An Etymological Dictionary of Astronomy and Astrophysics
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The largest and the sixth moon of → Saturn discovered by Christiaan Huygens in 1655. Called also Saturn VI. Titan has a diameter of 5,150 km, about half the size of Earth and almost as large as Mars. It orbits Saturn at a mean distance of 1,221,830 km every 15.945 days. It is the only moon known to have an → atmosphere. Its surface temperature is -179 °C, which makes water as hard as rocks and allows → methane to be found in its liquid form. Its surface pressure is slightly higher than Earth's pressure (1.6 bars against 1 bar at sea level). The Huygens probe released from → Cassini-Huygens landed on Titan on December 25, 2004. From the data obtained by Cassini-Huygens, we know that Titan is a world with lakes and seas composed of liquid methane and → ethane near its poles, with vast, arid regions not made of silicates as on Earth, but of solid water ice coated with → hydrocarbons that fall from the atmosphere. Titan's icy dunes are gigantic, reaching, on average, 1 to 2 km wide, hundreds kilometers long and around 100 m high. Titan is the only other place in the solar system known to have an Earth-like cycle of liquids flowing across its surface as the planet cycles through its seasons. Each Titan season lasts about 7.5 Earth years. The Huygens probe made the first direct measurements of Titan's lower atmosphere. Huygens also directly sampled → aerosols in the atmosphere and confirmed that → carbon and → nitrogen are their major constituents. Cassini followed up Huygens' measurements from orbit, detecting other chemicals that include → propylene and poisonous → hydrogen cyanide, in Titan's atmosphere. Cassini's gravity measurements of Titan revealed that this moon is hiding an internal, liquid water and → ammonia ocean beneath its surface. Huygens also measured radio signals during its descent that strongly suggested the presence of an ocean 55 to 80 km below the moon's surface.

In Gk. mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

The fourteenth and largest of → Uranus's known satellites. It has a diameter of 1578 km and orbits its planet at a mean distance of 436,270 km. Titania was discovered by Herschel in 1787. Also called Uranus IV.

Titania is the Queen of the Fairies and wife of Oberon in Shakespeare's Midsummer-Night's Dream.

A dark-gray or silvery, very hard, light metallic element, occurring combined in various minerals; symbol Ti. Atomic number 22; atomic weight 47.88; melting point 1,675°C; boiling point 3,260°C; specific gravity 4.54 at 20°C. It is used in metallurgy to remove oxygen and nitrogen from steel and to toughen it.

It was originally discovered by the English clergyman William Gregor in the mineral ilmenite (FeTiO₃) in 1791. It was rediscovered in 1795 by the German chemist Martin Heinrich Klaproth, who called it titanium because it had no characteristic properties to use as a name; from Titan + -ium.

Titan, loan from Fr., as above.

A → diatomic molecule made up of → titanium and → oxygen atoms. See → TiO band.

→ titanium; → band.

The empirical rule relating the approximate distances of the → solar system  → planets from the → Sun. The original formulation was: a = (n + 4) / 10, where a is the mean distance of a planet from the Sun in → astronomical units and n = 0, 3, 6, 12, 24, 48, 96, 192 (doubling for each successive planet). The planets were seen to fit this sequence quite well, provided the → asteroids between → Mars and → Jupiter are counted as one planet, as did → Uranus discovered in 1781. However, → Neptune and the ex-planet → Pluto do not conform to the rule. The question of whether there is any physical significance to the "law," i.e. some dynamical reason that will explain planetary orbit spacing has led to much discussion during the past two centuries. Today, many astronomers are very skeptical and consider this "laws" to be numerical coincidence.

Named after the German mathematician Johann Titius (1729-1796), who first found the law in 1766, and the German astronomer Johann Elert Bode (1747-1826), who published it in 1772; → law.

The distinguishing name of a book, poem, picture, piece of music, or the like (Dictionary.com).

M.E., from O.Fr. title and in part from O.E. titul, both from L. titulus "inscription, label, heading; honorable appellation," of unknown origin.

Sarâl, from sar, → head, + noun suffix -âl, → -al.

A measure of the explosive strength of a nuclear bomb, expressed in terms of the weight of → trinitrotoluene which could release the same amount of energy when exploded. The Hiroshima atomic (fission) bomb created a blast equivalent to 16 kilotons of TNT. The first hydrogen (thermonuclear) bomb test released an energy of about 10 megatons of TNT. See also → megaton of TNT.

→ trinitrotoluene (TNT); → equivalent.

A tailless amphibian with a short stout body and related to → frogs. In contrast to frogs, it has short legs and dry warty skin that can secrete a toxic, milky substance.

M.E. tode, toode, tade, tadde, from O.E. tadige, tadie, of unknown origin, cognate with Scots tade, taid, taed, ted "toad;" Dan. tudse, Swed. tassa, tossa, O.E. taxe, tosca "toad."

Vazaq "toad," variants Tabari, Aftari vak, Tabari vag, (prefixed) qurbâqé, Lori, Laki qorvâ, korvâx, Kurd. baq, Zâzâ baqa; Mid.Pers. vazak, vak; Av. vazaγa- "frog."

