An Etymological Dictionary of Astronomy and Astrophysics

1. A space completely empty of matter but not achievable in practice on Earth.
   
2. A state of lowest energy in a → quantum field theory.

Etymology (EN): L. vacuum “an empty space, void,” noun use of neuter of vacuus “empty,” related to vacare “to be empty.”

Etymology (PE): Xala’ loan from Ar.

1. A space completely empty of matter but not achievable in practice on Earth.
   
2. A state of lowest energy in a → quantum field theory.

Etymology (EN): L. vacuum “an empty space, void,” noun use of neuter of vacuus “empty,” related to vacare “to be empty.”

Etymology (PE): Xala’ loan from Ar.

A highly → magnetized vacuum behaving as a prism for the propagation of light, as predicted by → quantum electrodynamics (QED). Attempts to detect this phenomenon in the laboratory have not yet succeeded in the 80 years since it was predicted (Heisenberg & Euler, 1936, Z. Physik, 98, 714). This effect can be detected only in the presence of enormously strong → magnetic fields, such as those around → neutron stars.

Owing to the large inferred magnetic fields (B ~ 10¹³ G, → gauss), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the → thermal emission. The strongest magnetic field so far created in a laboratory is less than 10⁶ G lasting only for several tens of milliseconds. A large observed → polarization degree is, however, expected only if QED polarization effects are present in the magnetized vacuum around the star. The detection of a strongly → linearly polarized signal would therefore provide the observational evidence of QED effects in the strong-field regime.

Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have detected → linear polarization toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%). This finding is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding the neutron star.

See also: → vacuum; → birefringence.

A highly → magnetized vacuum behaving as a prism for the propagation of light, as predicted by → quantum electrodynamics (QED). Attempts to detect this phenomenon in the laboratory have not yet succeeded in the 80 years since it was predicted (Heisenberg & Euler, 1936, Z. Physik, 98, 714). This effect can be detected only in the presence of enormously strong → magnetic fields, such as those around → neutron stars.

Owing to the large inferred magnetic fields (B ~ 10¹³ G, → gauss), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the → thermal emission. The strongest magnetic field so far created in a laboratory is less than 10⁶ G lasting only for several tens of milliseconds. A large observed → polarization degree is, however, expected only if QED polarization effects are present in the magnetized vacuum around the star. The detection of a strongly → linearly polarized signal would therefore provide the observational evidence of QED effects in the strong-field regime.

Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have detected → linear polarization toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%). This finding is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding the neutron star.

See also: → vacuum; → birefringence.

An enclosure from which air is removed.

See also: → vacuum; → chamber.

An enclosure from which air is removed.

See also: → vacuum; → chamber.

In particle physics the lowest energy allowed by field quantization when all fields are in their → ground states. Vacuum energy is predicted to arise from → virtual particles that fluctuate in and out of existence, as manifested by the → Casimir effect. The cosmological → dark energy is postulated to be related to vacuum fluctuations. There is however an enormous discrepancy with the predictions of
quantum field theory. In this theory the value of vacuum energy density is expected to be roughly of the order ρ_v≅ E_max⁴, where E_max is the maximum energy at which the field theory is valid. At energies of the order of the → Planck energy, E_Pl≅ 10¹⁹ GeV, vacuum energy might be roughly: ρ_v≅ E_Pl⁴≅ 10⁷⁶ GeV⁴. On the other hand, the vacuum energy density in standard cosmological model is given by: ρ_Λ = Ω_Λ.ρ_crit, where Ω_Λ is the → density parameter for the → cosmological constant and ρ_crit is the → critical density. More explicitly, ρ_Λ = Ω_Λ . 3 H²/(8πG). Using present-day values of Ω_Λ (0.7) and H (70) leads to ρ_Λ = 10^-46 GeV⁴. Therefore, the discrepancy between the prediction and the observed value is 122 orders of magnitude.

See also: → vacuum; → energy.

