Fr.: longueur de Toomre
The scale beyond which for a thin, rotating disk, rotation stabilizes self-gravitational contraction. The Toomre length is given by: λT = 4π2GΣ / κ2, where G is the → gravitational constant, Σ is the mass → surface density, and κ is the → epicyclic frequency (Toomre 1964, ApJ 139, 1217).
Fr.: paramètre de Toomre
A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = csκ / πGΣ, where cs is the → sound speed, κ the → epicyclic frequency, G the → gravitational constant, and Σ the → surface density. The disk is linearly stable for Q > 1 and linearly unstable for Q < 1.
After Alar Toomre (1936-), an American astrophysicist of Estonian origin, professor of mathematics at the Massachusetts Institute of Technology; → parameter.
âzmâyeš-e carx-e dandâne-dâr
Fr.: expérience de la roue dentée
The experiment which provided the first accurate measurement of the speed of light. The experiment, conducted by the French physicist Armand H. L. Fizeau (1819-1896) in 1849, used a rotating wheel containing 720 teeth. The function of the wheel was to cut a light beam into short pulses and to measure the time required for these pulses to travel to a distant mirror and back (17.34 km). The round-trip time for each pulse could be calculated to be about 1/18,000 sec, which yielded the value of 315,300 km/sec for the speed of light. Leon Foucault (1819-1868) improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 km/s.
Âzmâyeš, → experiment; carx→ wheel; dandâne-dâr "toothed," from dandân "tooth," Mid.Pers. dandân; Av. dantan-; cf. Skt. dánta-; Gk. odontos; L. dens (Fr. dent); Lith. dantis, O.Ir. det, Welsh dent; PIE base *dont-/*dent- "tooth."
Fr.: sommet, du haut, haut
The highest point or part. The higher end of anything on a slope.
M.E., O.E. top "summit, crest, tuft;" cf. O.N. toppr "tuft of hair," O.Fris. top "tuft," O.Du. topp, Du. top, O.H.G. zopf "end, tip, tuft of hair," Ger. Zopf "tuft of hair."
Bâlâ "up, above, high, elevated, height" (variants boland "high, tall, elevated, sublime," borz "height, magnitude" (it occurs also in the name of the mountain chain Alborz), Laki dialect berg "hill, mountain;" Mid.Pers. buland "high;" O.Pers. baršan- "height;" Av. barəz- "high, mount," barezan- "height;" cf. Skt. bhrant- "high;" L. fortis "strong" (Fr. and E. force); O.E. burg, burh "castle, fortified place," from P.Gmc. *burgs "fortress;" Ger. Burg "castle," Goth. baurgs "city," E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg; PIE base *bhergh- "high."
top-down structure formation
diseš-e sâxtâr az bâlâ bé pâyin
Fr.: formation des structures du haut vers le bas
A cosmological model of → structure formation in which larger structures, such as galaxy → superclusters or perhaps even the vast → filaments and → voids, form earlier and then they fragment into smaller structures such as individual galaxies. Opposite of → bottom-up structure formation.
A star formation process in which → massive stars form more abundantly than that predicted by standard models, whereby the top end of the → initial mass function is significantly flatter than the canonical → Salpeter slope.
Fr.: coordonées topocentriques
A coordinate system that uses the observer's location as its central reference point. Usually, the difference in the position of an object in the sky measured using topocentric and geocentric coordinates is very small because most celestial objects are so far away. However, for nearby objects this is not true. The Sun, for example, may appear displaced as much as eight arcseconds from its geocentric position, and the Moon by as much as one degree.
Hamârâhâ, → coordinate; jâ-markazi "topocentric," from jâ "place" (from Mid.Pers. giyâg "place;" O.Pers. ā-vahana- "place, village;" Av. vah- "to dwell, stay," vanhaiti "he dwells, stays;" Skt. vásati "he dwells;" Gk. aesa (nukta) "to pass (the night);" Ossetic wat "room; bed; place;" Tokharian B wäs- "to stay, wait;" PIE base ues- "to stay, live, spend the night") + markazi, of, pertaining to markaz, → center.
Of or relating to → topology.
âk-e topošenâxti, ~ topošenâsik
Fr.: défaut topologique
In → cosmological models, a stable configuration of → matter formed when the → early Universe underwent → phase transitions during which fundamental symmetries were broken. There are a number of possible types of defects, such as domain walls, → cosmic strings, → magnetic monopoles, and → texture s. Same as → cosmic defect.
Fr.: espace topologique
A set X together with a collection of open subsets T that satisfies the three following conditions: 1) The empty set Ø and X are in T. 2) The intersection of a finite number of sets in T is also in T. 3) The union of an arbitrary number of sets in T is also in T.
The study of the properties of geometric figures that remain invariant under certain transformations, as bending or stretching. A circle is topologically equivalent to an ellipse (into which it can be deformed by stretching) and a sphere is equivalent to an ellipsoid.
