An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Thomson
  تامسون   
Thomson

Fr.: Thomson   

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.

Thomson atom
  اتم ِ تامسون   
atom-e Thomson (#)

Fr.: atome de Thomson   

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.

Thomson; → atom.

Thomson cross section
  سکنج‌گاه ِ تامسون   
sekanjgâh-e Thomson

Fr.: section efficace de Thomson   

The → cross section involved the → Thomson scattering of electromagnetic waves by a free electron. It is defined by: σT = 8πre2/3, where re is the classical → electron radius. Its value is 0.665 245 x 10-28 m2.

Thomson; → cross; → section.

Thomson effect
  اسکر ِ تامسون   
oskar-e Thomson

Fr.: effet de Thomson   

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.

Thomson; → effect.

Thomson scattering
  پراکنش ِ تامسون   
parâkaneš-e Thomson (#)

Fr.: diffusion de Thomson   

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.

Thomson; → scattering.

three-dimensional flow
  تچان ِ سه-وامونی   
tacân-e sé-vâmuni

Fr.: écoulement tri-dimensionnel   

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).

three; → dimensional; → flow.

threshold of reaction
  آستانه‌ی ِ واژیرش   
âstâne-ye vâžireš

Fr.: seuil de réaction   

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

threshold; → reaction.

throne
  کرسی   
korsi (#)

Fr.: trône   

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).

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.

Korsi "throne, chair, seat," from Ar. kursī.

tidal disruption
  گسیخت ِ کشندی   
gosixt-e kešandi

Fr.: rupture par effet de marée   

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

tidal; → disruption.

tidal disruption event (TDE)
  رویداد ِ گسیخت ِ کشندی   
ruydâd-e gosixt-e kešandi

Fr.: événement de rupture par effet de marée   

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).

tidal; → disruption; → event.

tidal disruption flare
  آلاو ِ گسیخت ِ کشندی   
âlâv-e gosixt-e kešandi

Fr.: éruption   

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.

tidal; → disruption; → flare.

tidal friction
  مالش ِ کشندی   
mâleš-e kešandi

Fr.: friction de marées   

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.

tidal; → friction.

time allocation
  تسک ِ زمان، ~ وقت   
tesk-e zamân, ~ vaqt

Fr.: attribution de temps de télescope   

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

time; → allocation.

time constant
  پایای ِ زمانی   
pâyâ-ye zamâni

Fr.: constante de temps   

Th speed of response of a detector, usually measured as 1/(2πν), where ν is the chopping frequency at which the responsivity fails to 1/√2 of its maximum value.

time; → constant.

time dilation
  فراخش ِ زمان   
farâeš-e zamân

Fr.: dilatation du temps   

A phenomenon related to special and general relativity.
1) In → special relativity, the apparent shortening of time that occurs at speeds approaching that of light. A clock moving relative to a stationary observer will appear to slow down by a factor √(1- v2/c2), where v is the velocity and c the speed of light. → twins paradox.
2) In → general relativity, a clock in a stronger gravitational field runs more slowly. The dilation factor is given by: √(1- 2GM/rc2), where G is the gravitational constant, M the mass of the object creating the gravitational field, r a radial coordinate of the observer, which is analogous to the classical distance from the center of the object, and c the speed of light.

time; dilation, verbal noun of dilate, from M.E. dilaten, from O.Fr. dilater, from L. dilatare "make wider, enlarge," from → dis- "apart" + latus "wide."

Farâxeš, → dilation; zamân, → time.

time resolution
  واگشود ِ زمانی   
vâgošud-e zamâni

Fr.: résolution temporelle   

Same → temporal resolution.

time; → resolution.

time zone
  زنار ِ زمان، زمان-زنار   
zonâr-e zamân, zamân-zonâr

Fr.: fuseau horaire   

Any of the 24 zones on the Earth surface delimited by → meridians at approximately 15° intervals. In each time zone a common standard time is used, and the time is one hour earlier than the zone immediately to the east.

time; → zone.

ton
  تن   
ton (#)

Fr.: tonne   

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.

tone
  تن   
ton (#)

Fr.: son   

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.

Toomre criterion
  سنجیدار ِ تومره   
sanjidâr-e Toomre

Fr.: critère d'Ostriker-Peebles   

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

Toomre criterion; → criterion.


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