; degenerate antineutrinos would suppress the number of protons relative to neutrons. Either case would affect BBNS (Peebles, P. et al., 2009, Finding the Big Bang, Cambridge: UK, Cambridge Univ. Press).

See also: → lepton; → degeneracy.

The era following the hadronic era, when the Universe consisted mainly of leptons and photons. It began when the temperature dropped below 10¹² degrees kelvin some 10^-4 seconds after the Big Bang, and it lasted until the temperature fell below 10¹⁰ degrees kelvin, at an era of about 1 second.

See also: → lepton; → era.

In particle physics, a quantum number attributed to elementary particles which is conserved in nuclear reactions. It is +1 for a lepton, -1 for an antilepton and 0 for other particles.

See also: → lepton; → number.

The Hare. A small → constellation in the southern hemisphere at about 6h right ascension, 20° south declination. Abbreviation: Lep; genitive: Leporis.

Etymology (EN): L. lepus “hare.”

Etymology (PE): Xarguš “rabbit, hare,” literally “ass-ear,” Mid.Pers. xargôš, from xar “donkey, ass” (Mid.Pers. xar “donkey;” Av. xara- “donkey;” cf. Skt. khára- “donkey, mule”)

• guš, → ear.

A bright blue star of → apparent visual magnitudeV = 2.70, that with → Shaula (Lambda Scorpii) makes up the Scorpion’s stinger. Among its other designations: HR 6508 and HIP 85696. Lesath is 580 → light-years away.

Lesath and Shaula appear very close on the sky (less than a degree apart), but they are not physically related. Lesath is a → subgiant of → spectral type B2 IV with a → luminosity of about 7,380 Msun. It has a radius of about 6 Rsun, and a → surface temperature of about 22,000 K.

See also: Lesath, from Ar. al-Las’ah (اللسعه) “the sting.”

1. A symbol or character representing a sound of which words are formed used in writing and printing a language.
   

2. A written or printed communication addressed to a person or organization.

Etymology (EN): M.E., from O.Fr. letre “character, letter; missive,” from L. littera “letter of the alphabet,” litterae (plural) “epistle, literature”

Etymology (PE): 1) Vât “letter, word,” from vat- “to speak, say;” cf. (Kurd.) wittin “to speak, say,” → article,
(Nâyin) vâte, (Qohrud) vâta, (Sangesar) -vât-/vân, (Zazaki) vâtiš, (Awromani) wâtay/wâc-, (Farizand) -vât/-vaj, (Gaz) vâ-/vât, related to Pers. vâžé “word;” Mid.Pers. vâc, vâcak “word, speech;” âva “voice, sound,” âvâz “voice, sound, song,” bâng “voice, sound, clamour” (Mid.Pers. vâng); Av. vacah- “word,” vaocanghê “to decalre” (by means of speech), from vac- “to speak, say;” cf. Skt. vakti “speaks, says,” vacas- “word;”
Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak.”

2. Nâmé, → book.

A position with respect to a given or specified base. → energy level.

Etymology (EN): From O.Fr. livel, from L. libella “a balance, level,” diminutive of libra “balance, scale.”

Etymology (PE): Tarâz “level; a level,” from 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).

Same as → significance level.

See also: → level; → significance.

A rigid bar that pivots about a point and is used to move or lift a load at one end by applying force to the other end. → lever arm.

Etymology (EN): M.E. levere, levour; O.Fr. levier “a lifter, a lever,” agent noun from lever “to raise,” from L. levare “to raise,” from levis “light” in weight.

Etymology (PE): Ahrom, of unknown etymology.

The distance between the point of application of a → force and the → axis around which the object could
→ rotate.

See also: → lever; → arm.

1. To rise or float in the air, especially by means of supposed magical powers.
   

   2. To cause to rise or float in the air.

Etymology (EN): From L. levitas “lightness,” from levis “light.”

Etymology (PE): Bâlâ kardan, literally “to raise, uplift,” from bâlâ “high, up,” → high, + kardan “to do, perform, make,” → effect; bâlâ šodan, from bâlâ + šodan “to go, to pass, to become,” → set.

1. The action of rising or causing something to rise and float in the air, typically by means of supposed magical powers.
   

   2. → radiative levitation.

See also: → levitate; → -tion.

A combining form meaning “left” and “counterclockwise,” used in the formation of compound words, such as → levorotation, levocardia, levoglucose, etc. The variant lev- occurs before vowels. Compare → dextro-.

Etymology (EN): From L. laevo-, from laevus, cf. Gk. laios, Russ. levyj,

• epenthetic vowel -o-.

Etymology (PE): Cap-, from cap “left,” of unknown etymology.

The counterclockwise rotation of the → plane of polarization of light (as observed when looking straight through the incoming light) by certain substances.

See also: Adj. related to → levorotation.

Relating to an → optically active substance that brings about → levorotation.

See also: Adj. related to → levorotation.

A branch of → linguistics that deals with the principle and methods of writing dictionaries.

See also: → lexicology; → -graphy.

A branch of → linguistics that studies the formation, meaning, and use of words and their idiomatic combinations. In contrast to → terminology, lexicology is based on words and does not conceive of meaning unless it is related to the word.

Etymology (EN): From lexic, from Gk. lexicon, noun use of lexikos “pertaining to words,” from lexis “word; speech” (+ -ikos, → -ics), from legein “to say, speak, recount,”

• -o-, + → -logy.

Etymology (PE): Vâžešnâsi, from vâžé, → word,

• -šenâsi, → -logy.

An early form of → capacitors which is a glass jar coated inside and outside about half way up the side with metal foil. A chain connects the inner coating to a rod which usually terminates in a small brass knob. The jar is charged by connecting the knob, that is the inner metal coating, to a charged body, meanwhile grounding the outer coating. Same as Leiden jar

Etymology (EN): Named after Leyden (Leiden) the city where it was invented by the Dutch scientist Pieter van Musschenbroek in 1745,
independently from the German Ewald Georg von Kleist; jar a usually “cylindrical vessel,” from M.Fr. jarre, from Provençal jarra, from Ar. jarrah.

Etymology (PE): Botri, → bottle.

→ 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.