An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



<< < B r bac Bal Bar Bar Bay Bec Ber bet bic bij bin bio Bir bla Bla blu blu bol Boo bou bra bre bri bub by > >>

Number of Results: 502
Biruni's method
  روش ِ بیرونی   
raveš-e Biruni

Fr.: méthode de Biruni   

A method devised by the Iranian astronomer Biruni (973-1048) to measure the Earth radius, using trigonometric calculations. In contrast to foregoing → Eratosthenes' method and → Mamun's method, which required expeditions to travel long distances, Biruni's method was on-site. He carried out the measurement when he was at Nandana Fort (at the southern end of the pass through the Salt Range, near Baghanwala in the Punjab). He first calculated the height of a hill (321.5 m). To do this he used the usual method of observing the summit from two places in a straight line from the hill top. He measured the distance, d, between the two places and the angles θ1 and θ2 to the hill top from the two points, respectively. He made both measurements using an astrolabe. The formula that relates these angles to the hill height is: h = (d. tan θ1 . tan θ2) / (tan θ2 - tan θ1). He then climbed to the hill top, where he measured the → dip angle (θ), that is the angle of the line of sight to the horizon. He applied the values he obtained for the dip angle and the hill's height to the following trigonometric formula to derive the Earth radius: R = h cosθ / (1 - cos θ). The result for the Earth radius was 12,851,369.845 cubits (or 6335.725 km, using favorable conversion units). Despite the fact that the method is very ingenious, such a precise value is only by chance, because of several drawbacks: The plane was not perfectly flat to serve as the smooth surface of the sea. A measuring instrument more accurate than the alleged 5 arc minutes was needed. And the method suffered from the → atmospheric refraction (See, e.g., Gomez, A. G., 2010, Journal of Scientific and Mathematical Research).

Abu Rayhân Mohammad Biruni (973-1048 A.D.), one of the greatest scholars of the medieval era, was an Iranian of the Khwarezm region; → method.

bismut (#)

Fr.: bismuth   

A white, crystalline, brittle metallic chemical element with a pinkish tinge; symbol Bi. → Atomic number 83; → atomic weight 208.9804; → melting point 271.3°C; → boiling point about 1,560°C; → specific gravity 9.75 at 20°C; → valence +3 or +5. Bismuth is the most → diamagnetic of all metals. Its thermal conductivity is lower than any metal, except → mercury. There is only one naturally occurring → isotope of bismuth, 209Bi. Bismuth is used in a number of very different applications, chiefly in bismuth alloys, and in pharmaceuticals and chemicals.

From Ger. Bismuth, Wismut, Wissmuth, probably from weisse Masse "white mass," indicating how the element appears in nature.


Fr.: bissextile   

Of or relating to a → leap year or to the extra day falling in a leap year.

L.L. bissextlis (annus) "year containing an intercalary day," from bisextus, from bis "twice, two, doubled" + sextus "sixth," because in the → Julian calendar the sixth day before the Calends of March was doubled every four years. Same as → leap and → intercalary day.

Andarheli, of or relating to andarhelintercalation.

Note. Some authors have used the Pers. term behizaki for "bissextile," however this is problematic as far as the calendar history is concerned. The term comes from Mid.Pers. vihezagig or vihezakik "movable," which was the name of the additional whole month of 30 days which Iranians used in their calendar system in the Sasanid era. In fact they did not use an intercalary day every four years, but instead added a whole month after a period of 120 years. The operation was called vihezag. The term originates from Mid.Pers. vihez- "to move, progress," vihezag(k) "movement, progression." Therefore, this dictionary cautions against equating behizaki and bissextile. → embolismic month.


