pulsâr-e Bâdbân, tapâr-e ~
Fr.: pulsar du Voile
A pulsar with a short period (89 milliseconds) associated with the → Vela supernova remnant. It is approximately 1500 light-years distant. The Vela pulsar is one of the few pulsars detectable optically. Its optical flashes, of visual magnitude 26, were detected in 1977. Also named PSR 0833-45.
Vela supernova remnant
bâzmânde-ye abar-now-axtar-e Bâdbân
Fr.: reste de supernova du Voile
A → supernova remnant located in the southern Milky Way in the constellation → Vela. It has a large angular diameter of about 8° and lies 250 ± 30 pc away (Cha et al. 1999, ApJ 515, L25). Its overall emission is dominated by the interaction of the → supernova blast wave with the → interstellar medium. This SNR is also notable for a number of protrusions extending well beyond its rim, which were suggested to be fragments of ejecta from the supernova explosion. X-ray spectroscopy has since confirmed several of these protrusions to indeed be strongly enriched with ejecta. The age of the SNR is estimated to be ~11,000 years, based on the spin-down rate of the associated → Vela pulsar, but ages as large as 20,000-30,000 years have also been argued.
Fr.: Vela X
A compact radio source about 1500 light-years distant associated with the Vela supernova remnant. It has a nonthermal radio spectrum and is about 20 percent polarized. It is associated with the Gum Nebula, the Vela pulsar, and the X-ray source 2U 0832-45.
The time rate of change of position in a given direction, measured as length per unit time. → speed.
L. velocitatem (nominative velocitas) "swiftness, speed," from velox (genitive velocis) "swift."
Tondâ, from tond "swift, rapid, brisk; fierce, severe" (Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through;" PIE base *(s)teud- "to thrust, to beat") + noun suffix -â.
Fr.: courbe de vitesse
A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines.
Fr.: dispersion de vitesses
The → standard deviation of a velocity → distribution. It indicates how objects of the sample move relative to one another. Objects with similar velocities have a small velocity dispersion, whereas objects with very different velocities have a large velocity dispersion.
Fr.: gradient de vitesse
Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.
Fr.: loi de vitesse
In the theory of → radiation-driven winds, an equation that describes the behavior of the → wind velocity of → hot stars as a function of distance from the star. This velocity β-law is given by the expression: v(r) = v∞(1 - R*/r)β, where v∞ is the → terminal velocity, R* is the stellar radius, and r the distance from the center. For → O-type stars, the exponent is estimated to be β = 0.8.
velocity of light
tondi-ye nur, tondâ-ye ~
Fr.: vitesse de la lumière
A → physical constant which represents the ultimate speed limit for anything moving through space, according to the theory of → special relativity. It is the speed of propagation of → electromagnetic waves in a vacuum, equal to 299,792.458 km/s (nearly 3 x 108 m/s). The velocity of light appears as the connecting link between mass and energy in the → mass-energy relation. Usually denoted by c, from L. celeritas "swiftness," from celer "swift," → acceleration.
Fr.: pression dynamique
Fr.: profil de vitesse
A plot of the fluid velocity as a function of position.
fazâ-ye tondâyi, ~ tondâhâ
Fr.: espace de vitesses
Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (vx, vy, vz). → phase space.
Fr.: relation vitesse-distance
The linear relation wherein all galaxies are moving away from one another, with velocities that are greater with increasing distance of the galaxy. Same as → Hubble's law.
Fr.: veine contractée
L. vena "channel;" contracta, "contracted," → contract.
nemudâr-e Venn (#)
Fr.: diagramme de Venn
A schematic diagram using circles to represent sets and the relationships between them. Each circle represents one set. Two or more may be overlapped. The areas of overlap indicate subsets.
Named after John Venn (1834-1923), a British logician and philosopher, who introduced the diagram; → diagram.
The subterranean conduit from the underlying → magma chamber through which a volcano ejects igneous material. Same as volcanic vent.
M.E. venten "to furnish (a vessel) with a vent," from O.Fr. esventer "to air," from es-, → ex-, + venter, from vent, from L. ventus "wind."
Dudkaš "chimney," literally "smoke extractor," from dud, → smoke, + kaš "to extract, to draw," present stem of kešidan/kašidan "to carry, draw, protract, trail, drag" (Mid.Pers. kešidan "to draw, pull;" Av. karš- "to draw; to plow," karša- "furrow;" cf. Skt. kars-, kársati "to pull, drag, plow;" Gk. pelo, pelomai "to move, to bustle;" PIE base kwels- "to plow").
Fr.: débitmètre de Venturi, tube de ~
Named after Italian physicist Giovanni Battista Venturi (1746-1822); → tube
The second → planet from the → Sun, at a mean distance of roughly 108.21 × 106 km (0.72 → astronomical units). Venus has the most circular orbit of any planet in the solar system. Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass). Its chemical composition and density are comparable to those of the Earth. It takes Venus just under 224.401 days to orbit the Sun, compared to the 365 day → orbital period of the Earth. Venus' rotation is → retrograde, that is it actually rotates from east to west, as opposed to west to east (→ prograde) which is the common rotating direction of most other planets. Seen from Venus, the sun would rise in the west and set in the east. Moreover, it takes about 244 Earth days for Venus to rotate once (→ sidereal rotation). This is longer than its orbital period. The length of its → solar day is about 117 Earth days. → Venus rotation. Its axial tilt is only three degrees, so there are no seasons on Venus. The → atmosphere on the surface of Venus consists mostly of → carbon dioxide, with a small trace of → nitrogen. Venus has a surface pressure about 90 times that of the Earth. → Venus visibility, → transit of Venus.
O.E., from L. Venus, the goddess of beauty and love in ancient Roman mythology, from venus "love, sexual desire, beauty, charm;" PIE base *wen- "to desire, love, wish;" cf. Av. vāunuš "lovingly," vantā- "beloved one, wife;" Skt. van- "to love, desire," vanánā- "desire," vanitā- "beloved one, wife;" O.H.G. wunsc(h) "wish," wunsken "to wish."
Nâhid, planet Venus, Mid.Pers. Anahid; O.Pers. anāhita- "immaculate, unstained," goddess of pure waters and fertility, from Av. arədvī-sūra-anāhita "valient and unsustained lord of waters," from arədvī- (Skt. Saravastī) probably "she who possesses water," + sūra- "strong, powerful" (Skt. śūra- "valiant, courageous") + anāhita- "unstained," from an- negation prefix + āhita "spotted."
Fr.: rotation de Vénus
The → sidereal rotation period of Venus, or its → sidereal day, is 243.025 Earth days (retrograde). The length of a → solar day on Venus (that is one entire day-night period) is 116.75 Earth days, that is significantly shorter than the sidereal day because of the retrograde rotation. One Venusian year is about 1.92 Venusian solar days.
Fr.: visibilité de Vénus
The conditions under which Venus can be seen from Earth as
it travels in its orbit around the Sun.
The → synodic period of Venus, that is the time Venus takes to
be seen again from the
Earth in the same position with respect to the Sun, is 583,92 days
or just over 19 months. When Venus is between Earth and Sun
(→ inferior conjunction) or on the far side of the sun
(→ superior conjunction), it is invisible in the Sun's glare. Since its
→ greatest elongation
from the Sun is never more than 47°, Venus appears only as
"the morning star" and "the evening star."
So at its greatest → western elongation
Venus will rise about
three hours ahead of the Sun and at its greatest → eastern elongation
it will set about three hours after sunset.
Its entire cycle is as follows: