# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 Kelvin scale   مرپل ِ کلوین   marpel-e KelvinFr.: échelle de Kelvin   A temperature scale, redefined in 1954, in which the zero point is equivalent to -273.16 °C. This fundamental fixed point, based on the → triple point of water, is considered to be the lowest possible temperature of anything in the Universe. Also known as the absolute temperature scale.→ kelvin (K); → scale. Kelvin's postulate   فراوس ِ کلوین   farâvas-e KelvinFr.: postulat de Kelvin   A transformation whose only final result is to transform into work heat extracted from a source which is at the same temperature is impossible. Kelvin's postulate is a statement of the → second law of thermodynamics and is equivalent to → Clausius's postulate.→ kelvin; → postulate. Kelvin-Helmholtz contraction   ترنگش ِ کلوین-هلمهولتس   terengeš-e Kelvin-HelmholtzFr.: contraction de Kelvin-Helmholtz   The contraction of a volume of gas under its → gravity, accompanied by the → radiation of the lost → potential energy as → heat.After the Scottish physicist William Thomson, also known as Lord Kelvin (1824-1907) and the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → contraction. Kelvin-Helmholtz instability   ناپایداری ِ کلوین-هلمهولتس   nâpâydâri-ye Kelvin-Helmholtz (#)Fr.: instabilité de Kelvin-Helmholtz   An → instability raised when there is sufficient velocity difference across the interface between two uniformly moving → incompressible fluid layers, or when velocity → shear is present within a continuous fluid. Kelvin-Helmholtz mechanism   ساز-و-کار ِ کلوین-هلمهولتس   sâzokâr-e Kelvin-HelmholtzFr.: mécanisme Kelvin-Helmholtz   The heating of a body that contracts under its own gravity. For a large body like a planet or star, gravity tries to compress the body. This compression heats the core of the body, which results in internal energy which in turn is radiated as → thermal energy. In this way a star could be heated by its own weight.William Thomson (Lord Kelvin) and Hermann von Helmholtz proposed that the sun derived its energy from the conversion of gravitational potential energy; → mechanism. Kelvin-Helmholtz time   زمان ِ کلوین-هلمهولتس   zamân-e Kelvin-HelmholtzFr.: échelle de temps de Kelvin-Helmholtz   The characteristic time that would be required for a contracting spherical cloud of gas to transform all its → gravitational energy into → thermal energy. It is given by the relation: tKH ≅ GM2/RL, where G is the → gravitational constant, M is the mass of the cloud, R the initial radius, and L the → luminosity. The Kelvin-Helmholtz time scale determines how quickly a pre-main sequence star contracts before → nuclear fusion starts. For the Sun it is 3 x 107 years, which also represents the time scale on which the Sun would contract if its nuclear source were turned off. Moreover, this time scale indicates that the gravitational energy cannot account for the solar luminosity. For a → massive star of M = 30 Msun, the Kelvin-Helmholtz time is only about 3 x 104 years. Kennelly-Heaviside layer   لایه‌ی ِ کنلی-هوی‌ساید   lâye-ye Kennelly-Heaviside (#)Fr.: couche de Kennelly-Heaviside   One of several layers in the Earth's ionosphere occurring at 90-150 km above the ground. It reflects medium-frequency radio waves whereby radio waves can be propagated beyond the horizon.Named after the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the English physicist Oliver Heaviside (1850-1925), who independently predicted the existence of the reflecting layer in 1902; → layer. Kepler   کپلر   Kepler (#)Fr.: Kepler   Johannes Kepler (1571-1630), a German mathematician and astronomer and a key figure in the 17th century astronomical revolution. He discovered that the Earth and planets travel about the Sun in elliptical orbits; gave three fundamental laws of planetary motion, and also did important work in optics and geometry. Kepler problem   پراسه‌ی ِ کپلر   parâse-ye KeplerFr.: problème de Kepler   1) Given the trajectory of a particle moving in a → central force field, determine the law governing the central force. 2) Inversely, considering a central force -k/r2, determine the trajectory a particle moving in the field will take.→ Kepler; → problem. Kepler spacecraft   تلسکوپ ِ فضایی ِ کپلر   teleskop-e fazâyi-ye KeplerFr.: télescope spatial de Kepler   A → NASA space telescope launched in March 2009 to discover Earth-size planets using the → transit method. The telescope has a diameter of 0.95 m and its only instrument is a → photometer that continuously monitors the brightness of over 145,000 → main sequence stars in a fixed field of view of 115 deg2 (about 12° diameter). The expected mission lifetime is 3.5 years extendible to at least 6 years.In honor of Johannes → Kepler; → spacecraft. Kepler's equation   هموگش ِ کپلر   hamugeš-e KeplerFr.: équation de Kepler   An equation that enables the position of a body in an elliptical orbit to be calculated at any given time from its orbital elements. It relates the → mean anomaly of the body to its → eccentric anomaly.Keplerian, adj. of → Kepler; → equation. Kepler's first law   قانون ِ نخست ِ کپلر   qânun-e naxost-e Kepler (#)Fr.: première loi de Kepler   Planets move in elliptical paths, with the Sun at one focus of the ellipse (year 1609).→ Kepler; → first; → law. Kepler's laws   قانون‌های ِ کپلر   qânunhâ-ye Kepler (#)Fr.: lois de Kepler   1) The planets move about the Sun in ellipses, at one focus of which the Sun is situated. 2) The → radius vector joining each planet with the Sun describes equal areas in equal times. 3) The ratio of the square of the planet's period of revolution to the cube of the planet's mean distance from the Sun is the same for all planets.→ Kepler; → law. Kepler's second law   قانون ِ دوم ِ کپلر   qânun-e dovom-e Kepler (#)Fr.: deuxième loi de Kepler   A line joining a planet to the Sun sweeps out equal areas in equal intervals of time (year 1609).→ Kepler; → second; → law. Kepler's star   ستاره‌ی ِ کپلر   setâre-ye Kepler (#)Fr.: étoile de Kepler   A → supernova in → Ophiuchus, first observed on 1604 October 9, and described by Johannes Kepler in his book De stella nova (1606). It reached a maximum → apparent magnitude of -3 in late October. The star remained visible for almost a year. The → light curve is that of a → Type Ia supernova. The → supernova remnant consists of a few filaments and brighter knots at a distance of about 30,000 → light-years. It is the radio source 3C 358. Also known as SN 1604 and Kepler's supernova.→ Kepler; → star. Kepler's third law   قانون ِ سوم ِ کپلر   qânun-e sevom-e Kepler (#)Fr.: troisième loi de Kepler   The ratio between the square of a planet's → orbital period (P) to the cube of the mean distance from the Sun (a) is the same for all planets: P2∝ a3 (year 1618). More accurately, P2 = (4π2a3) / [G(M1 + M2)], where M1 and M2 are the masses of the two orbiting objects in → solar masses and G is the → gravitational constant. In our solar system M1 = 1. The → semi-major axis size (a is expressed in → astronomical units and the period (P) is measured in years.→ Kepler; → third; → law. Keplerian   کپلری   KepleriFr.: keplerien   Of or pertaining to Johannes Kepler or to his works or discoveries.From → Kepler + -ian a suffix forming adjectives. Keplerian angular velocity   تندای ِ زاویه‌ای ِ کپلری   tondâ-ye zâviye-yi-ye KepleriFr.: vitesse angulaire keplérienne   The angular velocity of a point in a circular orbit around a central mass. It is given by: ΩK = (GM/r3)1/2, where G is the → gravitational constant, M is the mass of the gravitating object, and r is the radius of the orbit of the point around the object.→ Keplerian; → angular; → velocity. Keplerian disk   گرده‌ی ِ کپلری، دیسک ِ ~   gerde-ye Kepleri, disk-e ~Fr.: disque keplérien   A circumstellar disk (such as an → accretion disk or a → protoplanetary disk) in which the → angular velocity at each radius is equal to the angular velocity of a circular → Keplerian orbit at the same radius. The main characteristic of the Keplerian disk is that → orbital velocity varies as r-1/2. This means that an object on an orbit closer to the central mass turns more rapidly than that on a farther orbit. This velocity difference is at the origin of internal friction or kinematic viscous forces between disk particles, which heats up the material.→ Keplerian; → disk. Keplerian orbit   مدار ِ کپلری   madâr-e Kepleri (#)Fr.: orbit keplérienne   The orbit of a spherical object of a finite mass around another spherical object, also of finite mass, governed by their mutual → gravitational forces only.→ Keplerian; → orbit.