Fr.: bistabilité   

The condition in which a physical system is capable of assuming either of two stable states.

bi-; → stable.

bistability jump
  جهش ِ دو-پایداری   
jaheš-e do-pâydâri

Fr.: bistabilité par saut   

An abrupt discontinuity in the → stellar wind properties of → hot stars near → effective temperatures about 21,000 K and 10,000 K, corresponding to O9.5-B3 supergiants (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269). At these temperatures the → terminal velocity of the wind drops steeply by about a factor two and the → mass loss rate increases steeply by about a factor three to five, when going from high to low temperatures. Bistability jump is related to the degree of ionization in the wind. With a little drop in the temperature, the dominant driving element (Fe) will recombine to lower ionization stages which produces a lower terminal velocity and a relatively high density in the wind. → wind momentum. Additional bistability jumps may occur at higher temperatures where CNO may provide the dominant line driving, especially for low metallicity stars (Vink et al. 2001, A&A 369, 574). However, a recent study using a larger sample finds that there is a gradual decline in the wind terminal velocities of early B supergiants and not a "jump" (Crowther et al. 2006, A&A 446, 279).

bistability; → jump.

bistability mechanism
  سازوکار ِ دو-پایداری   
sâzokâr-e do-pâydâri

Fr.: mécanisme de bistabilité   

The mechanism that accounts for the → bistability jump.

bistability; → mechanism.

  بیت، رقم ِ درین   
bit, raqam-e dorin

Fr.: bit   

A contraction of → binary digit, either 0 or 1.

Bit, from binary + digit


Fr.: bivalent   

1) Chem.: Having a → valence of two or having two valences.
2) → bivalent logic.

bi-; → valence.

bivalent logic
  گوییک ِ دو-ارز   
guyik-e do-arz

Fr.: logique bivalente   

A logical system, such as → classical logic, in which every declarative sentence expressing a → proposition has exactly one → truth value, either → true or → false. Bivalent logic is just a sub-set of a more powerful type of logic known as → fuzzy logic. See also → polyvalent logic.

bivalent; → logic.

BL Lac object
  بر‌آخت ِ BL چلپاسه   
barâxt-e BL Calpâsé

Fr.: objet BL Lac   

A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.

BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the "star" was identified with a bright, variable → radio source and a very faint galaxy; → object.

Blaauw mechanism
  ساز-و-کار ِ بلاؤ   
sâzokâr-e Blaauw

Fr.: mécanisme de Blaauw   

A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.

Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.

  سیاه، سیه   
siyâh (#), siyah (#)

Fr.: noir   

Very dark in color; absorbing light without reflecting any of its various rays.

Black, from O.E. blæc "black," from P.Gmc. *blak-, from PIE *bhelg- "to shine, flash, burn" (cf. Gk. phlegein "to burn, scorch," L. flagrare "to blaze, glow, burn," fulgur "lightning").

Siyâh or siyah, from Mid.Pers. siyâ, siyâk, siyâvah "black," Av. sâma-, sayâva- "black, dark," cf. Skt. syama-, syava- "black, brown," Gk. skia "shadow."

black drop
  چکه‌ی ِ سیاه   
cekke-ye siyâh

Fr.: goutte noire   

The appearance of a band linking the solar limb to the disk of a transiting planet (Venus or Mercury) near the point of internal tangency. This effect increases the uncertainty in measuring the period from when the planet fully enters the solar disk to when it begins to depart. Historically, the black drop phenomenon limited the accuracy of the determination of the Astronomical Unit and the scale of the Solar System in the 18th and 19th centuries. While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury in 1999, which was observed by the NASA's TRACE satellite. Two effects could fully explain the black drop: the inherent blurriness of the image caused by the finite size of the telescope (→ point spread function), and an extreme dimming of the Sun's surface just inside the apparent outer edge (→ limb darkening). See Schneider et al. 2004, Icarus 168, 249.

black; → drop.

black hole
  سیه‌چال، ~ سوراخ   
siyah câl (#), ~ surâx (#)

Fr.: trou noir   

A fantastically → compact object, predicted by the theory of → general relativity, whose → gravity is so powerful that not even light can escape from it. A black hole forms when matter → collapses to → infinite  → density, producing a → singularity of infinite → curvature in the fabric of → space-time. Each black hole is surrounded by an → event horizon, at which the → escape velocity is the → speed of light. The → Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: → stellar black hole, → intermediate-mass black hole, → primordial black hole, → mini black hole, → supermassive black hole, → Schwarzschild black hole, → Kerr black hole.

