An Etymological Dictionary of Astronomy and Astrophysics

A → stellar association containing many → T Tauri stars. Examples include the Taurus-Auriga T association, the nearby → TW Hydrae association, and Vela T1 and T2.

See also: T, letter of alphabet; → association.

A type of → brown dwarf with an → effective temperature between about 1200 K and 500 K, i.e. colder than the preceding type → L dwarf. The spectra of T dwarfs are characterized by the presence of → methane (CH₄) bands in the → near infrared.
The presence of these bands, broad H₂O features, and H₂ collision-induced absorption radically alter the spectral energy distributions of T dwarfs compared to a black body at the same temperature. Hence near-infrared colors become increasingly blue (J - K ~ 0) as compared to L dwarfs. The first T dwarf, called
→ Gl 229B, was discovered by Nakajima et al. (1995, Nature 378, 463). The spectral classification scheme
(subtypes T0 to T9) currently
used was defined by Burgasser et al. (2002, ApJ 564, 421).

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

The most famous young stellar object, the prototype of → T Tauri stars which is located in the Taurus-Auriga star forming region at a distance of about 140 pc. T Tauri is about 2.4 solar masses and about 7 million years old. Complex structures of shock heated gas indicative of outflows surround the star, both on sub-arcsecond and larger scales. The → accretion rate is estimated to be (3-6) x 10^-8 solar masses per year. T Tauri has a companion (about 0.7 arcseconds apart) which is invisible in the optical and has been observed to be strongly variable in infrared. It is an intermediate mass
young star (2.5 solar masses) surrounded by a → circumstellar disk. This star is itself a → close binary, the component being a low mass star (0.6 solar masses) also surrounded by circumstellar material.

See also: T, letter of alphabet; Tauri genitive of → Taurus.

A member of a class of young stellar objects of roughly 1 solar mass showing strong → infrared excess emission attributed to → circumstellar disks and found within or close to molecular clouds. T Tauri stars are → protostars in the final stages of formation to become a stable → main sequence star. The nuclear reactions in their core have not yet stabilised and the stars are known for the variability of their brightness. See also → classical T Tauri star, → weak-line T Tauri star.

See also: → T Tauri; → star.

The symmetry of physical laws under a time reversal transformation.

See also: T for → time; → symmetry.

1. An arrangement of words, numbers, or signs, or combinations of them, in parallel columns, to exhibit a set of facts or relations in a definite, compact, and comprehensive form.
   
2. In particular, → zij

Etymology (EN): M.E.; O.E. tabule; O.Fr. table “board, plank, writing table”
(cf. O.H.G. zabel, Ger. Tafel), both from L. tabula “a board, plank, table,” originally “small flat slab or piece” usually for inscriptions or for games.

Etymology (PE): Jadval, loan from Ar. jadwal.

A flat slab or surface, especially one bearing or intended to bear an inscription, carving, or the like (Dictionary.com).

Etymology (EN): M.E. tablette, from M.Fr. tablete, diminutive from → table.

Etymology (PE): Parnik, literally “laminar, resembling a leaf, leaf-like,” variant parnix [Dehxodâ] “a stone or marble slab, a plane stone,” ultimately from Proto-Ir. *parnika-, from *parn-, *par- “feather; leaf, thin layer;” cf. Av. parəna- “feather,” Skt. parna- “feather; leaf (regarded as the plumage of a tree),” Mod.Pers. par(r) “feather; leaf;” PIE *pornos-, *pernom- “feather” (E. fern and Ger. Farn belong to this family). See also → slate.

A thin → transition  → layer inside → Sun, between the → differentially rotating 
→ convection zone and the uniformly rotating → radiative zone.
Discovered through → helioseismology, it has raised considerable interest, but its thinness remains still to be explained. Due to its strong → shear, it is believed to play a crucial role in the generation of the → solar magnetic field.

Etymology (EN): Tachocline, from tacho- a combining form meaning “speed,” → tachyon + → -cline “slope.” The term was first coined by Edward A. Spiegel and Jean-Paul Zahn (1992, A&A 265, 106), by analogy to the oceanic → thermocline.

Etymology (PE): Tondâšib, from tondâ, → velocity, + šib, → -cline.

A hypothetical subatomic particle that travels faster than the speed of light.

See also: From tachy- a combining form meaning “swift,” from Gk. tachys “swift” + → -on.

The aquatic larva of → frogs and → toads, having internal gills and a tail (TheFreeDictionary).

Etymology (EN): From M.E. taddepol, from tadde “toad” + polle “head,” from M.L.G. or M.Du. pol “head, top.”

Etymology (PE): Vagiz, from Tabari vag “→ frog”

• -iz, -ize diminutive suffix (variants -ce, -že, -ak, Mid.Pers -cak).

A shorter → horseshoe orbit confined to the → Lagrangian points L4 or L5.

See also: → tadpole; → orbit.

1. The part that sticks out at the back of an animal’s body, and that it can move.
   

2. Anything resembling such an appendage in form or position.
   

3. → cometary tail.

Etymology (EN): M.E.; O.E. tægl, from P.Gmc. *tagla- (cf. O.H.G. zagal, Ger. Zagel “tail,” O.N. tagl “horse’s tail”).

Etymology (PE): Dom, variant domb, “tail;” Mid.Pers. dumb “tail;” Av. duma- “tail”; donbâlé, from domb + -âlé, -âl resemblance suffix, → -al.

To get into one’s hold or possession by voluntary action; to hold, grasp, or grip (Dictionary.com).

Etymology (EN): M.E. taken “to take, strike, grasp,” O.E. tacan “to grasp, touch,” probably from Old Norse taka “to take, grasp;” cf. M.Du. taken “to grasp;,” Gothic tekan “to touch.”

Etymology (PE): Gereftan “to take, seize, hold;” Mid.Pers. griftan, gir- “to take, hold, restrain;” O.Pers./Av. grab- “to take, seize;”
cf. Skt. grah-, grabh- “to seize, take,” graha- “seizing, holding, perceiving;” M.L.G. grabben “to grab;” E. grab “to take or grasp suddenly;” PIE *ghrebh- “to seize;” see also → assumption, → concept.

A white, grey, or pale green soft → mineral with a greasy feel consisting of → hydrated  → magnesium  → silicate, Mg₃Si₄O₁₀(OH)₂.

See also: From M.Fr. talc, from M.L. talcus, talcum, from Ar. talq, from Pers. talk “talc.”

A narrative that relates the details of some real or imaginary event, incident, or case; story (Dictionary.com).

Etymology (EN): From M.E., from O.E. talu “series, list, narrative, story;” cognate with Du. taal “speech, language,” Ger. Zahl “number,” O.Norse tala “number, speech,”

Etymology (PE): Matal “tale, history,” of unknown origin.

The largest volcanic eruption in recorded history, which mainly occurred on April 10, 1815 in the Indonesian Sumbawa Island. An estimated 150 cubic kilometers of igneous material was ejected,
whereby the mountain lost ~ 1400 m in height during the blast (current height ~ 2900 m).
The eruption created a 6 km-wide, 1250 m-deep → caldera.
The ash put into the atmosphere produced high-latitude clouds which intercepted incoming sunlight. The resulting drop in → insolation caused a dramatic change in climate and weather patterns in the Northern Hemisphere during the following year. The year 1816 is known as the “year without a summer” because there was no warm season over much of the Northern Hemisphere.

See also: Tambora, proper noun; → volcano

1. A straight line or plane that touches a curve or curved surface at one and only one point. Conversely, a curve or curved surface that touches a straight line, curve, or curved surface at one and only one point.
   

2. In trigonometry, the function of an acute angle of a right triangle defined as the ratio of the length of the opposite side to that of the adjacent side.

Etymology (EN): From L. tangentem (nominative tangens), pr.p. of tangere “to touch,” from PIE base *tag- “to touch, to handle” (cf. L. tactus “touch,” Gk. tetagon “having seized,” O.E. þaccian “stroke, strike gently”); tânžânt, loan from Fr.

Etymology (PE): 1) Sâyân, pr.p. of sâyidan “to touch, to rub,” variants sâbidan, pasâvidan;
Khotanese sauy- “to rub;” Sogdian ps’w- “to touch;” ultimately
Proto-Iranian *sau- “to rub.”

2. Tânžânt, loan from Fr.

Pertaining to or of the nature of a tangent.

See also: → tangent + -ial variant of → -al.

That component of a an object’s motion which is perpendicular to the observer’s → line of sight.

See also: → tangential; → motion.

1. The instantaneous linear velocity of a body moving in a circular path. It is equal to the → angular velocity multiplied by the radius: v_t = ωr.
   

2. The component of the velocity of an object, such as a star, that is at right-angles to the observer’s → line of sight; also known as transverse velocity. See also → radial velocity.

See also: → tangential; → velocity.

The → linearly polarized light that vibrates perpendicularly to an imaginary line joining the source to the point of observation.

See also: Tangentially, adverb of → tangential; → polarized; → light.

A long, narrow strip of paper, plastic, metal, etc., as in → magnetic tape

Etymology (EN): M.E.; unexplained variant of tappe; O.E. tæppe “strip (of cloth),” akin to M.L.G. teppen “to tear, pluck.”

Etymology (PE): Navâr “a narrow, long piece, strip, rope,” cf. Ossetic nawar “tendon, sinew;” Av. snāvarə- “tendon, sinew;” cf. Skt. snāvan- “tendon, sinew;” Pali nahāru-, nhāru- “tendon, muscle;” Hindi nahāru “piece of leather;” Arm. neard “tendon;” Gk. neura “string, sinew;” L. neros “sinew, muscle, nerve;” Ir. sin “chain;” P.Gmc. *senawo (O.S. sinewa, O.N. sina, O.Fris. sine, M.Du. senuwe, O.H.G. senawa, Ger. Sehne, E. sinew)

The largest and brightest → H II region in the → Large Magellanic Cloud. This → giant H II region has a diameter of over 800 → light-years, and contains half a million → solar masses of ionized gas. The ionization is produced by several clusters of → O-type and → B-type stars, including the very powerful and compact cluster → R136 near its centre. The nebula’s name comes from its spider-like shape. Also known as → 30 Doradus and NGC 2070.

Etymology (EN): Tarantula “any of several large, hairy spiders of the family Theraphosidae,” from M.L. tarantula, from It. tarantola, from Taranto “seaport city in southern Italy in the region where the spiders are frequently found,” from L. Tarentum, from Gk. Taras; → nebula.

Etymology (PE): Miq, → nebula; roteyl “large, hairy spider, tarantula.”

The star Gamma, magnitude 2.72, in the constellation → Aquila.
It is a K3 → giant 460 light-years away. Other designations: HR 7525, HD 186791.

See also: Tarazed, from Pers. tarâzu “balance, scales,” from šâhin-e tarâzu “the beam of the balance,” the name given to the three aligned stars of Aquila, i.e. α, β, and γ (Abdolrahmân Sufi, Book of Fixed Stars, A.D. 964, Pers. translation by Nasireddin Tusi in 13th century).
The first word, šâhin, apart from “beam,” means “royal” and “falcon.” Tarâzu, from Mid.Pers. tarâzên-, tarâzênidan “to weigh;” Proto-Iranian *tarāz-, from *tarā- “balance, scales” (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).

An object to be observed with a telescope.

Etymology (EN): M.E., from M.Fr. targuete, from O.Fr. targe “light shield,” from Frank. *targa “shield” (cf. O.H.G. zarga “edging, border,” Ger. Zarge “edge, border”).

Etymology (PE): Âmâj “aim, goal,” from Proto-Iranian base *āma-, from prefix *ā- + *ma- “to measure;” cf. Av. mati- “point, tip;” O.Pers./Av. mā(y)- “to measure;” Pers. mun/mân “measure,” as in Pers. terms pirâmun “perimeter,”
âzmun “test, trial,” peymân “measuring, agreement,” peymâné “a measure; a cup, bowl;” cf. Skt. mati “measures,” matra- “measure,” Gk. metron “measure,” L. metrum; PIE base *me- “to measure.”

A reddish-brown deposit consisting mainly of potassium hydrogen tartrate, which forms during the fermentation of wine. Same as → argol.

Etymology (EN): From O.Fr. tartre, from L. tartarum, from late Gk. tartaron “tartar encrusting the sides of wine casks,” perhaps relating to Pers. dord (?).

Etymology (PE): Dord “lees, dregs, sediment, tartar of wine.”

An organic acid with general chemical formula C₄H₆O₆ that exists in four isomeric forms . The common form, d-tartaric acid, obtained from → tartar, is a white, soluble, crystalline solid. It occurs naturally in many plants, particularly in grapes, bananas, and tamarinds. It is also one of the main acids found in wine.

See also: Tartaric, from → tartar; → acid.

1. A definite piece of work required to be done as a duty or routine job.
   

2. A matter of considerable labor or difficulty.
   

3. In computer programming, a basic unit of programming that an → operating system controls.

Etymology (EN): M.E., from M.L. tasca, metathetic variant of taxa “tax,” from taxare “evaluate, estimate, handle,” also “censure, charge.”

Etymology (PE): Taš, created from Proto-Ir. root *taš- “to make, construct; to cut;” cf. Av. tāš- “to make, construct; to cut;” O.Pers. (ham)taxš- “to work with, effect;” Mid.Pers. tâš- “to cut, cleave; create;” Mod.Pers. taš, tišé “hatchet, axe, adze,”
tarâšidan “to shave;” Ossetic I. dasyn/dast “to shave;” Munji tiž-, Yidgha tiž- “to shear;” Pash. toq “to shave;” cf. Skt. taks- “to form (by cutting), to build, prepare;” Gk. tekton “carpenter.”

