An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines.

→ velocity; → curve.

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.

→ velocity; → dispersion.

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.

→ velocity; → gradient.

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; → law.

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.

→ velocity; → speed; → light.

→ dynamic pressure.

→ velocity; → pressure.

A plot of the fluid velocity as a function of position.

→ velocity; → profile.

Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (v_x, v_y, v_z). → phase space.

→ velocity; → space.

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.

→ velocity; → distance; → relation.

The location in a → fluid stream where the → cross section of the → stream is at a minimum, and fluid velocity is the highest, such as in the case of a → jet issuing out of a → nozzle.

L. vena "channel;" contracta, "contracted," → contract.

Šâre-gozar "fluid passage," → fluid; → passage, terengidé, → contracted.

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 k^wels- "to plow").

A → device used to → measure the → quantity of → fluid  → flowing through a → pipe.

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). The → greatest elongation of Venus is about 47°, making it visible at most about 3 hours after sunset or before sunrise. 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. See also: → 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."

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.

→ Venus; → rotation.

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:
Day 0: Superior conjunction, "full Venus."
Day 35: Venus appears in evening sky.
Day 221: Greatest → eastern elongation, "last quarter."
Day 271: Retrogression of Venus begins.
Day 286: Disappearance from the evening sky.
Day 292: Inferior conjunction, "new Venus."
Day 298: Venus appears in morning sky.
Day 313: Retrogression ends.
Day 362: Greatest → western elongation, "first quarter."
Day 549: Disappearance from morning sky.
Day 584: Superior conjunction, "full venus."
Therefore, Venus is visible as an evening star for 286 Earth days, as a morning star for 251 days, and is invisible for 47 days.

In addition, the orbital periods of Earth and Venus are closely correlated. After 8 Earth years or 13 Venus orbits, the two planets assume almost the same relative positions -- just 0.032 percent away from a perfect orbital resonance of 8:13. After this period of about 2920 Earth days, Venus appears just 1.5° (about 22 hours) in advance of its former position. Moreover, Venus exhibit → phases because its orbit lies within the Earth's. When Venus situated on the far side of the Sun from Earth, the planet is fully illuminated from our point of view. But its disk is small, just 10'' across, because it is nearly 300 million km away. When Venus is almost closest to Earth, on the near side of the Sun, it's about 60 million km away. Then it appears as a slender but much brighter crescent with a disk nearly 50'' across. See also → transit of Venus.

→ Venus; → visibility.

A new kind of optical telescope with a 8.4-m diameter → primary mirror currently under construction in Chile and scheduled to begin operations in October 2023. Initially named Large Synoptic Survey Telescope (LSST), it will use a 3200 megapixel camera and an automated data processing system. It will have a large → field of view almost 10 square degrees of sky, or 40 times the size of the full moon. The LSST will move quickly between images to rapidly → survey the sky. From its mountain top site in the Andes (Cerro Pachon, a 2,682-m high mountain in Coquimbo Region), the LSST will take more than 800 panoramic images each night with its 3.2 billion-pixel camera, recording the entire visible sky twice each week. Each patch of sky it images will be visited 1000 times during the survey, each of its 30-second observations will be able to detect objects 10 million times fainter than visible with the human eye. The LSST's combination of telescope, mirror, camera, → data processing, and survey will capture changes in billions of faint objects. Hence, the data it provides will be used to create an animated, three-dimensional cosmic map with unprecedented depth and detail. This map will serve many purposes, from locating the → dark matter and characterizing the properties of the → dark energy, to tracking transient objects, to studying our own Milky Way Galaxy in depth. It will even be used to detect and track → potentially hazardous asteroids that might impact the Earth.

Named after Vera C. Rubin (1928-2016) whose work on galaxy rotation rates supported the existence of dark matter in galactic halos.

A member of a major category of words that refers to an action or a state. Verbs present a complex system of forms in Indo-European languages. The set of → inflectional forms of a verb is called a → conjugation. Verbs are usually distinguished for person and number along with tense and mood (if applicable).

M.E., from O.Fr. verbe from L. verbum "verb," originally "a word," from PIE root *wer- "to speak;" cf. Av. urvāta- "command;" Skt. vrata- "command, vow;" Gk. rhetor "public speaker," eirein "to speak, say;" Lith. vardas "name;" Goth. waurd, O.E. "word."

Karvâz, literally "action word," from kar- present stem of kardan "to do, make" (Mid.Pers. kardan; O.Pers./Av. kar- "to do, make, build," Av. kərənaoiti "he makes;" cf. Skt. kr- "to do, to make," krnoti "he makes, he does," karoti "he makes, he does," karma "act, deed;" PIE base k^wer- "to do, to make") + vâz "word," variants vâž, âvâz, vâj, vât, vâ, → voice.

In exactly the same words; word for word.

From M.L. verbatim "word for word," from L. verbum "word," → verb.

Vâž-be-vâž, literally "word for word," from vâž, → word.