Historically, the Newtonian concept of such a celestial body appeared at the end of the 18th century when light was shown to have particle characteristics. In fact the English geologist John Mitchell (1724-1793) and French mathematician and astronomer Pierre Simon Laplace (1749-1827), independently, suggested that regions of space, where gravitational attraction was so strong that not even light could escape, may exist in the Universe. However, the term black hole was coined in 1967 by the Princeton physicist John A. Wheeler (1911-2008); → black; → hole.

black hole binary
  سیه‌چال ِ درین   
siyah câl-e dorin

Fr.: trou noir binaire   

A → binary system in which each component is a → black hole. The binary's evolution can be divided into three stages: → inspiral, → merger, and → ringdown.

black; → hole; → binary.

black hole candidate
  نامزد ِ سیه‌چال   
nâmzad-e siyah câl (#)

Fr.: candidat trou noir   

An object that seems likely to be a → black hole, but waits for more observational confirmations.

black; → hole; → candidate.

black hole corona
  تاج ِ سیه‌چال   
tâj-e siyah câl

Fr.: couronne du trou noir   

A spherical volume of hot plasma over a broader → accretion disk around a → black hole. The observation of energetic X-ray emission from black holes, which is inconsistent with → thermal emission from an accretion disk, is attributed to the presence of a putative hot corona. It has been widely postulated that the → hard X-rays are the product of → inverse Compton scattering of seed photons from accretion disks by hot ccoronae (See, e.g., F.L. Vieyro et al., 2010, arXiv:1005.5398 and R. C. Reis & J. M. Miller, 2013, arXiv:1304.4947).

black; → hole; → corona.

black hole merger
  تشک ِ سیه‌چالها   
tašk-e siyah-câlhâ

Fr.: fusion de trous noirs   

The collision of two → black holes in a → binary black hole system once they come so close that they cannot escape each other's gravity. They will merge in an extremely violent event to become one more massive black hole. The merger would produce tremendous energy and send massive ripples, called → gravitational waves, through the → space-time fabric of the Universe. Such an event (called GW150914) was first detected by the → Laser Interferometer Gravitational-Wave Observatory (LIGO) on September 14, 2015. The initial black hole masses were 36 and 29 Msun which gave a final black hole mass of 62 Msun, with 3 Msun radiated in gravitational waves. The event happened at a distance of 1.3 billion → light-years from Earth (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102). Black hole merger is preceded by → inspiral and followed by → ringdown.

black; → hole; → merger.

black hole surface gravity
  گرانی ِ رویه‌ی ِ سیه‌چال   
gerâni-ye ruye-ye siyah câl

Fr.: gravité de surface de trou noir   

The acceleration of gravity at the → event horizon of a → black hole. For a → Schwarzschild back hole it is given by κ = GM/RSch2 = c4/(4GM).

black; → hole; → surface; → gravity.

black hole's shadow
  سایه‌ی ِ سیه‌چال   
sâye-ye siyah-câl

Fr.: ombre de trou noir   

A gravitationally lensed image of a → black hole as seen by a distant observer if the black hole is in front of a bright background. According to → general relativity, photons circling the black hole slightly inside the boundary of the → photon sphere will fall down into the → event horizon, while photons circling just outside will escape to infinity. The shadow appears therefore as a rather sharp boundary between bright and dark regions and arises from a deficit of those photons that are captured by the event horizon. Because of this, the diameter of the shadow does not depend on the photons energy, but uniquely on the → angular momentum of the black hole. In a pioneering study, Bardeen (1973) calculated the shape of a dark area of a → Kerr black hole, that is, its "shadow" over a bright background appearing, for instance, in the image of a bright star behind the black hole.

black; → hole; → shadow.

<< < B r bac Bal Bar Bar Bay Bec Ber bet bic bij bin bio Bir bla Bla blu blu bol Boo bou bra bre bri bub by > >>