A → software → utility that enables a → user to view each of the tasks currently running on the → computer, each of the → processes, and the overall performance of the computer.

See also: → task; → manager.

An annual → meteor shower occurring in the constellation → Taurus. There are actually two distinct Taurid meteors: the South and North Taurids. The Southern peaks around 10 October and the Northern about 12 November. The Taurid meteor shower is created by debris left behind by → Encke’s comet.

See also: → Taurus; → -ids.

Th Bull. A large constellation of the → Zodiac, in the northern hemisphere at about 4h 20m right ascension, 16° north declination. Alpha Tauri or → Aldebaran is among the twenty brightest stars in the sky. Taurus contains several star clusters, including the → Pleiades and → Hyades. The famous → Crab nebula is situated to the west of Zeta Tauri. Abbreviation: Tau; Genitive: Tauri.

Etymology (EN): From L. taurus “bull,” from PIE *tauro- “bull” (cf. Gk. tauros;
O.C.S. turu “bull, steer;” Lith. tauras “aurochs;” O.Pruss. tauris “bison”), often said to be from PIE *steu-ro- “be big, be strong, be sturdy” (cf. Pers. sotur, Mid.Pers. stôr “horse, mount; large cattle;” Av. staora- “bovine animals;” O.Icelandic stjôrr; Goth. stiur “young bull;” O.E. steor; E. steer).

Etymology (PE): Gâv “bull, ox, cow;” Mid.Pers. gâw “ox, bull, cow; Taurus;” Av. gao- “cow, ox, bull;” cf. Skt. gaus; Gk. bous “ox;” L. bov-; Armenian kov; O.E. cu; E. cow; PIE base *gwou- “ox, bull, cow.”

1. Needless repetition of an idea, especially in words other than those of the immediate context, without imparting additional force or clearness, as in “lifeless dead.”
   

2. A → proposition that is → always  → true. On a → truth table a tautology is a → sentence that is true for every possible → truth value of its constituent parts.

Etymology (EN): L.L. tautologia “representation of the same thing in other words,” from Gk. tautologia, from tautologos “repeating what has been said,” from tauto “the same” (contraction of to auto “the same,” from to “the” + → auto + -logos “saying,” related to legein “to say,” → -logy.

Etymology (PE): Hamân “same” (Mid.Pers. ham “same; also; together,” → com-) + ân “that.”

A phenomenon in which the relative motion of a homogeneous rotating liquid tends to be the same in all planes perpendicular to the axis of rotation. When a rotating fluid comes into contact with a submerged object, the fluid flows around it as if it were a cylinder extending the entire depth of the fluid parallel to the axis of the system.

See also: → Taylor number; → column.

A → dimensionless number indicating the relative importance of the → centrifugal and → viscous forces in the → Taylor-Couette flow. It is also called rotational Reynolds number. Its value depends on the length scale of the convective system, the rotation rate, and → kinematic viscosity. The Taylor number Ta is expressed by

Ω²Rd³/ν²

where Ω is the → angular velocity of the inner cylinder, R = (R₁ + R₂)/2 is the mean radius of the two cylinders, d = R₂ - R₁ is the distance between the cylinders, and ν is → kinematic viscosity. If Ta is equal or greater than one, the rotational effects are significant.

See also: Named after Geoffrey Ingram Taylor (1886-1975), a British physicist, mathematician, and expert on fluid dynamics and wave theory; → number.

A series expansion of an infinitely differentiable function about a point a: Σ (1/n!) (x - a) ⁿ f ⁿ (a), where fⁿ(a) is the n-th derivative of f at a, and the sum over n = 0 to ∞. If a = 0 the series is called a → Maclaurin series.

See also: Named for the English mathematician Brook Taylor (1685-1731); → series.

The → Couette flow between two concentric cylinders with fluid filling the annular region. The flow is generated by the relative rotation of the two cylinders. Under some physical conditions the flow may undergo the → Taylor-Couette instability.

See also: → Taylor number; → flow.

A hydrodynamic instability in the → Taylor-Couette flow that arises when the rotation velocity of the fluid exceeds a critical value. The instability arises for → Taylor numbers larger than about 1700.
At the beginning the fluid elements will move in
simple rolls, but turbulence in the form of complex spirals will appear with increasing rotation velocity.

See also: → Taylor-Couette flow; → instability.

Fluid mechanics: A second order differential equation that governs the vertical structure of a perturbation in a stratified parallel flow.

See also: Named after G. I. Taylor (Effect of variation in density on the stability of superposed streams of fluid, 1931, Proc. R. Soc. Lond. A, 132, 499), → Taylor number, and S. Goldstein (On the stability of superposed streams of fluids of different densities, 1931, Proc. R. Soc. Lond. A, 132, 524); → equation.

In a rapidly rotating fluid, the fluid velocity is constant along any line parallel to the axis of rotation.

See also: → Taylor number; Joseph Proudman (1888-1975), British mathematician and oceanographer.

To impart knowledge or skill to; give instruction to.

Etymology (EN): M.E. techen, O.E. tæcan; cf. O.H.G. zihan, Ger. zeihen “to accuse,” Goth. ga-teihan “to announce;” cognate with Pers. dis, → form.

Etymology (PE): Âmuxtan, âmuz- “to teach; learn;” Mid.Pers. hammoxtan, hammoz- “to teach; learn;” cf. Sogd. muck “teacher;” Choresmian mxs “to learn,” mwcy “to teach, instruct;” Proto-Iranian *mauc- “to learn; teach.”

A radioactive chemical element which does not exist naturally on Earth; symbol Tc. Atomic number 43; mass number of most stable isotope 98; melting point 2,200°C; boiling point 4,877°C. Technetium is synthesised via the → s-process in deep layers of
→ asymptotic giant branch stars.

See also: From the Gk. technetos “artificial,” initially called masurium.

The body of specialized procedures and methods used in any specific field, especially in an area of applied science.

Etymology (EN): From Fr. technique “formal practical details in artistic expression,” noun use of adj. technique “of art, technical,” from Gk. tekhnikos,
from tekhne “art, skill, craft, method, system;” cognate with Pers. tarâš- “to cut, hew; scape; shave,” tišé “axe,” as below.

Etymology (PE): Tašnik, related to Pers. tarâšidan “to cut, hew; scape; shave;” Mid.Pers. tâšitan “to cut, cleave; create by putting together different elements;” from Av. taš- “to cut off, fashion, shape, create,” taša- “axe” (Mod.Pers. taš, tišé “axe;” tarâšidan “to shave”), tašan- “creator;” cf. Skt. taks- “to fom by cutting, tool, hammer, form,” taksan- “wood-cutter, carpenter;” Gk. tekton “carpenter,”
tekhne “art, skill, craft, method, system;” L. textere “to weave;” PIE *teks- “to fashion.”
Fann or fan, from Pers. fan “way, manner, mode, art, science,” related to Mod/Mid.Pers. pand “path, advice, counsel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;” O.Pers. paθi- “path, way;” Av. paθ- “path, way,” variants paθi-, paθā-, pantay-;
cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path;” P.Gmc. *finthanan “to find;” E. find; PIE base *pent- “to go, to tread.”

The use of scientific knowledge for the creation and development of devices, machines, and techniques to achieve a commercial, industrial, or scientific objective.

See also: From Gk. tekhnologia “systematic treatment of an art, craft, or technique,” originally referring to grammar, from tekhno-, from tekhne,
→ technique, + → -logy.

The science or art of assembling, shaping, or ornamenting materials in construction; the constructive arts in general. → plate tectonics

Etymology (EN): L.L. tectonicus, from Gk. tektonikos “pertaining to building,” from tekton (genitive tektonos) “builder, carpenter,” → technique.

Etymology (PE): Sâzânik, from sâzân pr.p. of sâz-, sâxtan “to build, make, fashion; to adapt, adjust, be fit” (Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”) + -ik, → -ics.

The first genuine → brown dwarf, discovered in 1995. It is located in the → Pleiades open cluster at approximately 400 → light-years. Teide 1 is a faint object of apparent magnitude I = 19.03, with a late → M dwarf spectral type (M8), corresponding to 55±15 → Jupiter masses (Rebolo et al. 1995, Nature 377, 129).

See also: Named for Observatorio del Teide, Teide Observatory (Tenerife, Canary Islands, Spain), where this object was first detected.

Small glassy bodies whose chemical composition is unrelated to the geological formation in which they are found. They are found mostly in Australia, Java, Philippines and Indochina. Tektites are now thought to have been produced by the impact of meteorites on the earth’s surface.

See also: From Gk. tekt(os) “molten” + -ite a suffix used in the name of minerals and fossils.

A combining form meaning “distant.”

Etymology (EN): From Gk. tele-, combining form of tele “far off, afar, at or to a distance,” related to teleos “end, goal, result, perfection.”

Etymology (PE): Dur- “far,” from Mid.Pers. dūr “far, distant, remote;” O.Pers. dūra- “far (in time or space),” dūraiy “afar, far away, far and wide;” Av. dūra-, dūirē “far,” from dav- “to move away;” cf. Skt. dūrá- “far; distance (in space and time);” PIE base *deu- “to move forward, pass;” cf. Gk. den “for a long time,” deros “lasting long.”

The science and technology of measurement and transmission of data by optical, acoustical, or radioelectric means from remote sources, as from space vehicles, to receiving stations for recording and analysis.

See also: → tele- + → -metry.

An instrument used to collect and amplify light or other energy. → Refracting telescopes gather light by means of a lens, → reflecting telescopes by means of a mirror. → Radiotelescopes gather radio energy
by using an antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy. → grazing incidence telescope.

Etymology (EN): From It. telescopio (used by Galileo, 1611), and Mod.L. telescopium (used by Kepler, 1613), both from Gk. teleskopos “far-seeing,” from → tele- “far” + -skopos “seeing,” from skopein “to watch, look, behold;” → -scope.

Etymology (PE): Durbin, from dur-, → tele-,

• -bin “to see; seer,” → -scope.

A covering, usually hemispherical, that is rotatable about a central axis. There is a slit opening along one side wide enough to allow a telescope to be directed at any vertical angle up to 90°.

See also: → telescope; → dome.

The accuracy with which a telescope can be pointed to a particular coordinate in the sky.

See also: → telescope; → pointing; accuracy.

The Telescope. An inconspicuous constellation situated in the southern hemisphere, at 19h right ascension, 50° south declination. Abbreviation: Tel; genitive: Telescopii.

See also: Telescopium was named by Abbé Nicolas Louis de Lacaille (1713-1762); → telescope.

The tenth of Saturn’s known satellites. It is irregularly-shaped and has a diameter of 29 x 22 x 20 km. Telesto orbits Saturn at a distance of 294,660 km. Telesto is co-orbital with Tethys, residing in Tethys’ leading Lagrangian point (L4). The images taken by the Cassini probe during its distant flyby on October 11, 2005 show that its surface is surprisingly smooth, devoid of small impact craters. Telesto was discovered by B. Smith, H. Reitsema, S. Larson, J. Fountain in 1980 from ground-based observations.

See also: In Gk. mythology Telesto was a daughter of Oceanus and Tethys.

1. Pertaining to the Earth, as a planet, or the earth or soil.
   
2. Derived from or containing → tellurium.

Etymology (EN): From L. tellur-, from tellus “earth” + → -ic.

Etymology (PE): Zamini, of or pertaining to zamin, → earth.

A band seen in the spectra of celestial objects, which is due to absorption by gases such as oxygen, water vapor, and carbon dioxide in the Earth’s atmosphere

Etymology (EN): → telluric; → band.

A brittle metallic element usually found in combination with → gold and other → metals, used to → alloy stainless → steel and → lead, and, as bismuth telluride, in thermoelectric devices; symbol Te. → Atomic number 52;
→ atomic weight 127.60; → melting point 450°C; → boiling point 990°C; → specific gravity 6.24 at 20°C. It was discovered by the Roumanian mine director Franz Joseph Muller von Reichenstein in 1782 and overlooked for sixteen years until it was first isolated by German
chemist Martin-Heinrich Klaproth in 1798. The Hungarian chemist Paul Kitaibel independently discovered tellurium in 1789, prior to Klaproth’s work but after von Reichenstein.

See also: From L. tellur-, from tellus “earth” + -ium a L. suffix occurring in the name of some chemical elements.

A → periodic comet that is the progenitor of the
→ Leonids meteor shower. It has a period of 33 years, a → perihelion of 0.982 → astronomical units, an → eccentricity of 0.904, and an → inclination of 162.7°. It was first discovered in 1865 though its past appearances have been traced back to 1366.
Tempel-Tuttle is estimated to have a nucleus of radius 1.8 km and a mass of
1.2 × 10¹³ kg. Also designated 55P/Tempel-Tuttle.

See also: Named after the German astronomer Ernst Wilhelm Tempel (1821-1889) and the American astronomer Horace Parnell Tuttle (1837-1923), who independently discovered the comet on December 19, 1865 and January 6, 1866 respectively.

A physical quantity characterizing the mean random motion of molecules in a physical body. In other words, a measure of the average kinetic energy of the particles of a system.

Etymology (EN): From L. temperatura “a tempering, moderation,” from temperatus, p.p. of temperare “to moderate, to mix.” Sense of “degree of heat or cold” first recorded 1670 (Boyle), from L. temperatura, used in this sense by Galileo.

Etymology (PE): Damâ, from dam “breath of an owen; bellows; smoke; air,” also “moment, time,” from Mid./Mod.Pers. damidan “to blow, breathe;” Av. dāδmainya- “blowing up;” cf.
Skt. dahm- “to blow,” dhámati “blows;” Gk. themeros “austere, dark-looking;” Lith. dumti “to blow;” PIE dhem-/dhemə- “to smoke, to blow.”

Cosmology: Minute temperature variations of the cosmic microwave background radiation.

See also: → temperature; → anisotropy.

A physical quantity that describes the rate of change of temperature with displacement in a given direction from a given reference point. Same as → thermal gradient.

See also: → temperature; → gradient.

Meteo.: A reversal in the normal temperature decrease, the temperature rising with increased elevation in the atmosphere instead of falling. A layer in which temperature increases with altitude.

See also: → temperature; → inversion.

An elementary unit of time defined as the duration which is necessary for light to travel a distance equal to the classical radius of an electron. Thus, one tempon (τ) is equal to (e²/mc²)(1/c)≅ 10^-23 seconds.

See also: From tamp, from L. tempus “time” + → -on.

1. Of or pertaining to time.
   

2. Pertaining to or concerned with the present life or this world; worldly.
   

3. Enduring for a time only; → temporary; transitory; → transient (opposed to eternal).
   

4. Secular, lay, or civil, as opposed to ecclesiastical.
   

5. Grammar: Of, pertaining to, or expressing time.

Etymology (EN): M.E., from O.Fr., from L. temporalis “of a time, but for a time, temporary,” from tempus (genitive temporis) “time, season, proper time or season,” of unknown origin.

Etymology (PE): Tâmeni, from tâmen, → time.

A measure of the correlation between the phases of an → electromagnetic wave at different points along the direction of propagation. Temporal coherence indicates to what extent a source is monochromatic. Imagine a source emitting waves with wavelength λ ± Δλ. Waves with wavelength λ and λ + Δλ, which at some point in space constructively interfere, will no longer constructively interfere after some path length l_c = λ²/(2πΔλ); l_c is called the → coherence length.

See also: → temporal; → coherence.

A unit of time used in the Roman and Ottoman empires that divided the day from sunrise to sunset into 12 equal numbers of hours, resulting in long summer hours and short winter hours.

Etymology (EN): M.E., from O.Fr. temporal, from L. temporalis “of time, temporary,” from tempus (genitive temporis) “time, season, proper time or season,” → time; → hour.

Etymology (PE): Sâ’at→ hour; fasli, of or pertaining to fasl→ season.

The measure of the ability of an observing system to clearly separate events in time. In other words, the shortest time interval that can be determined between two different events.

See also: → temporal hour; → resolution.

Temporal character or nature.

See also: → temporal; → -ity.

Lasting, existing, serving, or effective for a time only; not permanent.

Etymology (EN): From L. temporarius “according to circumstances, of seasonal character, lasting a short time,” from tempus (genitive temporis) “time, season.”

Etymology (PE): Tâmenvâr, from tâmen “time”, → temporal, + -vâr suffix denoting suiting, befitting, resembling, in the manner of.

To be indecisive or evasive to gain time or delay acting (Dictionary.com).

Etymology (EN): From M.Fr. temporiser “to pass one’s time, wait one’s time,” from M.L. temporizare “to pass time,” from L. tempus (genitive temporis), → temporal.

Etymology (PE): Tâmenidan from tâmen “time,” → temporal, + -idan, → -ize.

A cardinal number, nine plus one.

Etymology (EN): M.E. ten(e), tenn(e), O.E. ten(e), tien(e); from P.Gmc. *tekhan (cf. O.S. tehan, O.N. tiu, Dan. ti, Du. tien, O.H.G. zehan, Ger. zehn “ten”), cognate with Pers. dah, as below.

Etymology (PE): Dah, from Mid.Pers. dah “ten;” Av. dasa “ten;” cf. Skt. dáśa- “ten;” Gk. deka “ten;” L. decem “ten;” O.Ir. deich; Lith. dešimtis “ten;” PIE base *dekm.

Stretched tight, as a cord, fiber, etc.; drawn taut; rigid.

Etymology (EN): From L. tensus, p.p. of tendere “to stretch,” → tension.

Etymology (PE): Tanu “stretched, strained,” from tan + -u suffix of excess. The first element tan, from tanidan
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
PIE base *ten- “to stretch”), Pers. târ “string,” tân “thread,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string”).

Of or pertaining to → tension.

Etymology (EN): From M.L. tensilis “capable of being stretched,” from L. tensus, p.p. of tendere “to stretch,” → tension.

Etymology (PE): Taneši, related to taneš, → tension.

The force tending to stretch (or produce tension in) an object

See also: → tensile; → force.

General: The act of stretching or straining; the state of being stretched or strained.
Mechanics: The longitudinal deformation of an elastic body that results in its elongation.
Electricity: Voltage or potential; electromotive force.

Etymology (EN): M.E., from M.Fr. tension, from L. tensionem (nominative tensio) “a stretching,” from tensus, p.p. of tendere “to stretch,” cognate with Pers. taneš, as below.

Etymology (PE): Taneš, verbal noun from tanidan
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
PIE base *ten- “to stretch”), Pers. târ “string,” tân “thread,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string.”

A system of numbers or functions where components obey a certain law of transformation when the variables undergo a linear transformation. A tensor may consist of a single number, in which case it is referred to as a tensor of order zero, or simply a → scalar. The tensor of order one represents a → vector. Similarly there will be tensors of order two, three, and so on.

See also:
→ absolute tensor, → calculus of tensors, → contravariant tensor, → covariant tensor, → Einstein tensor, → energy-momentum tensor, → metric tensor, → order of a tensor, → relative tensor, → Ricci tensor, → Riemann curvature tensor, → scalar-tensor theory, → skew-symmetric tensor, → symmetric tensor, → tensor analysis, → tensor contraction, → tensor density, → tensor field, → tensor perturbation, → tensor rank, → tensor-vector-scalar (TeVeS) theory, → weight of a tensor.

See also: Agent noun of tense (v.) → tension.

A method of calculation in higher mathematics based on the properties of tensors.

See also: → tensor; → analysis.

An operation of tensor algebra that is obtained by setting unlike indices equal and summing according to a summation convention.

See also: → contraction; → tensor.

A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.

See also: → tensor; → density.

A field of space and time each point of which has multiple directionality, and is describable by a tensor function.

See also: → tensor; → field.

The perturbation in the → primordial Universe plasma caused by → gravitational waves. These waves stretch and squeeze space in orthogonal directions and bring about → quadrupole anisotropy in incoming radiation temperature.

See also: → tensor; → perturbation.

The total number of → contravariant and → covariant indices (→ index) of a → tensor.

See also: → tensor; → rank.

A theory put forward to provide a basis for a relativistic generalization of the → MOdified Newtonian Dynamics (MOND) paradigm. TeVeS is based on three dynamical fields: a tensor field, a vector field, and a scalar field. In contrast to general relativity, it has two metrics, an Einstein metric and a physical metric. TeVeS has attracted considerable attention, since it can explain many galactic and cosmological observations without the need for → dark matter. Proposed by J. D. Bekenstein, 2004, “Relativistic gravitation theory for the modified Newtonian dynamics paradigm”, Phys. Rev. D, 70, 083509, arXiv:astro-ph/0403694.

See also: → tensor; → vector; → scalar; → theory.

A general term for materials of all types and sizes that are ejected by volcanic eruptions. It includes particles as tiny as volcanic ash and as large as bombs and blocks.

See also: From Gk. tephra “ashes.”

Prefix denoting one million million (10¹²).

See also: From Gk. teras “monster.”

A metallic chemical element; symbol Tb. Atomic number 65; atomic weight 158.9254; melting point 1,356°C; boiling point 3,123°C; specific gravity about 8.25. Terbium was discovered by the Swedish surgeon and chemist Carl-Gustav Mosander in 1843 in an yttrium salt, which he resolved into three elements. He called one yttrium, a rose colored salt he called terbium and a deep yellow peroxide he called erbium.

See also: From the “village of Ytterby” in Sweden, where the mineral ytterbite (the source of terbium) was first found.

1. A word or group of words that has a precise meaning and expresses a definite idea,
   used in a particular science, art, or profession. See also
   → terminology; → determine.
   

2. Math.: In an expression, a number or a letter standing alone; or a combination of such representing a unit, e.g.: a, b, 5, 4ab, 3a/b.
   

3. Physics: A set of atomic states having a definite → configuration and → spin and → orbital angular momentum  → quantum numbers. In the → LS coupling scheme, the entity ^2S+1L_J, in which 2S+1 is called the → multiplicity of the term.
   

4. Logic: The → subject or → predicate of a → categorical proposition. See also → syllogism.
   

5. In → first-order logic, an → individual constant, → individual variable, or → function.

Etymology (EN): M.E. terme, from O.Fr., from L. terminus “boundary, limit, end; boundary stone or marker,” variant termen “boundary, end;” cognate with Gk. termon “limit, boundary;” Skt. tarman “the top of the sacrificial (usually tripod) post; passage;”
Irish tearmann “a refuge, sanctuary, asylum;” this Irish word would point to the sacredness of the sacrificial post in primitive IE customs; Hittite tarma- “peg, plug, nail;” PIE base *ter- “to cross;” cf. Pers. tarm, târem, tarâ-, Av. tar- “to cross over,” as below.

Etymology (PE): Tarm, variant târem “boundary, limit,” more specifically “a wooden palisade to exclude people from a garden,” also “a wooden building of a circular form with an arched roof” (cf. Irish tearmann, as above), Tabari talm “pole, stick” (that marks a boundary),
Tâleši/Tâti talmi “pole, stick,” Garkâni taram “lever,” Lori, Laki tarm “poles fastened together in order to carry a corpse to the village cemetery;” O.Pers./Av. tar- “to cross over,” O.Pers.
vi-tar- “to go across,” Mid.Pers. vitartan “to pass,” Mod.Pers. gozar, gozaštan “to pass, cross;” cf. Skt. tarman “the top of the sacrificial post; passage,” tar- “to pass (through), overcome,” tárati “crosses, passes,” tirás “through, across, beyond;” see also → trans-.

1. Forming or found at the extreme point or limit of something, or relating to the very end of something.
   
2. Computers: An input/output device having a keyboard for communicating with a computer and usually a display.

Etymology (EN): M.E., from L. terminalis “pertaining to a boundary or end, final,” from terminus “end, boundary line,” → term.

Etymology (PE): Pâyâni, pâyâné, noun and adj. from pâyân “end, extremity; limit, boundary,” from pâ(y) “foot; step” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-).

The locus of stars on the → Hertzsprung-Russell diagram that are at the point of exhausting hydrogen in their cores. TAMS forms the upper luminosity boundary of the → main sequence strip. See also
→ zero age main sequence (ZAMS).

See also: → terminal; → age; → main; → sequence.

1. The constant maximum velocity reached by a body falling under gravity through a liquid or gas, especially the atmosphere. The body ceases to accelerate downward because the force of gravity is equal
   to the opposing force of resistance by the medium.
   

2. The velocity acquired at the end of a body’s motion.
   

3. The velocity attained by → stellar wind at very large distance from the star. The material that escapes from the outer layers of the stars is accelerated outward from a small radial velocity at the → photosphere of the star, to some high velocity at large distance from the star.
   The theory of → radiation-driven winds predicts that the
   terminal velocity scales with the → escape velocity as: v_∞  ≅ 2-3 v_esc. The winds of hot → O stars experience fast acceleration and reach 80% of their terminal velocity near the star (some 3 R_*).
   The terminal velocity ranges from about 10 km s^-1 for a cool → supergiant star to 3000 km s^-1 for a luminous hot star. Terminal velocities are quite accurately measured from the violet trough of saturated → P Cygni line profiles in the ultraviolet (mainly N V λ1239, C IV λ1548, Si IV λ1394).

See also: → terminal; → velocity.

A → shock wave inside the → heliopause where the → supersonic → solar wind abruptly slows from an average speed of 500 km s^-1 to → subsonic and becomes denser and hotter.

Etymology (EN): Termination, verbal noun from terminate, from → term; → shock.

Etymology (PE): Toš, šok, → shock; pâyâni, → terminal.

The dividing line between the illuminated and the un-illuminated part of the Moon’s or a planet’s disk.

Etymology (EN): From L. terminator, from terminare, from terminus→ term.

Etymology (PE): Šid-marz, literally “light boundary,” from šid “light, sunlight” (Mid.Pers. šêt “shining, radiant, bright;” Av. xšaēta- “shining, brilliant, splendid, excellent”)

• marz “boundary, limit” (Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
  Ger. Mark; E. mark, margin).

1. The system of terms belonging to a particular science, art, specialized subject, or social group. Terminology is the way of naming concepts, which generally precede the corresponding terms. See also → lexicology.
   

2. The theory or science dealing with the relations between → terms and → concepts.

Etymology (EN): A hybrid word coined first in Fr., before 1764, by Yves Marie André (1675-1764), a Jesuit mathematician and philosopher, from termin, from L. terminus, → term, + epenthetic vowel -o- + Gk. -logia, → -logy. Recoined or borrowed in Ger. Terminologie in 1786, by C.G. Schütz of Jena; first appeared in E. in 1801.

Etymology (PE): From tarm, → term, + -šenâsi, → -logy.

To alter the environment of a planet or moon in a → terraforming process in order to make it habitable for life forms.

Etymology (EN): Probably taken from noun terraform,
from L. terra “earth,” → terrestrial; → form.

Etymology (PE): From zamin, → earth, + disidan “to → form.”

The hypothetical process of altering the environment (atmosphere, temperature, surface topography, or ecology) of another planet or moon to improve the chances of survival of an indigenous biology or to allow habitation by terrestrial life forms. See also → ecopoiesis.

See also: Verbal noun of → terraform. The term first appeared in a science fiction novel, Seetee Shock (1949) by Jack Williamson, an American science fiction writer; but the actual concept pre-dates this work.

Pertaining to, consisting of, or representing the Earth as distinct from other planets.

Etymology (EN): From L. terrestris “earthly,” from terra “earth,” literally “dry land” (as opposed to “sea”); from PIE base *ters- “to dry” (cf. Pers. tešné “thirsty;” Mid.Pers. tašnak “thirsty;” Av. taršu- “dry,” taršna- “thirst;” Skt. trsta- “dry,” tars- “to be thirsty;”
Gk. teresesthai “to become or be dry,” L. torrere “to dry up, roast,” Goth. þaursus “dry, barren,” O.H.G. derren “to make dry,” durst “thirst;” Ger. dürr “arid;” O.E. þurstig “thirsty”).

Etymology (PE): Zamini adj. of zamin, variant zami “earth, floor, land;”
Mid.Pers. zamig; Av. zam- “the earth;” cf. Skt. ksam- “the ground, earth;” Gk. khthôn, khamai “on the ground;” L. homo “earthly being” (as in homo sapiens,
homicide, humble, humus, exhume), humus “the earth;” O.Russ. zemi “land, earth;” PIE root *dh(e)ghom “earth”.

A uniform atomic time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution, which replaced Ephemeris Time. TDT is independent of the variable rotation of the Earth, and the length of the tropical year is defined in days of 86,400 seconds of international atomic time. In 1991 it was replaced by Terrestrial Time.

See also: → terrestrial; → dynamical; → time.

A parameter representing the product of the → gravitational constant by the Earth’s mass. It is 3.987 x 10¹⁴ m³s^-2 or 3.987 x 10⁵ km³s^-2.

See also: → terrestrial; → gravitational; → constant.

The four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth’s. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. These planets are approximately the same size, with the Earth the largest. They are considerably denser than the Jovian planets, ranging from a specific gravity of 4 for Mars to 5.5 for the Earth.

See also: → terrestrial; → planet.

The modern astronomical standard for the passage of time on the surface of the Earth. It is
the → coordinate time scale consistent with the theory of general relativity for an observer on the surface of the Earth. TT was renamed from Terrestrial Dynamical Time (TDT)
in 1991. The fundamental unit of TT is the day of 86,400 SI seconds. It is related to the International Atomic Time by the relation: TDT = TAI + 32.184 sec.

See also: → terrestrial; → time.

1. Third in order, rank, level, stage, formation, etc.
   

2. Chem.: Being or containing a carbon atom having bonds to three other carbon atoms.
   

3. Geology: (usually with capital initial) A term formerly used to distinguish a major period of → geologic time, from the end of the → Cretaceous period about 65 million years ago to the start of the Quaternary period about 1.6 millions years ago. The International Commission on Stratigraphy no longer sustains this term as part of the formal stratigraphic nomenclature. Instead, the → Paleogene and → Neogene periods are recommended as the primary subdivisions of the Cenozoic era.

See also: From L. tertius, → third; → period.

The unit of → magnetic induction flux density in the SI system. It is the induction of a field in which each meter of conductor with a current of one → ampere and arranged perpendicular to the direction of the vector of induction is acted upon by a force of one → newton. 1 tesla = 1 → weber/m² = 10⁴→ gauss.

See also: Named after Nikola Tesla (1856-1943), American physicist, electrical engineer, and inventor, born in Smiljan, Lika, which was then part of the Austo-Hungarian Empire, region of Croatia.

A procedure by which the presence, quality, or genuineness of anything is determined.

Etymology (EN): M.E. test, originally “a small vessel for refining or assaying metals,” from O.Fr. test, from L. testum “earthen pot,” related to testa “piece of burned clay, earthen pot, shell,” related to texere “to weave;” → technique.

Etymology (PE): Âzmun, from âzmudan, âzmây- “to try, experiment, test;” Mid.Pers. uzmudan, ôzmutan “to test, try, prove;” from O.Pers./Av. *uz-mây-, from uz-, → ex-,

• mť(y)- “to measure;” from PIE *me- “measure;” cf. Skt. mati “measures,” matra “measure;” Gk. metra “lot, portion;” L. metri “to measure.”

Any of a pair of identical 1.96 kg gold-platinum cubes measuring 4.6 cm on a side that are planned to be used in the → eLISA experiment to detect → gravitational waves. The test masses will be released in → free fall in the the → LISA Pathfinder spacecraft. The aim is to test methods and techniques that will be used in the eLISA experiment to detect gravitational waves. Even in space there are forces capable of disturbing the cubes, including the radiation and wind from the Sun. Hence the test masses need be isolated from all of these non-gravitational influences. To do so, LISA Pathfinder continually measures their positions and manoeuvres around them with microthrusters to avoid ever touching them.

See also: → test; → mass

Same as → significance testing and → rule of decision.

See also: → test; → significance.

In → plasma physics, → fluid dynamics, and → self-gravitating systems, a particle or ensemble of particles which are affected by the evolution of the rest of the system, but do not affect the rest of the system.
The test particle concept is used, e.g.,
for revealing a flow, describing the path of fluid elements, and so on.

See also: → test; → particle.

That part of the working time of a telescope devoted to tests of coupled instruments or the telescope itself.

See also: → test; → time.

The state or fact of being → testable. See also → falsifiability.

See also: → testable; → -ity.

The quality of a claim, hypothesis, or theory that can be verified by tests or experiments. See also → falsifiable.

See also: → test; → -able.

The ninth of Saturn’s known satellites. It it 1060 km across and orbits at a mean distance of 294,660 km from Saturn. It has a rotational period of 1.89 days that equals its orbital period. Tethys is thought to be composed almost entirely of water-ice because of its low density (1.21 gm/cm³). The western hemisphere of Tethys is dominated by a huge impact crater called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself. Tethys has two moons named Telesto and Calypso. It was discovered by J.-D. Cassini in 1684.

See also: In Gk. mythology Tethys was a Titaness and sea goddess who was both sister and wife of Oceanus.

Combining form meaning four.

Etymology (EN): From Gk. tetra-, combining form of tettares, tessares “four,” cognate with Pers. cahâr, → four.

1. A group of four.
   

2. A sequence of four → total lunar eclipses occurring at approximately six month intervals. Tetrads are relatively rare events. The first → total eclipse of a tetrad occurred on 15 April 2014, the next eclipse of the series will happen on 8 October 2014 and the next two eclipses on 4 April and 28 September 2015, respectively. The previous tetrad happened in 2003-2004 and the next one will be in 2032-2033.

Etymology (EN): From Gk. tetrad-, stem of tetras “group of four.”

Etymology (PE): Cahârgân, from cahâr, → four, + -gân suffix of plurality.

Same as → quadrangle.

Etymology (EN): From Gk. tetragonon, from → tetra- + -gon “angled, angular,” from gonia “angle,”
related to gony “knee;” L. genu “knee;” Mod.Pers. zânu “knee;” Av. žnav-, žnu- “knee;” Skt. janu-; PIE base *g(e)neu-.

Etymology (PE): Cahârguš “four-cornered,” from cahâr, câr “four,” cognate with L. quattuor, → four,

• gušé, guš “corner, angle;” Mid.Pers. gôšak “corner.”

A solid figure having four plane faces.

See also: → tetra-; → -hedron.

1. The main body of matter in a manuscript, book, newspaper, etc., as distinguished from notes, appendixes, headings, illustrations, etc.
   

2. The actual wording of anything written or printed.
   

3. Any of the various forms in which a writing exists (Dictionary.com).

Etymology (EN): M.E., from L. textus “text, terms, structure,” originally, “pattern of weaving,”
→ texture, from texere “to weave.”

Etymology (PE): Bâfé, from bâftan “to weave,” → texture.

A type of hypothetical → topological defect in the structure of → space-time that forms when large, complicated symmetry groups are completely broken.

Etymology (EN): M.E., from M.Fr., from L. textura “web, texture, structure,” from stem of textere “to weave;” PIE base *tek- “to make” → technique.

Etymology (PE): Bâft, verbal noun, past stem of bâftan “to weave;” Mid.Pers. waf- “to weave;” Av. ubdaēna- “consisting of woven texture,”
from *ubda- “woven,” p.p. from vaf- “to weave; to sing,” literally “to weave the songs;” cf. Skt. vabh- “to bind, fetter,” ubhnāti “he laces together;” Gk. hyphainein “to weave,”
hyphos “web;” P.Gmc. *webanan (cf. O.N. vefa, M.L.G., M.Du., Du. weven, O.H.G. weban, Ger. weben; O.E. wefan; E. weave), from PIE *webh-/*wobh- “to weave.”

The second innermost satellite of → Neptune. It was discovered using NASA’s Voyager 2 mission in 1989. It orbits 50,070 km from the center of Neptune and is about 80 km in diameter.

See also: Named after a daughter of Aether and Hemera from Gk. mythology. Thalassa is also the Greek word for “sea”.

The fourth-closest of → Jupiter’s known → satellites, also known as Jupiter XIV. Thebe is 100 x 90 km in diameter and orbits its planet at 222,000 km in 0.6745 (Earth) day. It is in synchronous rotation, i.e. always keeps the same side facing Jupiter. Thebe was discovered by Stephen Synnott (Voyager 1) in 1979.

See also: In Gk. mythology, Thebe was a nymph, daughter of the river god Asopus.

A hypothetical → protoplanet that collided with → earth some 4.5 billion years ago. Debris from the collision, a mixture of material from both bodies, spun out into Earth orbit and → coalesced into the → Moon. This scenario explains many aspects of → lunar geology including the size of the Moon’s → core and the → density and → isotopic  → composition of Moon rocks.

See also: Named for Theia, the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

The belief in one God as the creator and ruler of the universe, without rejection of revelation (distinguished from → deism).

2. Belief in the existence of a god or gods (opposed to → atheism) (Dictionary.com).

Etymology (EN): From the- variant of theo- before a vowel, from Gk. theos “god,” from PIE root *dhes-, root of words applied to various religious concepts, such as L. feriae “holidays,” festus “festive,” fanum “temple.”

Etymology (PE): Yzadân-bâvari, from yazdân “god,” from Mid.Pers. yazetân “gods,” ultimately from Proto-Ir. *iaz- “to sacrifice, worship, venerate,” → deity.

A person who believes in → theism.

See also: → theism; → -ist

Of or relating to a → theme.

See also: → thematics.

A body of topics for study or discussion.

See also: From L. themat-, from thema, → theme,

• → -ics.

The act or process of thematizing.

See also: → thematize; → -tion.

Present or select as a theme.

Etymology (EN): From L. themat-, from thema, → theme,

• E. suffix → -ize.

Etymology (PE): Verbal noun of nedanidan, → thematize.

1. Generally, a topic or subject of discourse, discussion, meditation, or composition.
   

   2. A unifying or dominant idea, motif, etc., as in a work of art. More specifically, an implicit central concept or message found in a literary work.
      

   3. A recurrent melodic element in a musical composition.
      

   4. Linguistics: The stem of a noun or verb; the part to which inflections are added, especially one composed of the root and an added vowel.

Etymology (EN): M.E. teme, theme, from O.Fr. tesme, from L. thema “a subject, thesis,” from Gk. thema “a proposition, subject, deposit,” literally “something set down,” from PIE root *dhe- “to set, put.”

Etymology (PE): Nedan, literally “(something) set down,” from prefix ne- “down,” → ni- (PIE), + da-, variant of dâ-/dâdan “to give, set” (dah-/dahad “gives”); Mid.Pers. dâdan, dah- “to give, set; create;” O.Pers. dā- “to give;” Av. dā- “to place upon, bestow;” Proto-Ir. *da- “to give” (cf. Skt. dā- “to give, present, offer;” Gk. thema; PIE *dhe- “to set, put,” as above) + word forming suffix -an (as in rowzan, rasan, anjoman, barzan).

A small satellite of → Jupiter, ninth in order from the planet. It is about 8 km in diameter and orbits Jupiter at a mean distance of 7 500 000 km every 130 days. It was discovered in 1975, lost, and then rediscovered in 2000. Also known as Jupiter XVIII.

See also: Named after Themisto, daughter of the river god Inachus, who became the mother of Ister (the river Danube) by Zeus (Jupiter).

1. A form of government in which → God or a → deity is recognized as the supreme civil ruler, the God’s or deity’s laws being interpreted by the ecclesiastical authorities.
   

2. A system of government by priests claiming a divine commission (Dictionary.com)

See also: → theism; → -cracy.

An instrument for the measurement of angles, used in surveying. It consists essentially of a telescope moving along a circular scale graduated in degrees.

See also: The first occurrence of the word theodolite is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges, which was published posthumously by his son, Thomas Digges. The etymology of the word is unknown. The first part of the New Latin theo-delitus might stem from the Gk. theaomai “to behold, view attentively, contemplate,” but the second part is more puzzling and is often attributed to an un-scholarly variation of delos “evident, clear.”

A person versed in theology, especially Christian theology (Dictionary.com)

See also: → theology.