1) The present day, time, or age.
2) The present epoch in the cosmic evolution since the → Big Bang characterizd by the → redshift  z = 0.

M.E. today, from O.E. todæge, to dæge "on (this) day," from to "at, on" + dæge, dative of dæg "→ day."

Emruz "today," from Mid.Pers. imrôc, imrôz, from im "this; here" + rôz, ruz, → day.

Each of the five digits on the end of the foot.

M.E. to, from O.E. tā akin to Ger. Zehe "toe," Du. teen.

Pângol, literally "foot finger," from pâ, → foot, + ângol (Dehxodâ) "finger," variant of angošt, → finger.

A machine producing a → toroidal magnetic field for confining a → plasma in → nuclear fusion research.

From Rus. Tokamak, acronym from toroidal'naya kamera s magnitnymi katushkami "toroidal chamber with magnetic coils." It was invented in the 1950s by Soviet physicists Igor Yevgenyevich Tamm and Andrei Sakharov (who had been inspired by an original idea of Oleg Lavrentyev).

A set of astronomical tables drawn up by a group of astronomers in Toledo, Spain, mainly Zarqâli, and compiled after 1068. This work, which represents the first original development of Andalusian astronomy, was extremely influential in Europe for three centuries until the advent of the → Alfonsine Tables. The main sources for the bulk of the table collections were those of the Persian astronomer Khwârizmi (mainly planetary latitudes), Battâni (planetary equations), and Ptolemy. In fact the oldest version of the Toledan Tables was mainly modeled on Khwârizmi's Sindhind, but had admixture from Battâni. In addition, the oldest versions of the Toledan Tables preserve some tables of Khwârizmi that are rare or absent elsewhere. The Toledan Tables also incorporated the theory of → trepidation. The original Arabic version of the Toledan Tables has been lost, but two Latin versions have survived, one by Gerard of Cremona (12th century) and one by an unknown author.

Toledo, a city in central Spain, 70 km south of Madrid; → table.

The maximum permissible error or variation in a dimension of an object.

M.E., from O.Fr. tolerance, from L. tolerantia "endurance," from tolerans, pr.p. of tolerare "to bear, endure, tolerate."

Ravâdâri, noun from ravâdâr "consenter; judging right; lawful," from ravâ "admissible; allowable; tolerated" (from raftan "to go, walk; to flow;" Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack" + -dâr "having, possessor" (from dâštan "to have, to possess," Mid.Pers. dâštan, O.Pers./Av. root dar- "to hold, keep back, maitain, keep in mind," Skt. dhr-, dharma- "law," Gk. thronos "elevated seat, throne," L. firmus "firm, stable," Lith. daryti "to make," PIE *dher- "to hold, support").

A colorless, flammable liquid, insoluble in water and soluble in alcohol and ether, used as a solvent and in the manufacture of other organic chemicals and explosives. Chemical formula C₆H₅CH₃. Same as methylbenzene and phenylmethane. See also → trinitrotoluene.

From tolu, from the older name toluol, which refers to tolu balsam, an aromatic extract from the tropical Colombian tree Myroxylon balsamum, from which it was first isolated, + -ene suffix used to form names of unsaturated hydrocarbons, from Gk. -ene denoting origin or source.

Any of several techniques, such as → Doppler tomography, for constructing a spatial distribution of physical quantity given measurements that are essentially line-integrals ("projections") through the distribution. Most famously, in medical tomography, the absorption of X-rays by a specimen is directly related to the line integral to make detailed images of a predetermined plane section of a solid object while blurring out the images of other planes.

From Gk. tomo- combining form of tomos "a cut, section, slice" tome "cutting" + → -graphy.

Borešnegâri, from boreš "section, slice, cutting," from boridan "to cut" (Mid.Pers. britan, brinitan "to cut off;" Av. brī- "to shave, shear," brin-; cf. Skt. bhrī- "to hurt, injure," bhrinanti "they hurt") + -negâri, → -graphy.

A → metric unit of → mass, equal to 1000 → kilograms.

M.E. tunne unit of weight or capacity (cf. O.Fris. tunne, M.Du. tonne, O.H.G. tunna, Ger. tonne), also found in M.L. tunna and O.Fr. tonne, perhaps from a Celtic source.

A musical sound of definite pitch, consisting of several relatively simple constituents called partial tones, the lowest of which is called the fundamental tone and the others harmonics or overtones.

M.E., from O.Fr. ton, from L. tonus "a sound, tone, accent," literally "stretching," from Gk. tonos "vocal pitch, raising of voice," related to teinein "to stretch," cognate with Pers. tanidan "to spin, weave," → tension.

Ton, loan from Fr., as above.

A criterion for the stability of the disk of a → spiral galaxy. It is expressed by the → Toomre parameter.

→ Toomre criterion; → criterion.

The scale beyond which for a thin, rotating disk, rotation stabilizes self-gravitational contraction. The Toomre length is given by: λ_T = 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).

→ Toomre parameter; → length.

A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = c_sκ / πGΣ, where c_s 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.