In particle physics the lowest energy allowed by field quantization when all fields are in their → ground states. Vacuum energy is predicted to arise from → virtual particles that fluctuate in and out of existence, as manifested by the → Casimir effect. The cosmological → dark energy is postulated to be related to vacuum fluctuations. There is however an enormous discrepancy with the predictions of
quantum field theory. In this theory the value of vacuum energy density is expected to be roughly of the order ρ_v≅ E_max⁴, where E_max is the maximum energy at which the field theory is valid. At energies of the order of the → Planck energy, E_Pl≅ 10¹⁹ GeV, vacuum energy might be roughly: ρ_v≅ E_Pl⁴≅ 10⁷⁶ GeV⁴. On the other hand, the vacuum energy density in standard cosmological model is given by: ρ_Λ = Ω_Λ.ρ_crit, where Ω_Λ is the → density parameter for the → cosmological constant and ρ_crit is the → critical density. More explicitly, ρ_Λ = Ω_Λ . 3 H²/(8πG). Using present-day values of Ω_Λ (0.7) and H (70) leads to ρ_Λ = 10^-46 GeV⁴. Therefore, the discrepancy between the prediction and the observed value is 122 orders of magnitude.

See also: → vacuum; → energy.

A quantum field theory a process in which an electromagnetic field gives rise to virtual electron-positron pairs that in turn exert electromagnetic fields of their own, in a manner similar to classical dielectric polarization.

See also: → vacuum; → polarization.

A quantum field theory a process in which an electromagnetic field gives rise to virtual electron-positron pairs that in turn exert electromagnetic fields of their own, in a manner similar to classical dielectric polarization.

See also: → vacuum; → polarization.

1. Not clearly or explicitly stated or expressed.
   

2. Indefinite or indistinct in nature or character, as ideas or feelings.
   

3. Not definitely established, determined, confirmed, or known; uncertain (Dictionary.com).

→ ambiguous, → nebulous, → indefinite, → fuzzy, → unclear.

Etymology (EN): M.E., from M.Fr. vague “empty, vacant; wild, uncultivated; wandering,” from L. vagus “wandering, rambling,” of unknown origin.

Etymology (PE): Gereng, from Kalidari gereng “wandering,” Kurd. (Kurmanji) garin, (Sorani) garân “to wander,” Dehxodâ gereng “shattered;” ultimately from Proto-Ir. *gar-an-ka-, from *gar- “to turn, to wind” (Cheung 2007).

1. Not clearly or explicitly stated or expressed.
   

2. Indefinite or indistinct in nature or character, as ideas or feelings.
   

3. Not definitely established, determined, confirmed, or known; uncertain (Dictionary.com).

→ ambiguous, → nebulous, → indefinite, → fuzzy, → unclear.

Etymology (EN): M.E., from M.Fr. vague “empty, vacant; wild, uncultivated; wandering,” from L. vagus “wandering, rambling,” of unknown origin.

Etymology (PE): Gereng, from Kalidari gereng “wandering,” Kurd. (Kurmanji) garin, (Sorani) garân “to wander,” Dehxodâ gereng “shattered;” ultimately from Proto-Ir. *gar-an-ka-, from *gar- “to turn, to wind” (Cheung 2007).

A year of 365 days that overlooks the fraction of less than 0.25 days corresponding to the whole length of the → tropical year. The vague year was used in the → calendars of ancient Egypt, Iran, Mayas, and some other civilizations. Typically the vague year was divided into 12 months of 30 days each plus 5 → epagomenal days.

See also: From annus vagus “wandering year;” → vague→ year.

A year of 365 days that overlooks the fraction of less than 0.25 days corresponding to the whole length of the → tropical year. The vague year was used in the → calendars of ancient Egypt, Iran, Mayas, and some other civilizations. Typically the vague year was divided into 12 months of 30 days each plus 5 → epagomenal days.

See also: From annus vagus “wandering year;” → vague→ year.

A measure of the number of chemical bonds formed by the atoms of a given element. It represents the relative ability of an atom of an element to combine with other atoms. For example, the valence of O in water, H₂O, is 2.
Also called valency.