The field of study and analysis that treats of → God and of God’s attributes and relations to the universe; study of divine things or religious truth; divinity (Dictionary.com).

See also: → theism; → -logy.

A → proposition, → statement, or → formula in → mathematics or → logic deduced from → axioms, other propositions, → assumptions, → premises, or formulas. Theorems are statements which can be proved. For example, → Fourier theorem; → Liouville’s theorem; → Woltjer’s theorem.

Etymology (EN): From M.Fr. théorème, from L.L. theorema, from Gk. theorema “spectacle, speculation,” in Euclid “proposition to be proved,” from theorein “to look at, speculate, consider.”

Etymology (PE): Farbin, from far- intensive prefix “much, abundant; elegantly; forward” (Mid.Pers. fra- “forward, before; much; around;”
O.Pers. fra- “forward, forth;” Av. frā, fərā-, fra- “forward, forth; excessive;” cf. Skt. prá- “before; forward, in front;” Gk. pro “before, in front of;” L. pro “on behalf of, in place of, before, for;” PIE *pro-)

• bin, present stem of didan “to see,” from
  Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
  cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”

Of, pertaining to, or consisting in theory.

Etymology (EN): From L.L. theoreticus “of or pertaining to theory,” from Gk. theoretikos “contemplative, pertaining to theory;” → theory.

Etymology (PE): Negarik, contraction of negaréik, from negaré→ theory + -ik, → -ic.

An astrophysical study or research group mainly concerned with theory rather than observation.

See also: → theoretical; → astrophysics.

One who formulates or is expert in the theoretical side of a subject.

Etymology (EN): From theoretic, from theoretics, from → theory

• -ian.

Etymology (PE): Negare-pardâz, from negaré, → theory,

• pardâz, present stem of pardâxtan “to accomplish; bring to perfection; to attempt, to care; to clean; to free;” Mid.Pers. pardâxtan, pardâzidan “to accomplish; to be done with, freed of” ultimately from Proto-Iranian *para-tāxta-, *para-tāca- “to take away; to expel,”
  from *para- “along, forth,” → para-, + tāxta-, tāca- “to run, to flow;” cf. Av. tak- “to run, to flow;” Mod.Pers. tâxtan, tâz- “to flow, to cause to walk,” → flow.

Same as → theoretician.

Etymology (EN): From theor-, from → theory + -ist a suffix of nouns, often corresponding to verbs ending in -ize or nouns ending in -ism.

Etymology (PE): → theoretician.

A coherent set of verified facts, propositions, or principles analyzed in their relation to one another and used to explain and predict phenomena, e.g. the → theory of relativity.
The criterion of the scientific status of a theory is its → falsifiability, → refutability, or → testability. See also → hypothesis, → model.

Etymology (EN): From L.L. theoria, from Gk. theoria “contemplation, speculation, a looking at, things looked at,” from theorein “to consider, view, look at,” from theoros “spectator,” from thea “a view” + horan “to see.”

Etymology (PE): Negaré, from negar present stem of negaridan, negaristan “to look, observe;” Mid.Pers. nigeridan, niger-, nikiritan, nikir- “to look, to watch, to notice, to consider;” ultimately from Proto-Iranian *ni-kar-, from *ni- “down, in, into,” → ni- (PIE), + *kar- “to observe, to consider;” cf. Av. kar- “to remember; to impress on memory;” Skt. kal- “to observe, consider,” kalayati “considers, observes;” Mid.Pers. kartan “to establish; to declare; to found,” (h)angârtan “to consider, to bear in mind, to regard as,” us-kâritan “to consider, deliberate, discuss,” sikâl, sigâl “thought;”
Mod.Pers. engâridan, engâštan “to suppose,”
segâl “thought,” segâlidan “to think, to resolve to injure, to deceive.”

Any theory that attempts to describe all the forces of nature including gravity in a single mathematical formalism; e.g. → grand unified theory. → string theory.

Etymology (EN): → theory; every; M.E. every, everich; O.E. æfre ælc “ever each;” → thing.

Etymology (PE): Negaré, → theory; hamé, → all; ciz, → thing.

Any of the two theories put forward by Albert Einstein:
→ special relativity (1905) and → general relativity (1916).

See also: → theory; → relativity;

Any of several commercial units of heat energy, as one equivalent to 10⁶ calories.

See also: From Gk. therme “heat,” → thermal.

Of, pertaining to, or caused by heat or temperature.

Etymology (EN): From M.Fr. thermal, from Gk. therme “heat,” cognate with Pers. garm “warm,” as below.

Etymology (PE): Garmâyi, adj. of garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

1. The random movement of the molecules of a substance, the energy of which is, by kinetic theory, synonymous with the heat content of the substance.
   

2. Solid state physics: The random motion of free electrons in a conductor due to heat energy.

See also: → thermal; → agitation.

The emission of electromagnetic radiation from high temperature plasma, produced as electrons are deviated by positive ions. Same as → free-free emission

See also: → thermal; → bremsstrahlung.

A process that occurs in a medium where a → temperature gradient exists:

dQ = -κ(dT/dx)dA.dt, where dQ is the amount of heat passing through the time dt across an area dA in the direction of the normal x to this area and toward the reduction in temperature, κ is the → thermal conductivity, and (dT/dx) the temperature gradient.

See also: → thermal; → conduction.

In → thermal conduction, the amount of heat passing across unit area per unit time and per unit → temperature gradient.

See also: → thermal; → conductivity.

A detector that senses the change of temperature due to the absorption of photons.

See also: → thermal; → detector.

A physical process resulting from → temperature gradients in stellar interiors, whereby more highly charged and more massive chemical species are concentrated toward the hottest region of the star, its center. Therefore, thermal diffusion and → gravitational settling tend to make heavier species sink relative to the light ones.

See also: → thermal; → diffusion.

→ thermal radiation.

See also: → thermal; → emission.

1. The energy in the form of heat emitted by an object by virtue of its temperature.
   

2. The total potential and kinetic energies associated with the random motions of the particles of a material. The quantity of thermal energy possessed by a body determines
   its temperature. The thermal energy which is absorbed, given up, or transferred from one material to another is heat.
   

3. The characteristic energy of → thermal neutrons at room temperature, about 0.025 eV.

See also: → thermal; → energy.

In thermodynamics, the state of a system all parts of which have attained a uniform temperature and no net heat exchange is taking place between it and its surroundings. If two bodies are in thermal equilibrium, they have the same temperature. Thermal equilibrium is the central criterion of the → zeroth law of thermodynamics. See also → local thermodynamic equilibrium (LTE).

See also: → thermal; → equilibrium.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space.

In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent.

Thermal escape has two types: → Jeans’ escape and → hydrodynamic escape

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to

Space, pp. 129-167. Cambridge University Press).

See also: → thermal; → escape.

A process in which collisions that occur between particles cause atoms or molecules to obtain additional kinetic energy.

See also: → thermal; → excitation.

The change in dimensions of a material resulting from a change in temperature.

See also: → thermal; → expansion.

A vector quantity that depends on the distribution of temperature in three dimensions with respect to a given point. The magnitude and orientation of the maximum thermal gradient are given by:

∇T = (∂T/∂x)i + (∂T/∂y)j

• (∂T/∂z)k, where T is the temperature distribution function in three dimensions, and i, j, and k are the unit vectors along the x, y, and z axes defining the temperature field.
  Same as → temperature gradient.

See also: → thermal; → gradient.

A mechanism for the → transport of → electrons which occurs when the → Fermi level lies
below a low but wide energy → barrier. The → tunneling probability across the barrier is considerably suppressed due to the width of the barrier. However, at higher temperatures, the electron can raise its energy with the assistance of a vibrational mode. The electron is said to hop from one side of the barrier to the other side via an intermediate state.

See also: → thermal; → hop; → -ing.

The tendency of a body to resist a change in temperature. A body with a low thermal inertia requires very few calories to change its surface temperature. A low thermal inertia material tends to be thermally insulating, so that the surface temperature changes readily, but those changes are not conducted to depth within the material (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → thermal; → inertia.

The → Jeans mass when → turbulence is insignificant.

See also: → thermal; → Jeans; → mass.

The random motions and collisions of molecules, atoms, electrons, or other subatomic particles constituting an object at all temperatures above → absolute zero. The thermal motion of particles rises with the temperature of those particles and is governed by the laws of → thermodynamics. The most convincing experimental proof of thermal motion → Brownian motion.

See also: → thermal; → motion.

A neutron of very slow speed and consequently of low energy. The energy of thermal neutrons is of the same order as the → thermal energy of the atoms and molecules of the substance through which they are passing.

See also: → thermal; → neutron.

Electric noise signals that are produced by the random thermal motion of charges in circuit.

See also: → thermal; → noise.

The ordinary pressure in a gas that is due to motions of particles and can be attributed to the object’s → temperature.

See also: → thermal; → pressure.

Repeated instabilities of the He burning shell which ignites in sudden burst during the final phases of the → AGB evolution.

See also: → thermal; → pulse.

The energy radiated from an object in the form of → electromagnetic waves as a result of its → temperature. Thermal radiation ranges in → wavelength from the longest → infrared radiation
through the → visible light spectrum to the shortest → ultraviolet rays. In opposition, → non-thermal radiation is caused by energetic particles.

See also: → thermal; → radiation.

Stresses induced in a material because of rapid temperature change or a → thermal gradient .

See also: → thermal; → shock.

A → transient → rise in → temperature above the normal level in a medium.

See also: → thermal; → spike.

In star formation models, the gas pressure that counters the collapsing pull of gravity.

See also: → thermal; → support.

1. The process by which a system reaches → thermal equilibrium.
   Thermalization results from energy exchange between the components constituting the system and their exchange with the outside medium. In a gas at a given temperature, molecules move with different velocities. The gas temperature corresponds to the mean velocity of the molecules, but individual molecules may deviate largely from the mean velocity. Some move very fast others slowly and change velocity upon collisions. Collisions reduce the energy of fast moving molecules and increase that of slow ones.

In the process of thermalization → matter and → radiation are in constant interaction such that their → temperatures become identical.

The process goes on until energy distribution reaches → equilibrium. The system is said to be → thermalized.

2. The condition where in an → atomic or → molecular → transition the → Boltzmann factor for the two → levels of transition takes on the value it would have in → thermodynamic equilibrium.

See also: Verbal noun of → thermalize.

To bring neutrons into → thermal equilibrium with their surroundings; to produce → thermal neutrons.

Etymology (EN): From → thermal + → -ize.

Etymology (PE): Yekgarmâyidan, literally “equal warming,” from yek-,
→ one, + garmâyidan, infinitive from garmâ, → thermo-.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature
of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

See also: Thermalized, p.p. of → thermalize; → line.

An electron that has been emitted from a heated body such as the hot cathode of an electron tube.

See also: From therm- variant of → thermo- before a vowel + → ion.

Electrons gaining enough thermal energy to escape spontaneously from the cathode or dynodes and mimic photoelectrons.

See also: thermion; → emission.

A combining form meaning “heat, hot,” used in the formation of compound words. Also therm- before a vowel.

Etymology (EN): From Gk. therme “heat,” thermos “hot;” cf. L. fornax “oven, kiln,” related to fornus, furnus “oven,” and to formus “warm;” cognate with Pers. garm “warm,” as below; P.Gmc. *warmaz (O.E. wearm; E. warm; O.H.G., Ger. warm).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- “warm.”

A layer in a large body of water, such as a lake, in which temperature changes more rapidly with depth than it does in the layers above or below.

See also: → thermo- + → -cline.

Electrical circuit consisting of two dissimilar metals, in which an electromotive force is produced when the two junctions are at different temperatures.

See also: → thermo- + → couple.

Of or pertaining to → thermodynamics.

See also: → thermo-; → dynamic.

The condition of a → thermodynamic system in which the available → energy is distributed uniformly among all the possible forms of energy. Furthermore,
all → thermodynamic process es must be exactly balanced by their reverse processes. For example, inside a star there will be as many → ionizations of helium per second as there are → recombinations of free electrons and helium ions. Se also
→ local thermodynamic equilibrium (LTE).

See also: → thermodynamic; → equilibrium.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

See also: → thermodynamic; → path.

A measure of the energy level of a → thermodynamic system. It represents the amount of → work obtainable when the system undergoes a → change. The main types of thermodynamic potential are: → internal energy,
→ enthalpy, the → Helmholtz free energy, and the
→ Gibbs free energy.

See also: → thermodynamic; → potential.

An ordered set of → equilibrium states undergone by a
→ thermodynamic system. Thermodynamics processes have various types:
→ cyclic process, → reversible process, and → irreversible process, → isothermal process, → adiabatic process, → isentropic process.

See also: → thermodynamic; → process.

A quantity of substance or a working machine which in a well-defined way is set apart from its → environment. The boundary between the system and its surroundings can be real or an imaginary mathematical envelope. A thermodynamic system is not necessarily bound to a predefined geometry. Thermodynamic systems can be divided into three types: → open systems, → closed systems, and → isomated systems.

See also: → thermodynamic; → system.

A temperature scale, measured in → kelvin (K), that is related to the energy possessed by matter; it was formerly known as → absolute temperature. The zero point on the scale (0 K) is absolute zero. Thermodynamic temperature can be converted to temperature on the → Celsius scale by subtracting 273.15.

See also: → thermodynamic; → temperature.

A branch of physics concerned with the relations between heat and other forms of energy and how these affect temperature, pressure, volume, mechanical action, and work.

