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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Number of Results: 50 Search : point
Alfven point
  نقطه‌ی ِ آلفون   
noqte-ye Alfvén

Fr.: point d'Alfvén   

In magnetized disk models, the point where the → poloidal velocity equals the → Alfven speed. Within this point, the magnetic energy density dominates, and the gas is forced to flow along the field lines. Well beyond this point, the kinetic energy acquired by the flowing gas prevails and the field is forced to follow the flow.

Alfven wave; → point.

antisolar point
  نقطه‌ی ِ پادخورشیدی   
noqte-ye pâdxoršidi

Fr.: direction antisolaire   

Meteo.: The point on the → celestial sphere that lies directly opposite the Sun from the → observer, observer, that is, on the line from the Sun through the observer. The antisolar point is the center of the rainbow, and can be easily found on a sunny day: it it located at the shadow of one's head; it is 180° away from the Sun. If the Sun is in the sky, the antisolar point is below the horizon. If the Sun has set, the antisolar point is above the horizon.

anti-; → solar; → point.

Arago point
  نقطه‌ی ِ آراگو   
noqte-ye Arago

Fr.: point d'Arago   

A → neutral point located at about 20° above the → antisolar point.

Named for François Arago (1786-1853), French physicist; → point

Babinet point
  نقطه‌ی ِ ببینه   
noqte-ye Babinet

Fr.: point de Babinet   

One of several points on the sky where the degree of → linear polarization is zero in skylight. See also → neutral point; → Arago point, → Brewster point.

Babinet compensator; → point.

bifurcation point
  نقطه‌ی ِ دوگلش   
noqte-ye dogaleš

Fr.: point de bifurcation   

The point or moment in the evolution of a → dynamical system that occurs if a parameter passes through a critical point. At this point the system branches into any number of qualitatively new types of behavior.

bifurcation; → point.

boiling point
  نقطه‌ی ِ جوش   
noqte-ye juš (#)

Fr.: point d'ébullition   

The temperature at which a liquid changes to a gas (vapor) at normal atmospheric pressure. In other words, the temperature at which the vapor pressure of a liquid is equal to the external pressure.

M.E. boillen; O.Fr. boillir, from L. bullire "to bubble, seethe," from bulla "a bubble, knob;" → point.

Noqté, → point; juš "boiling," present stem of jušidan "to boil;" Khotanese jis- "to boil;" Av. yaēšiiant- "boiling;" cf. Skt. yas- "to boil, become hot," yasyati "boils, seethes;" Gk. zein "to bubble, boil, cook;" O.H.G. jesan "to ferment, foam;" Ger. Gischt "foam, froth," gären "to ferment;" O.E. gist; E. yeast.

Brewster point
  نقطه‌ی ِ بروستر   
noqte-ye Brewster

Fr.: point de Brewster   

A → neutral point located 15 to 20° directly below the Sun.

Brewster's law; → point

conjugate points
  نقطه‌های ِ همیوغ   
noqtehâ-ye hamyuq

Fr.: points conjugués   

Two points positioned along the principal axis of a mirror or lens so that light coming from one focuses onto the other.

conjugate; → point.

decimal point
  جداگر، ممیز   
jodâgar, momayez (#)

Fr.: séparateur décimal, signe décimal   

A symbol (usually a point or dot) used to separate the integer part from the fractional part of a → decimal number.

decimal; → point.

Jodâgar, → separator.
Momayez, from Ar. mumaiyiz "distinguisher, discerner, separator."

dew point
  نقطه‌ی ِ شبنم   
noqte-ye šabnam (#)

Fr.: point de rosée   

The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur.

dew; → point.

equinoctial points
  نقطه‌ها‌ی ِ هموگانی   
noqtehâ-ye hamugâni

Fr.: points équinoxiaux   

One of the two points of intersection of the ecliptic and the celestial equator. Same as equinox.

equinoctial; → point.

First Point of Aries
  نقطه‌ی ِ آغاز ِ برّه   
noqte-ye âqâz-e barré (#)

Fr.: premier point de Bélier   

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

first; → point; → Aries.

First Point of Libra
  نقطه‌ی ِ آغاز ِ ترازو   
noqte-ye âqâz-e tarâzu (#)

Fr.: permier point de Balance   

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

focal point
  نقطه‌ی ِ کانونی   
noqte-ye kânuni

Fr.: point focal   

Same as → focus.

focal; → point.

freezing point
  نقطه‌ی ِ یخ‌بست   
noqte-ye yaxbast

Fr.: point de congélation   

1) The temperature at which a liquid of specified composition changes into a solid under a specified pressure.
2) The temperature at which the liquid and solid phases of a substance of specified composition coexist in equilibrium.

freezing; → point.

ice point
  نقطه‌ی ِ یخ   
noqte-ye yax (#)

Fr.: point de congélation   

The temperature at which a mixture of air-saturated pure water and pure ice may exist in equilibrium at a pressure of one standard atmosphere.

ice; → point.

image point
  نقطه‌ی ِ تصویر   
noqte-ye tasvir

Fr.: point image   

One of the points constituting an extended image formed in an → optical system.

image; → point.

inner Lagrangian point
  نقطه‌ی ِ لاگرانژ ِ درونی   
noqte-ye Lagrange-e daruni (#)

Fr.: point de Lagrange interne   

One of the five → Lagrangian points, denoted L1, which lies between the two bodies on the line passing through their center of mass. In a → close binary star system mass transfer occurs through this point.

inner; → Lagrangian points.

Lagrangian point
  نقطه‌های ِ لاگرانژ   
noqtehâ-ye Lagrange (#)

Fr.: points de Lagrange   

On of the five locations in space where the → centrifugal force and the → gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body, located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m's orbit approximately 60° ahead of it, while L5 lies in the m's orbit approximately 60° behind m. See also → Trojan asteroid; → Roche lobe; → equipotential surface; → horseshoe orbit.

Lagrangian; → point.

lambda point
  نقطه‌ی ِ لامبدا   
noqte-ye lâmbdâ

Fr.: point lambda   

The temperature (roughly 2.17 K) at which → liquid helium (→ helium I) becomes → superfluid (→ helium II).

The name was given by the Dutch physicist Willem Hendrik Keesom (1876-1956), who discovered the behavior of helium near this transition point and successfully solidified helium in 1926 (under an external pressure of 25 atmospheres). The name was originally suggested by Paul Ehrenfest (1880-1933), who was inspired by the shape of the → specific heat curve, which resembles the Gk. letter → lambda; → point.

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