Etymology (EN): M.E., from O.Fr., from L valentia “strength, worth,” from valentem (nominative valens), pr.p. of valere “to be strong.”

Etymology (PE): Arzâyi, noun from arzâ, agent noun/adj. from arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth” (Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value”).

A measure of the number of chemical bonds formed by the atoms of a given element. It represents the relative ability of an atom of an element to combine with other atoms. For example, the valence of O in water, H₂O, is 2.
Also called valency.

Etymology (EN): M.E., from O.Fr., from L valentia “strength, worth,” from valentem (nominative valens), pr.p. of valere “to be strong.”

Etymology (PE): Arzâyi, noun from arzâ, agent noun/adj. from arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth” (Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value”).

The range of energy states in the spectrum of a solid crystal which includes the energies of all the electrons binding the crystal together.

See also: → valence; → band.

The range of energy states in the spectrum of a solid crystal which includes the energies of all the electrons binding the crystal together.

See also: → valence; → band.

An electron of an atom lying farthest from the nucleus. Valence electrons are shared when atoms combine to form molecules.

See also: → valence; → electron.

An electron of an atom lying farthest from the nucleus. Valence electrons are shared when atoms combine to form molecules.

See also: → valence; → electron.

Same as → valence.

Same as → valence.

Logic: Of an argument, if the premises are true, then the
conclusion cannot be denied without contradiction. The truth of the premises logically guarantees the truth of the conclusion.

Etymology (EN): M.E., from M.Fr. valide, from L. validus “strong, effective,” from valere “to be strong.”

Etymology (PE): Pâymand, from pâ, pây “foot; base, foundation, firmness” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -mand possession suffix.

Logic: Of an argument, if the premises are true, then the
conclusion cannot be denied without contradiction. The truth of the premises logically guarantees the truth of the conclusion.

Etymology (EN): M.E., from M.Fr. valide, from L. validus “strong, effective,” from valere “to be strong.”

Etymology (PE): Pâymand, from pâ, pây “foot; base, foundation, firmness” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -mand possession suffix.

Logic: Of an argument, whether or not the conclusion follows logically from the premises and the allowable syllogisms of the logical system being used.

See also: Quality noun from → valid.

Logic: Of an argument, whether or not the conclusion follows logically from the premises and the allowable syllogisms of the logical system being used.

See also: Quality noun from → valid.

A system of canyons located just south of the Martian equator. The system is about 4000 km long. The central individual troughs, generally 50 to 100 km wide, merge into a depression as much as 600 km wide. In places the canyon floor reaches a depth of 10 km, 6 to 7 times deeper than the Grand Canyon on Earth.

See also: L. Valles Marineris “Mariner’s Valleys,” named after the Mars orbiter Mariner 9, which discovered the Martian canyon in 1971-72. → valley.

A system of canyons located just south of the Martian equator. The system is about 4000 km long. The central individual troughs, generally 50 to 100 km wide, merge into a depression as much as 600 km wide. In places the canyon floor reaches a depth of 10 km, 6 to 7 times deeper than the Grand Canyon on Earth.

See also: L. Valles Marineris “Mariner’s Valleys,” named after the Mars orbiter Mariner 9, which discovered the Martian canyon in 1971-72. → valley.

A long, narrow region of low land between ranges of mountains, hills, or other high areas, often having a river or stream running along the bottom.

Etymology (EN): M.E. valeie, valey, from O.Fr. valee “valley,” from V.L. *vallata, from L. vallis “valley,” of unknown origin.

Etymology (PE): Darré “valley,” from Mid.Pers. dar, darrak “ravine, cleft;” cf. Skt. pradará- “cleft.”

A long, narrow region of low land between ranges of mountains, hills, or other high areas, often having a river or stream running along the bottom.

Etymology (EN): M.E. valeie, valey, from O.Fr. valee “valley,” from V.L. *vallata, from L. vallis “valley,” of unknown origin.