See also: → thermo-; → dynamics, coined by the Scottish physicist William Thomson (Lord Kelvin, 1824-1907), in 1849.

Of, relating to, or produced by electric phenomena occurring in conjunction with a flow of heat.

See also: → thermo- + → electric.

A phenomenon occurring when temperature differences exist in an electrical circuit, such as
the → Peltier effect, the → Seebeck effect, and the → Thomson effect,

See also: → thermoelectric; → effect.

The electricity produced by heat or temperature difference in a conductor.

See also: → thermo- + → electricity.

An instability in the ocean water that occurs when
a layer of warm salt water is above a layer of fresh cold water of slightly higher density. In this process the hot salt water cools off and then, after having reached a higher density than the fresh water, sinks down even in the presence of stabilizing temperature gradients. This phenomenon explains the large-scale water movements in the oceans called themohaline circulation. First discussed by Melvin E. Stern (1960, Tellus 12, 172). → thermohaline mixing.

Etymology (EN): Thermohaline, from → thermo- + haline, from Gk. hals (genitive halos) “salt, sea;” cf. L. sal; O.Ir. salann; Welsh halen;
O.C.S. sali “salt;” O.E. sealt; cf. O.N., O.Fris., Goth. salt, Du. zout, Ger. Salz from PIE *sal- “salt.”

Etymology (PE): Garmâšur, from garmâ-→ thermo- + šur “salty” (Mid.Pers. šôr “salty,” šorag “salt land;” cf. Skt. ksurá- “razor, sharp knife;” Gk. ksuron “razor;” PIE base *kseu- “to rub, whet”).

In stars, an instability phenomenon, reminiscent of the
→ thermohaline convection in the oceans, that takes place
when layers of higher molecular weight occur above a region of lower molecular weight. A situation of heavier material being above lighter gas in a star can occur during the → helium flash when → helium burning does not start in the center but in the shell. Similarly, in → close binary systems it may happen that helium-rich material is transferred to a → main sequence star. Then a helium-rich outer layer is formed and the instability occurs at the interface between that layer and the original stellar material. This process can explain several surface abundance variations in stars. First discussed by S. Kato (1966, Publ. Astron. Soc. Japan 18, 374).

See also: → thermohaline; → mixing.

A theoretical interpretation for the → X-ray bursts observed toward → low-mass X-ray binary (LMXB) stars. According to models, X-ray bursts are produced on the surface of → neutron stars as a result of violent thermonuclear processes in a → hydrogen or → helium rich → layer. It is the → nuclear energy released in the → fusion of hydrogen and helium to heavier elements (e.g., Ni, Zn, and Se) in the → accreted matter which heats the upper layers of the neutron star so that X-rays are emitted from the surface (see, e.g., Taam, R.E., 1984, AIP Conf. Proc. 115, 263).

See also: → thermonuclear; → flash.

A nuclear reaction in which two or more atomic nuclei fuse into a single heavier nucleus by a collision of the interacting particles at extremely high temperatures.
Chains of thermonuclear reactions, such as the → proton-proton chain and the → CNO cycle, account for the energy radiated from the Sun and more massive stars.

See also: → thermo- + → nuclear; → reaction.

1. The uncontrolled → fusion of hydrogen into helium.
   

2. A → thermonuclear reaction process occurring at electron → degenerate conditions in stellar material, such as in → Type Ia supernovae.

Etymology (EN): → thermonuclear; → runaway.

Etymology (PE): Vâžireš, → reaction; garmâhaste-yi, → thermonuclear; legâm gosixté literally “rampant, unrestrained,” from legâm “bridle, rein” + gosixté “broken off, torn away,” p.p. of gosixtan “to tear away, to break off.”

Same as → type Ia supernova

See also: → thermonuclear; → supernova.

The region of the upper atmosphere in which temperature increases continuously with height, starting at roughly 100 km. The thermosphere includes the exosphere and most of the ionosphere.

See also: → thermo-; → sphere.

A device for maintaining a system at constant temperature by automatically terminating or restoring the heating or cooling source. It consists of a temperature sensing instrument connected to a switching device. The sensing
device is often a bimetallic strip which triggers a simple electric switch.

See also: → thermo- + → -stat.

1. A controlled and structured list of terms or descriptors usually with a cross-reference system used in subject analysis and information retrieval in a particular field.
   

2. More generally, a work that lists words arranged and grouped according to their semantic similarities, including synonyms and sometimes antonyms. This is different from the dictionary, which
   contains definitions and pronunciations. The first major work of this kind in English is Peter Mark Roget’s Thesaurus of English words and phrases, Classified and Arranged so as to Facilitate the Expression of Ideas (1852).

Etymology (EN): From L. thesaurus “treasury, treasure,” from Gk.  thesauros  “treasure, treasury, storehouse,” from root of  tithenai “to put, to place,” → thesis.

Etymology (PE): Vâžganj, from vâž, → word, + ganj “treasure,” from Mid.Pers. ganj “treasure.”

1. A proposition put forward for consideration, especially one to be discussed and proved or to be maintained against objections.
   Philo.: The first of three stages in Hegelian dialectic; the inevitable transition of thought, by contradiction and reconciliation, from an initial conviction to its opposite and then to a new, higher conception that involves but transcends both of them. → antithesis; → synthesis.
   

2. A dissertation based on original research, especially as work toward an academic degree.

Etymology (EN): M.E., from L., from Gk. thesis “a proposition; a setting down, something set down,” from root of tithenai “to place, put, set,” cognate with Pers. dâdan “to give,” as below.

Etymology (PE): 1) Dâyan “giving, setting down,” from O.Pers./Av. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give; to put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”)

• -y- epenthetic vowel + -an noun/adjective suffix appearing in many words (such as rowzan, mihan, barzan, rasan, barzan, rowšan).
  

2. Dâyan-nâme, from dâyan “thesis” + nâme “diploma, letter” (Mid.Pers. nâmag “book, letter, inscription,” from O.Pers./Av. nāman- “name;” cf. Skt. nama-;
   Gk. onoma, onuma; L. nomen; PIE *nomen-).
   Pâyân-nâme, literally “ending, final diploma,” from pâyân “end,” → terminal + nâme.

The proper name of the → extrasolar planet  → Pollux b.

See also: In Greek and Roman mythology, Thestias was the patronym of Leda, → Pollux’s mother.

Relatively great in extent from one surface to the opposite; deep or profound. &rarroptically thick

Etymology (EN): M.E. thikke, O.E. thicce “not thin, dense,” from P.Gmc. *theku-, *thekwia- (cf. O.S. thikki, O.H.G. dicchi, Ger. dick), from PIE *tegu- “thick.”

Etymology (PE): Setabr, from Mid.Pers. stabr “strong, big,” stambag “pugnacious, opposing;” O.Pers. (mā) stabava [2sg.inj.] “to revolt;” Av. stabra- “strong, firm;” cf. Skt. stabh- “support,” stambh- “to support, fix firmly,” stabhnāti “supports;” Gk. astemphes “steadfast,” stephein “to tie around, encircle,” astemphes “firm, rigid;” Lith. stebas “staff, pillar,” stembti “to oppose.”

A disk component of a → spiral galaxy that lies above the → thin disk and mainly consists of stars. The thick disk of our → Galaxy makes up about 10-50% of the stellar mass of the → Milky Way and has a scale height of ~ 1,000-3,000 → light-years.
Thick disk stars are, on average, moving faster in a vertical direction with respect to the → galactic plane than thin disk stars. In contrast to thin disk, the stars within the thick disk are almost all older than 10 billion years and typically have a smaller → metallicity than the average values for the thin disk stars. These facts suggest that the formation scenarios for the thin and thick disks were different. In particular, it is thought that the thick disk is much older than the thin disk.

See also: → thick; → disk;

A lens whose thickness is not small compared with its focal length. The thick lens may include several components, which may or may not be in contact. → thin lens.

See also: → thick; → lens.

The state or quality of being thick. → optical thickness.

Etymology (EN): M.E. thiknesse, O.E. thicnes, from → thick + -nes(s) suffix of action, quality or state, cf. M.Du. -nisse, O.H.G. -nissa, Ger. -nis, Goth. -inassus.

Etymology (PE): Setabrâ, from setabr→ thick + -â a suffix forming nouns from adjectives.

The part of the human → leg between the → hip and the → knee.

Etymology (EN): M.E. thi, thigh(e), the(h), O.E. thioh, theoh; cognate with Du. dij, O.H.G. dioh, O.Norse thjo.

Etymology (PE): Rân, from Mid.Pers. rân “thigh;” Av. rāna- “the outer part of the thigh.”

Having relatively little extent from one surface or side to the opposite; not thick. → optically thin; → superthin galaxy.

Etymology (EN): M.E. thyn(ne), O.E. thynne, from P.Gmc. *thunnuz, *thunw- (cf. W.Fris. ten, M.L.G. dunne, Du. dun, O.H.G. dunni, Ger. dünn), from PIE *tnus-, *tnwi-, from base *ten- “stretch;” cf. Pers. tonok “thin, slender,” → attenuate.

Etymology (PE): Nâzok, from Mid.Pers. nâzuk, nâzik “tender, gentle.”

A disk component of a → spiral galaxy containing → stars, → gas, and → dust
which are confined to the galaxy’s → plane of rotation. In contrast to → thick disks, thin disks contain the bulk of the → baryonic matter in spiral galaxies. For example, on the order of 60-90% of the baryonic matter in the → Milky Way is located in the thin disk. The scale height of the thin disk in the Milky Way is about 400 → light-years,
whereas its scale length is about 10,000 light-years. Moreover, the outer regions of
thin disks appear to be bent by the → warp phenomenon.
The thin disks of spiral galaxies are active sites of
→ star formation, especially in the spiral arms. For this reason, stars in the thin disk tend to be relatively young. Thin disk stars also tend to be → metal-rich compared with thick disk and → halo stars, and typically have a → metallicity similar to that of the Sun.

See also: → thin; → disk;

A lens whose thickness is considered small in comparison with the distances generally associated with its optical properties. Such distances are, for example, radii of curvature of the two spherical surfaces, primary and secondary focal lengths, and object and image distances. → thick lens.

See also: → thin; → lens.

1. A material object without life or consciousness; an inanimate object.
   

   2. Some entity, object, or creature that is not or cannot be specifically designated or precisely described.
      

   3. Anything that is or may become an object of thought (Dictionary.com).

Etymology (EN): M.E. thing; O.E. þing “meeting, assembly, discussion;” cf. O.Norse þing “assembly, meeting, council;” O.Frisian thing “assembly, action, matter, thing;” O.Saxon thing; O.Du. dinc “law suit, matter, thing;” M.Du. ding; Du. ding; O.Ger. ding, dinc “assembly;” M.H.G. dinc “assembly;” Ger. ding “matter, affairs, thing.” Hence, the word originally meant “assembly, meeting,” then came to mean a specific issue discussed at such an assembly, and finally came to indicate “an object.”

Etymology (PE): Ciz, from Mid.Pers. ciš, tis “thing, affair;” O.Pers. cišciy “anything.”

To employ one’s mind rationally and objectively in evaluating or dealing with a given situation.

Etymology (EN): M.E. thinken, variant of thenken, O.E. thencan;
P.Gmc. *thankjan (cf. O.Fris. thinka, O.S. thenkian, O.H.G. denchen, Ger. denken, Goth. thagkjan).

Etymology (PE): Andišidan, infinitive from andiš-; Mid.Pers. handeš-,
handešidan “to think, consider, reflect,” ultimately from Proto-Iranian *ham-dis-, from *ham- “together, with, same,” → syn- + *dis- “form, appearance,” cf. Av. daēs- “to show,” daēsa- “sign, omen;” Mod.Pers. dis, disé “form, appearance,” variants -diz, -diš; Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Sogd. andiš “to seem,” andêš “to show,” andêšik “appearing;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).

Next after the second; the ordinal number for three. → Newton’s third law of motion; → third contact; → third dredge-up; → third law of thermodynamics.

Etymology (EN): M.E. thirde, O.E. (north) thirda, variant of ridda, from P.Gmc. *thridjas (cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, Goth. thridja).

Etymology (PE): Sevom, ordinal number for sé, → three.

The end of the total phase of a solar eclipse marked by the trailing edge of the Moon first revealing the Sun.

See also: → third; → contact.

A → dredge-up process that occurs in the stellar interior during He shell burning, as in → asymptotic giant branch (AGB) stars.
These stars consist of a degenerate carbon-oxygen core, surrounded by a helium-rich region, above which lies a hydrogen-rich convective envelope. Following thermal pulses of the helium-burning shell, the convective envelope moves inward in mass, penetrating the hydrogen-exhausted regions. This is known as third dredge-up. As convection moves inward, nuclear processed materials are carried to the surface.

See also: → third; → dredge-up.

The → entropy of an idealized state of maximum order is zero at the temperature of → absolute zero. Another version of this law: As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

See also: → third; → law; → thermodynamics.

The phase of the Moon when half the visible disk of the waning Moon is illuminated. This quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun’s.

See also: → third; → quarter.

A cardinal number, 10 times 3.

Etymology (EN): M.E. thritty, O.E. thritig, from thri, threo “three” + -tig “group of ten, -ty.”

Etymology (PE): Si, from Mid.Pers. sih; O.Pers. *ciθas nominative singular of *ciθant-; Av. θrisant- “thirty;” cf. Skt. trimśát- “thirty.”

A fundamental system for the classification of → asteroids based on → albedo and → spectral characteristics. The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types.
→ C-type asteroid, → M-type asteroid, → S-type asteroid.

See also: David J. Tholen (1984) Ph.D. thesis, University of Arizona; → classification.

The British physicist Sir Joseph John Thomson (1856-1940), discoverer of the electron (1897), Nobel Prize in Physics (1906). → Thomson atom, → Thomson cross section, → Thomson effect, → Thomson scattering, → Joule-Thomson effect.

The earliest theoretical description of the inner structure of atoms whereby an atom consists of a sphere of positive electricity of uniform density, throughout which is distributed an equal and opposite charge in the form of electrons. The diameter of the sphere was supposed to be of the order of 10^-8 cm, the magnitude found for the size of the atom. → Rutherford atom.

See also: → Thomson; → atom.

The → cross section involved the → Thomson scattering of electromagnetic waves by a free electron. It is defined by: σ_T = 8πr_e²/3, where r_e is the classical → electron radius. Its value is 0.665 245 x 10^-28 m².

See also: → Thomson; → cross; → section.

The absorption or emission of heat when current is passed through a single conductor whose ends are kept at different temperatures. If current is passed from hotter end to colder end of a copper wire, then heat is evolved along the length of the wire. When current is passed from colder end to the hotter end, then heat is absorbed.

See also: → Thomson; → effect.

The classical, → non-relativistic scattering of photons by free charged particles. When an electromagnetic wave is incident on a charged particle, the
electric and magnetic components of the wave exert a force on the particle, setting it into motion. As it accelerates, it in turn radiates in all directions. Such scattering is independent of wavelength and equal numbers of photons are scattered forward and backward. Thomson scattering occurs in stellar atmospheres and in any non-relativistic → plasma. Thomson scattering is normally taken as the minimum → opacity.

See also: → Thomson; → scattering.

A soft, ductile, lustrous, silver-white, → radioactive metal; symbol Th. → Atomic number 90; → atomic weight 232.0381; → melting point about 1,750 °C;
→ boiling point about 4,790 °C; → specific gravity 11.7 at 20 °C. It was discovered in the mineral thorite (ThSiO₄) by the Swedish chemist Jons Jacob Berzelius in 1828. It was first isolated by the chemists D. Lely Jr. and L. Hamburger in 1914.

Thorium-232 is a → fertile isotope, naturally occurring, from which the → fissile isotope uranium-233 can be bred.

See also: From Thor, the “Scandinavian god of thunder.”

A theoretical class of → stellar objects in which a → neutron star core is surrounded by a large and diffuse envelope.

TŻOs are expected to form as a result of the evolution of two → massive stars in a → close binary, with the neutron star forming when the more massive star explodes as a → supernova. During subsequent evolution of the system, the expanding envelope of the companion may lead to a common envelope state and the spiral-in of the neutron star into the core of its companion. Alternately, a TŻO may be produced when a newly-formed neutron star receives a supernova “kick” velocity in the direction of its companion and becomes embedded.

Supergiant TŻOs are predicted to be almost identical in appearance to → red supergiants (RSGs). The best features that can be used at present to distinguish TŻOs from the general RSG population are the unusually strong → heavy element and → lithium lines present in their spectra, products of the star’s fully → convective envelope linking the → photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core.

These objects are thought to be extremely rare, with as few as 20-200 of them predicted to exist in the Galaxy at present, though some authors have doubted whether such an object could survive the merger with the envelope intact. A candidate is HV 2112 (Levesque et al., 2014, MNRAS, arXiv:1406.0001;
Beasor et al., 2018, MNRAS, arXiv:1806.07399).

See also: Thorne K. S., Żytkow A., 1975, ApJ 199, L19.

The product of mental activity; that which one thinks; the act or process of thinking.

Etymology (EN): M.E. thoght; O.E. (ge)thoht, from stem thencan “to think;” cf. O.Fris. thinka, O.S. thenkian, O.H.G. denchen, Ger. denken “to think.”

Etymology (PE): Andišé, noun from andišidan, → think.

A demonstration which is carried out in the realm of the imagination, rather than in a laboratory. Thought experiments are designed to test ideas, theories, and hypotheses which cannot physically be tested, at least with current scientific equipment. Some examples: → Maxwell’s demon; → Einstein’s elevator; Heisenberg’s gamma-ray microscope; → Schrodinger’s cat. Also called Gedanken experiment.

See also: → thought; → experiment. Based on both the Ger./L. compound Gedankenexperiment and its Ger. equivalent Gedankenversuch.

1. A declaration of an intention or determination to inflict punishment, injury, etc., in retaliation for, or conditionally upon, some action or course; menace.
   

2. An indication or warning of probable trouble (Dictionary.com).

Etymology (EN): M.E. threte, O.E. threat “pressure, oppression;” cognate with O.N. thraut “hardship, bitter end,” Du. verdreiten, Ger. verdrießen “to vex,” L. trudere “to press, thrust.”

Etymology (PE): Harš, from Kurd. haraša “threat,” haraša kirdan “threaten,” related to Mid/Mod.Pers. rašk “envy, jealousy;” Lori, Laki erešt “assault, attack;” Tabari ârâšt “curse, anathema;”
Av. arš- “to be envious;” Skt. īrs- “to be envious, envy;” Arm. her “anger, quarrel;” O.E. eorsian “to be malicious;” Proto-Ir. Harš- “to be envious.”

1. To utter a threat against; menace.
   

2. To be a menace or source of danger to (Dictionary.com).

Etymology (EN): From M.E. thretnen, from O.E. thrêatnian, → threat

Etymology (PE): Infinitive from harš, → threat.

A cardinal number, 2 plus 1.

Etymology (EN): M.E.; O.E. threo, thrib, feminin and neuter of thri(e);

cf. O.Fris. thre, M.Du., Du. drie, O.H.G. dri, Ger. drei, Dan. tre), cognate with Pers. sé, as below.

Etymology (PE): Sé, from Mid.Pers. sé; Av. θrayô, θrayas, tisrô, θri; cf. Skt. tráya, tri, trini; Gk. treis, L. tres, Lith. trys, O.C.S. trye, Ir., Welsh tri, O.E. threo, as above; PIE base *trei-.

The mathematical problem of studying the positions and velocities of three mutually attracting bodies (such as the Sun, Earth and Moon) and the stability of their motion. This problem is surprisingly difficult to solve, even in the simple case, called → restricted three-body problem, where one of the masses is taken to be negligibly small so that the problem simplifies to finding the behavior of the mass-less body in the combined gravitational field of the other two. See also → two-body problem, → n-body problem.

See also: → three; → body; → problem.

A flow whose parameters (velocity, pressure, and so on) vary in all three coordinate directions. Considerable simplification in analysis may often be achieved, however, by selecting the coordinate directions so that appreciable variation of the parameters occurs in only two directions, or even only one (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

See also: → three; → dimensional; → flow.

The level that must be reached for a physical effect to begin or be noticeable.

Etymology (EN): M.E. threschold, O.E. threscold, threscwald “doorsill, point of entering.”

Etymology (PE): Âstâné “threshold; a place of rest or sleeping,” variant âstân; Mid.Pers. âstânak; ultimately from Proto-Iranian *ā-stānaka-, from *stā- “to stand;” cf. O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; Mid.Pers. êstâtan “to stand;” Mod.Pers. istâdan “to stand;” cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand.”

The minimum energy necessary for the occurrence of some chemical/physical effect.

See also: → threshold; → energy.

The minimum energy, for an incident particle or photon, below which a particular reaction does not occur.

See also: → threshold; → reaction.

The minimum intensity of a signal that can be detected and recognized.

See also: → threshold; → signal.

The front part of the neck. → nozzle throat.

Etymology (EN): M.E. throte, O.E. throte, throta, throtu; cognate with O.H.G. drozza “throat,” O.N. throti “swelling.”

Etymology (PE): Galu “throat,” related to geri, geribân “collar,” gerivé “low hill,” gardan “neck;” Mid.Pers. galôg, griv “throat,” gartan “neck;” Av. grīvā- “neck;” cf. Skt. gala- “throat, neck;” Gk. bora “food;” L. gula “throat” (Fr. gueule “(animal) mouth”), gluttire “to gulp down,” vorare “to devour;” PIE base *g^wer- “to swallow, devour.” L. gula; cf. Mod.Pers. galu “throat,”

A jointed ring placed at the upper end of a → planispheric astrolabe astrolabe. By slipping one’s thumb into the ring, one raises the instrument so that its weight and symmetrical design keeps it perpendicular to the ground (online museo galileo, VirtualMuseum).

Etymology (EN): From L. thronus, from Gk. thronos “elevated seat, chair, throne,” from PIE root *dher- “to hold firmly, support;” cf. L. firmus “firm, steadfast, strong, stable,” Skt. dharma- “statute, law;” Pers. dâr-, dâštan “to have, to possess,” → property.

Etymology (PE): Korsi “throne, chair, seat,” from Ar. kursī.

1a) To stop the breath of by compressing the throat; strangle.

1b) To compress by fastening something tightly around.

2a) To obstruct or check the flow of (a fluid), as to control the speed of an engine.

2b) To reduce the pressure of (a fluid) by passing it from a smaller area to a larger one (Dictionary.com).

Etymology (EN): M.E. throtelen, from throten “to cut the throat of (someone), strangle,”
from → throat.

Etymology (PE): Tâsenidan, from Dezfuli tâsenidan “to choke, compress the throat,” cf. Laki, Šuštari tâsenan “to strangle,” Ilâmi tâsânen “to strangle,” Baxtiyâri tâsest “suffocated,” Fini Bandar-Abbâs tâsaki “exhausted,” (Dehxodâ) tâsidan “to be afflicated or sad,” tâsidé “tired, emaciated,” Baluci ta(n)sit “to be out of breath, pant;” Skt. tam- “to become suffocated, exhausted;” L. temulentus “drunken;” PIE *temH- “to faint, be exhausted” (Cheung 2007).

1. Thermodynamics: A process in which a gas, originally at a constant high pressure, passes → adiabatically
   through a porous wall or a narrow opening into a region of constant lower pressure. The throttling process is → irreversible and is accompanied by an increase of → entropy. The → enthalpy of the gas is the same in the → initial and → final → states. Also called → Joule-Thomson expansion. The change in the temperature of the gas in throttling is known as the → Joule-Thomson effect.
   

2. A technique implemented in the architecture of computers whereby the frequency produced by the microprocessor is automatically adjusted. Throttling process allows the computer to conserve
   power and also reduce the heat generated by its chip.

See also: → throttle; → process.

1. In at one end, side, or surface and out at the other.
   

2. From one to the other of; between or among the individual members or parts of.
   

3. Over the surface of, by way of, or within the limits or medium of.
   

4. During the whole period of (Dictionary.com).

Etymology (EN): M.E. (preposition and adv.), metathetic variant of thourgh, O.E. thurh, (cognates O.S. thuru, OFris. thruch, Du. door, O.H.G. thuruh, Ger. durch), cognate with Av. tarô, tarə “over, across, beyond,” L. trans-, → trans-.

Etymology (PE): Târu, related to tarâ-, → trans-, and from Av. tarô, tarə, as above.

To propel something through the air in any way, especially by
swinging the arm and releasing the object from the hand; → ejecter.

Etymology (EN): M.E. throwen, thrawen, “to twist, turn writhe” (cf. O.S. thraian, M.Du. dræyen, Du. draaien, O.H.G. draen, Ger. drehen “to turn, twist”).

Etymology (PE): Andâxtan, andâz-, from Mid.Pers. handâxtan, handâz-; ultimately from Proto-Iranian *ham-tak-, from *ham- “together, with, same,” → syn- + *tak- “to run, to flow;” cf. Av. tak- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
tacar- “course,” tacan “current, streaming;” Mod.Pers. tacidan, tâxtan, tâzidan “to run; to hasten; to assault,” tajan name of a river (initially “flowing, streaming, stream”), tâzi “swift (greyhound),” tak “running, rush,” from
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;” 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.

The force that is exerted by a rocket, propeller, or jet engine to propel an aircraft. It is directed forward along the axis of the engine. → drag; → lift.

Etymology (EN): M.E. thrusten, thrysten (v.); O.N. thrysta “to thrust, force.”

Etymology (PE): Pišrâné, from
piš “before; in front” (Mid.Pers. pêš “before, earlier,” O.Pers. paišiya “before; in the presence of”) + râné, from rândan “to push, drive, cause to go”
(causative of raftan “to go, walk, proceed,”
present tense stem row-, Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

A fourth magnitude star (V = 3.65), called also α Draconis,
in the constellation → Draco. Despite its designation as Alpha (α), it is the seventh brightest star of the constellation. Thuban is a → giant star of → spectral type A0 III lying 310 → light-years away. It has an faint → companion in an orbit with a 51 day period. Thuban was the → pole star at about 2700 BC. Other designations: HR 5291, HD 123299, and SAO 16273.

See also: Thuban, from Ar. Ath-thu’bân (الثعبان) “a large male snake; basilisk.”

A soft, malleable, ductile, lustrous silver-white metal; symbol Tm. Atomic number 69; atomic weight 168.9342; melting point about 1,545°C; boiling point 1,947°C; specific gravity 9.3.
Thulium was discovered in 1879 by the Swedish chemist Per Theodor Cleve in a sample of erbium mineral. It was first isolated by the American chemist Charles James in 1911.

See also: From Thule, the earliest name for Scandinavia.

A loud rumbling sound emitted by rapidly expanding air along the path of the electrical discharge of lightning.

