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.
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.
Fr.: point d'Arago
Named for François Arago (1786-1853), French physicist; → point
Fr.: point de Babinet
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.
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.
Fr.: point de Brewster
A → neutral point located 15 to 20° directly below the Sun.
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.
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.
Jodâgar, → separator.
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.
Fr.: points équinoxiaux
One of the two points of intersection of the ecliptic and the celestial equator. Same as equinox.
First Point of Aries
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 of Libra
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.
Fr.: point focal
Same as → focus.
Fr.: point de congélation
1) The temperature at which a liquid of specified composition changes into a solid under
a specified pressure.
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.
Fr.: point image
One of the points constituting an extended image formed in an → optical system.
inner Lagrangian point
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.
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.
Fr.: point lambda
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.