Etymology (PE): Darré “valley,” from Mid.Pers. dar, darrak “ravine, cleft;” cf. Skt. pradará- “cleft.”

Math.: Magnitude; quantity; a numerical quantity assigned to a mathematical symbol.

Etymology (EN): M.E., from O.Fr. value “worth, value,” noun use of feminine p.p. of valoir “be worth,” from L. valere “be strong, be well, be of value.”

Etymology (PE): Arzeš, verbal noun of arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth;” Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value.”

Math.: Magnitude; quantity; a numerical quantity assigned to a mathematical symbol.

Etymology (EN): M.E., from O.Fr. value “worth, value,” noun use of feminine p.p. of valoir “be worth,” from L. valere “be strong, be well, be of value.”

Etymology (PE): Arzeš, verbal noun of arzidan “to be worth,” arzân “worthy; of small value, cheap,” arj “esteem, honour, price, worth;” Mid.Pers. arz “value, worth,” arzidan “be worth,”
arzân “valuable;” Av. arəjaiti “is worth,” arəja- “valuable,” arəg- “to be worth;” cf. Skt. arh- “to be worth, to earn,” árhant- “worthy person;” Gk. alphanein “to bring in as profit,” alphein “to ear, obtain;” Lith. algà “salary, pay;” PIE base *alg^wh- “to earn; price, value.”

Any device that halts or regulates the flow of a fluid through a passage, pipe, etc.

Etymology (EN): From L. valvae “leaves of a door.”

Etymology (PE): Daricé, leterally “small door, window,” → stop. Supâp, loan from Fr. soupape, from O.Fr. sourpape, probably from souspape “a blow under chin,” from sous “under,” from L. subtus, → sub-,

• *pape “jaw,” from paper, from L. pap(p)are “to eat.”

Any device that halts or regulates the flow of a fluid through a passage, pipe, etc.

Etymology (EN): From L. valvae “leaves of a door.”

Etymology (PE): Daricé, leterally “small door, window,” → stop. Supâp, loan from Fr. soupape, from O.Fr. sourpape, probably from souspape “a blow under chin,” from sous “under,” from L. subtus, → sub-,

• *pape “jaw,” from paper, from L. pap(p)are “to eat.”

A mechanism proposed by Eddington to explain → stellar pulsations. Same as the → kappa mechanism. In this analogy the stellar layer acts like a heat engine with radiation taking the role of stream. The expanding and contracting layer acts as the piston, and the opacity of the layer behaves as the valve mechanism (Eddington, 1917, The pulsation theory of → Cepheid variables, The Observatory 40, 290).

See also: → valve; → mechanism.

A mechanism proposed by Eddington to explain → stellar pulsations. Same as the → kappa mechanism. In this analogy the stellar layer acts like a heat engine with radiation taking the role of stream. The expanding and contracting layer acts as the piston, and the opacity of the layer behaves as the valve mechanism (Eddington, 1917, The pulsation theory of → Cepheid variables, The Observatory 40, 290).

See also: → valve; → mechanism.

The ring-shaped regions of charged particles surrounding the Earth from 1 to 6 Earth radii into space. The charged particles are trapped in by the Earth’s magnetic field. The inner belt is between 1.2 and 4.5 Earth radii and contains high-energy electrons and protons which originate mainly from interactions between cosmic rays and the upper atmosphere. The outer belt, located between 4.5 and 6.0 Earth radii, contains lower-energy charged particles mainly coming from the solar wind.

See also: Named after James Van Allen (1914-2006), who discovered the belts
in 1958 based on measurements made by Explorer 1, the USA’s first successful artificial satellite; → belt.

The ring-shaped regions of charged particles surrounding the Earth from 1 to 6 Earth radii into space. The charged particles are trapped in by the Earth’s magnetic field. The inner belt is between 1.2 and 4.5 Earth radii and contains high-energy electrons and protons which originate mainly from interactions between cosmic rays and the upper atmosphere. The outer belt, located between 4.5 and 6.0 Earth radii, contains lower-energy charged particles mainly coming from the solar wind.