Etymology (EN): M.E. thonder, thunder, O.E. thunor, from P.Gmc. *thunraz (cf. O.Fris. thuner, M.Du. donre, Du. donder, O.H.G. donar, Ger. Donner “thunder”), cognate with Pers. tondar, as below.

Etymology (PE): Tondar; Mid.Pers. tndwr, Sogdian twntr; cf. Skt. stan- “to thunder, resound,” tanyati “thunders, roars,” tanyu- “thundering,” stanatha- “thunder;” L. tonare “to thunder,” tonitrus “thunder” (Fr. tonnerre); PIE base *(s)tene- “to resound, thunder.”

A → storm of → thunder and → lightning. Thunderstorms are associated with → convective clouds (Cumulonimbus) and are often accompanied
by → precipitation. They are usually short-lived and hit on only a small area.

Etymology (EN): → thunder; → storm.

Etymology (PE): Tondar-tuf, tufân-e tondari, from tondar, → thunder + tuf stem of tufidan “to roar, to raise a tumult,” tufân “storm, the roaring of the sea, the confused hum of men or animals.” This Persian word may be related to Gk. typhon “whirlwind, mythical monster associated with tempests.”

Anatomy: The inner of the two bones of the leg, that extend from the knee to the ankle and articulate with the femur and the talus; shinbone (Dictionary.com).

Etymology (EN): From L. tibia “shinbone,” also “pipe, flute,” of unknown origin.

Etymology (PE): Dorošt-ney, literally “large reed,” from dorošt “large,” → macro-, + ney “reed, pipe, flute.”

1. A small dot, mark, check, or electronic signal, as used to mark off an item on a list, serve as a reminder, or call attention to something (Dictionary.com).
   

2. To mark with a tick or ticks (usually followed by off).

Etymology (EN): M.E. tek “little touch,” akin to Du. tik “a touch, pat,”
M.H.G. zic.

Etymology (PE): Sof, sofidan, related to sufâr “the groove at the end part of an arrow,” → nock, on the model of Fr. coche “notch, score.”

Of, pertaining to, characterized by, or subject to → tides.

See also:
→ tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal disruption event (TDE), → tidal disruption flare, → tidal dwarf galaxy, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching, → tidal tail.

Etymology (EN): A hybrid formation from → tide + Latin-derived suffix → -al.

Etymology (PE): Adj. from kašand, → tide.

The physical process that slows the → Earth’s rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its
rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth’s → day and → month are equal, at which point the Moon will never seem to move in Earth’s sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

See also: → tidal; → braking.

Either of the two swells of land or water (on Earth) created by the pull of another object (Moon) orbiting around it. The → gravitational attraction between the → Earth and the → Moon is strongest on the side of the Earth that happens to be facing the Moon. This attraction causes the water on this “near side” of Earth to be pulled toward the Moon. As gravitational force acts to draw the water closer to the Moon, → inertia attempts to keep the water in place. But the gravitational force exceeds it and the water is pulled toward the Moon, causing a “bulge” of water on the near side toward the Moon.

On the opposite side of the Earth, or the “far side,” the gravitational attraction of the Moon is less because it is farther away. Here, inertia exceeds the gravitational force, and the water tries to keep going in a straight line, moving away from the Earth, also forming a bulge. In this way the combination of gravity and inertia creates two bulges of water (Ross, D.A. 1995. Introduction to Oceanography. New York, NY: Harper Collins. pp. 236-242).

See also: → tidal; → bulge.

A process in which two stars remain → bound after their → close encounter, leading to the formation of a → binary system. Tidal capture becomes possible when two stars pass each other so closely (within a few stellar radii) that their → tidal forces are able to absorb the excess energy of → unbound → orbital motion. The process was originally suggested by Fabian et al. (1975) to explain the origin of → low-mass X-ray binary systems observed for the first time in → globular clusters.

See also: → tidal; → capture.

In a system composed of one celestial body orbiting another, the synchronization of the orbital and rotational motions of the two bodies under the action of → tidal forces. For example, Pluto is tidally coupled to its moon Charon. As for the → Earth-Moon system,
billions of years from now, the Earth and the Moon will have the same period of rotation, and these will also exactly equal the orbital period of the Moon around the Earth. → tidal friction.

See also: → tidal; → coupling.

The water current brought about by the → tides.

See also: → tidal; → current.

The disruption of an extended astronomical object under the action of the → tidal forces exerted by another nearby object.

See also: → tidal; → disruption.

The process in which a star is torn apart by the → tidal forces of a → supermassive black hole. About 50% of the star’s mass is eventually → accreted by the → black hole, generating a flare, which, in extreme cases of very high (→ super-Eddington) mass → accretion rates, can result in a → relativistic jet. TDEs have been proposed as sources of → ultra-high-energy cosmic rays and suggested as sources of high energy astrophysical → neutrinos (W. Winter and C. Lunardini, 2021, Nature Astronomy, arXiv:2005.06097 and references therein).

See also: → tidal; → disruption; → event.

A luminosity enhancement in the → light curve of a galaxy observed in X-rays or ultraviolet surveys supposed to be associated with the → tidal disruption of a star that has passed close to a → supermassive black hole in the core of a → host galaxy. An → accretion disk forms after the tidal disruption. The flare event marks the beginning of the accretion process onto the black hole.

See also: → tidal; → disruption; → flare.

A self-gravitating entity which has been formed from tidal material expelled during interactions between larger galaxies. TDGs are typically found at the tip of tidal tails at distances between 20 and 100 kpc from the merging galaxies, of which at least one should be a gas-rich galaxy. They are gas-rich objects that can be as massive as the Magellanic Clouds, form stars at a rate which might be as high as
in blue compact dwarf galaxies and seem dynamically independent from their parent galaxies.

See also: → tidal; → dwarf;
→ galaxy.

The → gravitational force exerted on an extended body
as a result of the difference in the strength of gravity between near and far parts of the body. The ocean tides on Earth result from the varying gravitational force of the Moon exerted on the Earth’s oceans closest and farthest from the Moon. Tidal force, which is the → gradient of the gravitational force, varies as 1/r³. More specifically, F_tidal = dF/dr = (2GMm)/r³, where M is mass of the → primary body,
m is mass of the → secondary body, r is distance between objects, and G the → gravitational constant. The total tidal force experienced across a body is equal to the tidal force (force per unit distance) multiplied by the diameter of that body:

F_tt = F_tidal x 2R (provided that radius
R is much smaller than r). It is obvious that the tidal force experienced by Earth at Moon’s → perigee is larger than that at the → apogee. If the tidal force is stronger than a body’s cohesiveness, the body will be disrupted. The minimum distance that a secondary comes to a primary before it is shattered by tidal force is called its → Roche limit. Tidal forces create → tidal heating.

See also: → tidal; → force.

The → friction exerted on a → primary body (Earth) because of the → phase lag between the → tides and the → gravitational attraction of the → secondary body (Moon).
The Earth’s → rotation is faster than the Moon’s orbital motion; therefore the Earth’s → tidal bulges lead the Moon on its orbit. This has two important effects: The Earth is being pulled slightly “back” from its sense of rotation. So the Earth’s rotation slows (by about 1 second every 50,000 years). Moreover, the Moon is being pulled slightly “forward” on its orbit. So it is harder for the Earth to hold it in place, and it moves further away from the Earth (by about 3-4 cm per yr). Tidal friction tends to synchronize the rotation period of a close-in companion with the period of its orbital motion around the primary. → tidal coupling.

See also: → tidal; → friction.

The heating of the → interior of a → planet or → satellite due to the → friction caused by → tidal forces. For example, the huge tidal forces by → Jupiter heat its close satellite → Io, making it a seismically very active body.

See also: → tidal; → heating.

The process whereby the → rotation period of a → primary body becomes identical to the → orbital period of a → secondary body. Tidal locking results from → tidal braking and leads to → synchronous rotation.

See also: → tidal; → lock; → -ing.

Same as → Roche limit.

See also: → tidal; → radius.

The stretching of an object under → tidal force. Tidal stretching results from a difference in the gravitational pull felt on two sides of a body. It is proportional to the inverse cube of the distance to the source of gravity (1/r³). As a consequence, nearby objects, even small ones like the Moon, raise high tides, whereas distant giants like Jupiter do not produce much of an effect.

See also: → tidal; → stretching.

The phenomenon whereby gas and stars are ripped out from a gravitationally → bound system, such as a galaxy or → globular cluster, by the action of → tidal forces from an external, more massive object. See also → ram pressure stripping.

See also: → tidal; → strip.

A long stream of stars and gas, often in the form of a spectacular tail, thrown off a galaxy when it collides with another galaxy. → interacting galaxies; → merger. Two tidal tails form in each galaxy, and they are more spectacular when the masses of the two galaxies are comparable, and when their relative orbit is in the same sense as the rotation inside each spiral galaxy.

See also: → tidal; → tail.

The description of a → system of two bodies undergoing → tidal locking.

See also: → tidal; → -ly; → lock.

Describing a → stellar system that has undergone → tidal stripping.

See also: → tidal; → -ly; → strip.

1. The periodic rising and falling of the waters of the ocean and its inlets. The tides result from the → gravitational attraction of the → Moon and → Sun acting upon the rotating → Earth. See also: → ebb tide, → high tide, → low tide, → neap tide, → spring tide, → tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal current, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.
   

2. → tidal force.

Etymology (EN): M.E.; O.E. tid “time, hour” (cf. O.S. tid, Du. tijd, O.H.G. zit, Ger. Zeit “time”).

Etymology (PE): Kešand, from Mod./Mid.Pers. kešidan/kašidan “to draw, protract, trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
Av. karš- “to draw; to plow,” karša- “furrow;” Proto-Iranian *kerš-/*xrah- “to draw, plow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow.”

Firmly or closely fixed in place. → compact.

Etymology (EN): M.E. thight, from O.N. thettr “watertight, close in texture, solid” (cf. second element in O.E. metethiht “stout from eating;” M.H.G. dihte “dense, thick,” Ger. dicht “dense, tight,” O.H.G. gidigan, Ger. gediegen “genuine, solid, worthy”), from PIE base *tenk- “to become firm, curdle, thicken;” cf. Ir. techt “curdled, coagulated,” Lith. tankus “close, tight;” cognate with Pers. tang “tight,” as below.

Etymology (PE): Tang “tight; narrow, straight; tight,” also “horse girth, a strap for fastening a load” (Mid.Pers.
tang “tight, narrow”), tanjidan “to squeeze, press, pull together;”
cf. Skt. tanákti “draws together, contracts;” cognate with E. tight, as above; PIE base *tenk- “to become firm, curdle, thicken.”

A cluster of stars in which members are closely situated so that high resolution observations are required to distinguish them individually.

See also: → tight; → star cluster.

→ close binary star.

See also: → tight; → bound system; → binary star.

Optics: A deviation in the propagation direction of a beam of light.
Tilt quantizes the average slope in both the X and Y directions of a → wavefront or phase profile across the pupil of an optical system.

Etymology (EN): M.E. tylten “to upset, tumble,” from tealt “unsteady” (cf. O.N. tyllast “to trip,” Swed. tulta “to waddle,” Norw. tylta “to walk on tip-toe,” M.Du. touteren “to swing”).

Etymology (PE): Gerâ, present stem of gerâyidan “to incline toward; to intend; to make for.”
Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

The angle a rocket makes with the vertical as it curves along its trajectory.

See also: → tilt; → angle.

1. A non-spatial sequential relation in which events occur in apparently irreversible succession from the past through the present to the future. → time’s arrow.
   

2. A limited period or interval, as between two successive events.

Etymology (EN): M.E.; O.E. tima “limited space of time,” from P.Gmc. *timon “time” (cf. O.N. timi “time,” Swed. timme “an hour”), akin to L. tempus (genitive temporis) “time” (Fr. temps, Sp. tiempo, It. tempo); maybe related to Pers. Tabari tum, tomon, temen “time;” Aftari ton “time.”

Etymology (PE): Zamân “time,” from Mid.Pers. zamân, jamân “time,” zamânak “period, epoch;”
loaned into Aramaic and Ar., loaned into Arm. žam, žamanak “time;” prefixed Sogdian nγm “time, moment, hour;” Proto-Iranian *gām- “to go, to come;”
cf. Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
“step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE base *g^wem- “to go, come.”
Gâh “time; place;” Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”
Vaqt, pronounced vaxt (وخت), but written vaqt (وقت),
is a Pers. word meaning “portion (of time)”. Its variants and related words in Mod./Mid.Pers. are: baxt “what is allotted, fate, fortune,” baxš “portion, part, division,” baxšidan, baxtan “to divide, distribute, grant,” Av. base bag- “to attribute, allot, distribute,” baxš- “to apportion, divide, give to,”
baxta- “what is allotted (luck, fortune),” baxədra- “part, portion,” baγa- “master, god,” O.Pers. bāji- “tribute, tax,” cf. Skt. bhaj- “to share, divide, distribute, apportion,” bhájati “divides,” bhakta- “allotted; occupied with; a share; food or a meal, time of eating?,” Gk. phagein “to eat (to have a share of food)”; PIE base *bhag- “to share out, apportion.”
Tâmen “time,” from Tabari temen, tumun, tum “time,” pərtəmən “long time;” Lori temen “age, length of life;” Aftari ton; Lâri, Garâši taim “time span” (related to L. tempus?).

The assignment of telescope time by an expert panel to proposals after evaluating the merits of the observation projects.

See also: → time; → allocation.