An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

→ thermal; → gradient.

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.

→ thermal; → radiation.

Same as → Roche limit.

→ tidal; → radius.

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."

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.
Tufân-e picandé, from tufân→ storm + picandé agant noun/adj. of picidan

An inherited or common body of beliefs or practices belonging to a particular people, family, or institution over a relatively long period. Also their transmission over time.

M.E. tradicion, from  O.Fr. tradicion, from L. traditionem "delivery, surrender, a handing down," from traditus, p.p. of tradere "to deliver, hand over," from → trans- "over" (time) + dare  "to give," → datum.

Tarâdâd, from tarâ- "over time," → trans-, + dâd past stem of dâdan "to give," → datum.

A thought experiment in special relativity, according to which if one of a pair of twins (A) remains on Earth, and the other (B) travels at a speed near the speed of light, B will be younger than A upon returning to Earth. In fact there is no paradox, because the two perspectives, A and B's, are actually not completely symmetric. There is no fixed time difference between the events, and different observers experience different intervals of time between the same two events. In fact, B returns younger than A because only B travels in a non-inertial (accelerating) reference frame. From A's point of view, B experiences time dilation, but from B's point of view the distance traveled is shortened because of length contraction. If B leaves in the year 2000 and returns in 2020, for A 20 years have elapsed. For B it depends on his travel speed. If has has moved as fast as 86% of the speed of light for him 10 years have passed. If his speed has been 99.5% of the speed of light the travel duration for him has been 2 years. This effect has been verified experimentally by measurements with atomic clocks.

Twin M.E.; O.E. twinn; cf. O.N. tvinnr, O.Dan. tvinling, Du. tweeling, Ger. zwillung; → paradox.

Pârâdaxš, → paradox; hamzâdhâ, plural of hamzâd "twin," literally "born together," from ham- "together" → syn- + zâd "born," from zâdan "to bring forth, give birth" (Mid.Pers. zâtan; Av. zan- "to bear, give birth to a child, be born," infinitive zazāite, zāta- "born;" cf. Skt. janati "begets, bears," janitár "progenitor, father;" Gk. genetor "progenitor;" L. gignere "to beget," nasci "to be born," as above, PIE base *gen- "to give birth, beget").

1a) An incline going up in the direction of movement.
1b) An increase or improvement.
1c) A new version, improved model, etc.
2a) To promote to a higher grade or rank.
2b) To improve or enhance the quality or value of (Dictionary.com).

→ up; → grade.

To send data from one's computer to another computer. See also: → loading, → download.

→ up-; → load.

Bârgozâštan, literally "to put load," from bâr, → load, + gozâštan "to put, to place," → passage.

A soft, ductile, silver-grey metal; symbol V. → Atomic number 23; → atomic weight 50.9415; → melting point about 1,890°C; → boiling point 3,380°C; → specific gravity about 6 at 20°C; and → valence +2,+3, +4, or +5. → It is used in various alloys to increase its shock resistance.

Named 1830 by Swedish chemist Nils Gabriel Sefström (1787-1845), from Old Norse Vanadis, epithet of the goddess Freya, + → -ium.

Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.

→ velocity; → gradient.

The radius centered on a galaxy containing matter at 200 times the → critical density of the Universe.

→ virial; → radius.

A → rotating star cannot simultaneously achieve → hydrostatic equilibrium and → rigid body rotation. The paradox can be solved if → baroclinic flows (essentially a → differential rotation and a → meridional circulation) are included. For a broader view of the subject see: M. Rieutord, 2006, in Stellar Fluid Dynamics and Numerical Simulations: From the Sun to Neutron Stars, ed. M. Rieutord & B. Dubrulle, EAS Publ., 21, 275, arXiv:astro-ph/0608431.

→ von Zeipel theorem; → paradox.

To walk through water, snow, sand, or any other substance that impedes free motion or offers resistance to movement (Dictionary.com).

M.E. waden "to go, wade;" O.E. wadan "to go;" cf. Dan. vade, O.Fris. wada, Du. waden, Ger. waten, O.Norse vatha; akin to O.E. wæd "ford, sea," L. vadere "to go, rush," vadum "shoal, ford."

Gampidan, related to Proto-Ir. *gamp-, *gamb- "to move," cf. Dezfuli gomba, Bardesiri gopak "jump with two feet;" Sogd. (+ *â-) âγamp "walking;" (+ *uz-) Yighda žib-/žibi- "to rise, to stand," jib- "to awake;" (+ *ham-) Wakhi gəfs-/gəfst- "to run."

Same as → paradox of youth.