From topo- combining form of Gk. topos "place" + → -logy.
Topošenâsi, from topo-, loan from Gk., as above, + šenâsi→ -logy.
A light to be carried in the hand, consisting of some combustible substance, as resinous wood, or of twisted flax or the like soaked with tallow or other flammable substance, ignited at the upper end (Dictionary.com).
M.E. torche, from O.Fr. torche "torch," originally "twisted thing," then "torch formed of twisted tow dipped in wax," probably from V.L. *torca, alteration of L.L. torqua, from torquere "to twist," → torque.
Âhir, from Kurd. âhir "torch," variant of âzar, → fire.
Fr.: échelle de Turin
A scale used to assess the threat posed by the → impact of a → near-Earth object (NEO). It considers the impact energy as well as the probability of impact. It is designed to communicate to the public the risk associated with a NEO in more qualitative form than the → Palermo scale. The → impact hazard is expressed by a number between 0 and 10 depending on the probability that an impact will occur and the kinetic energy of the potential → impactor. The extent of damage ranges from inconsequential (0) to catastrophic (10). The scale is color-coded such that white = no consequence; green = meriting careful monitoring; yellow = meriting concern; orange = threatening events; red = impact is certain.
Named for Torino, because the scale was adopted by that city in Italy in 1999; → scale.
pic-tuf, tufân-e picandé
A mass of rotating air with high wind speeds at its center. It is produced in a very severe thunderstorm and appears as a funnel cloud extending from the base of a Cumulonimbus to the ground.
Tornado, metathesis from Sp tronada "thunderstorm," from tronar "to thunder," from L. tonare "to thunder," → thunder.
Pic-tuf, from pic present stem of picidan
"to twist, entwine, coil" (Mid.Pers. pecidan "to twist, entwine") +
tuf short for tufân, → storm.
A surface of revolution obtained by rotating a closed plane curve about an axis parallel to the plane which does not intersect the curve. The simplest toroid is the → torus. The solid enclosed by such a surface.
toroidal magnetic field
meydân-e meqnâtisi-ye cambarvâr
Fr.: champ magnétique toroïdal
A magnetic field which is generated in a → plasma inside a → toroid, as in a → tokamak, by the electric current which spirals around the toroid. Toroidal field has no radial component. → poloidal magnetic field.
The tendency of a → force applied to an object to cause the object to → rotate about a given → axis or → point. Torque is the rotational analogue of or the turning effect of a force. It is equal to the product of the force and its distance from the reference axis. More specifically, if a force F acts on a single particle at a point P whose position with respect to the origin O of the inertial reference is given by the distance vector r, the torque T acting on the particle with respect to the origin O is defined as: T = r × F. Torque is a → vector quantity. Its magnitude is given by: rF sin θ, where θ is the angle between r and F; its direction is normal to the plane formed by r and F. The sense is given by the → right-hand screw rule for the → vector product of two vectors. Torque has the same dimensions as work, but work is a scalar.
From L. torquere "to twist;" cf. Skt. tarku- "spindle;" maybe Mod.Pers. duk "spindle;" Mid.Pers. dôk "spindle;" O.C.S. traku "band, girdle," O.H.G. drahsil "turner," Ger. drechseln "to turn on a lathe;" O.E. thweorh "transverse, perverse, angry, cross;" E. thwart; PIE base *twork-/*twerk- "twist"
Gaštâvar literally "that which makes turn, turning agent," from gašt "turning," past stem of gaštan, gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- "to turn, bend") + âvar agent noun of âvardan "to bring; to cause, produce" (Mid.Pers. âwurtan, âvaritan; Av. ābar- "to bring; to possess," from prefix ā- + Av./O.Pers. bar- "to bear, carry," bareθre "to bear (infinitive)," bareθri "a female that bears (children), a mother;" Mod.Pers. bordan "to carry;" Skt. bharati "he carries;" Gk. pherein; L. fero "to carry").
A unit of pressure used in the field of high vacuum, equivalent to 1 mm of mercury
After the Italian scientist Evangelista Torricelli (1608-1647), who invented the first barometer.
A stream of water flowing with great rapidity and violence. See also → rapids.
M.E., from M.Fr. torrent, from L. torrentem (nominative torrens) "a rushing (stream)," from torrere, "to parch, dry up, roast," → torrid.
Lâhez, from Tabari lahez "an overwhelming flow," Baxtiyâri lâhiz "flood." The first component lâh, lah maybe related to lur, Lori, Kordi laf, Tabari lé "flood," → cataclysm.
Fr.: loi de Torricelli
In fluid dynamics, a theorem that relates the speed of fluid flowing out of an opening to the height of fluid above the opening: v = (2gh)1/2, where v is the exit velocity of the water, h is the height of the water column, and g is the acceleration due to gravity (9.81 m/s2). It was later shown to be a particular case of → Bernoulli's theorem.
After the Italian scientist Evangelista Torricelli (1608-1647), who found this relationship in 1643.