See also: Named after James Van Allen (1914-2006), who discovered the belts
in 1958 based on measurements made by Explorer 1, the USA’s first successful artificial satellite; → belt.

In → Young’s experiment of → interference with double apertures, if a monochromatic source is a considerable distance from the → aperture plane and aperture separation is small, → fringe visibility from an extended source is proportional to the → Fourier transform of the source’s spatial distribution. The transform variable is the angular separation of the aperture-plane sampling points divided by the wavelength. The van Cittert-Zernike Theorem is at the heart of → aperture synthesis.

See also: Developed independently by Dutch physicists Pieter Hendrick van Cittert (1889-1959) in 1934 and Frits Zernike (1888-1966) in 1939; → theorem.

In → Young’s experiment of → interference with double apertures, if a monochromatic source is a considerable distance from the → aperture plane and aperture separation is small, → fringe visibility from an extended source is proportional to the → Fourier transform of the source’s spatial distribution. The transform variable is the angular separation of the aperture-plane sampling points divided by the wavelength. The van Cittert-Zernike Theorem is at the heart of → aperture synthesis.

See also: Developed independently by Dutch physicists Pieter Hendrick van Cittert (1889-1959) in 1934 and Frits Zernike (1888-1966) in 1939; → theorem.

A catalog of → reflection nebulae containing 158 objects. The catalog gives information for all BD and CD stars north of δ = -33 deg which are surrounded by reflection nebulosity visible on both the blue and red prints of the → Palomar Observatory Sky Survey .

See also: van den Bergh, S., 1966, AJ, 71, 990; → catalog

A catalog of → reflection nebulae containing 158 objects. The catalog gives information for all BD and CD stars north of δ = -33 deg which are surrounded by reflection nebulosity visible on both the blue and red prints of the → Palomar Observatory Sky Survey .

See also: van den Bergh, S., 1966, AJ, 71, 990; → catalog

An → equation of state that satisfactorily describes the behavior of → real gass over a wide range of temperatures and pressures. It is derived from considerations based on kinetic theory, taking into account to a first approximation the size of a molecule and the cohesive forces between molecules: (P + a / V²) (V - b) = RT, where P, V, and T are pressure, volume, and temperature and R the gas constant. a and b are characteristic constants for a given substance. For a = b = 0, the van der Waals equation reduces to the characteristic equation of an → ideal gas. See also → Dieterici equation.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → equation.

An → equation of state that satisfactorily describes the behavior of → real gass over a wide range of temperatures and pressures. It is derived from considerations based on kinetic theory, taking into account to a first approximation the size of a molecule and the cohesive forces between molecules: (P + a / V²) (V - b) = RT, where P, V, and T are pressure, volume, and temperature and R the gas constant. a and b are characteristic constants for a given substance. For a = b = 0, the van der Waals equation reduces to the characteristic equation of an → ideal gas. See also → Dieterici equation.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → equation.

A weak attractive force between neutral atoms and molecules arising from polarization induced in each particle by the presence of other particles. All molecules contain electrically charged particles, and even though the molecule as a whole is electrically neutral there do exist between molecules van der Waals attractive forces of electric origin.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → force.

A weak attractive force between neutral atoms and molecules arising from polarization induced in each particle by the presence of other particles. All molecules contain electrically charged particles, and even though the molecule as a whole is electrically neutral there do exist between molecules van der Waals attractive forces of electric origin.

See also: Named after Dutch physicist Johannes Diderik van der Waals (1837-1923), Nobel Prize in Physics 1910; → force.

A soft, ductile, silver-grey metal; symbol V. → Atomic number 23; → atomic weight 50.9415; → melting point about 1,890°C; → boiling point 3,380°C; → specific gravity about 6 at 20°C; and
→ valence +2,+3, +4, or +5. → It is used in various alloys to increase its shock resistance.

See also: Named 1830 by Swedish chemist Nils Gabriel Sefström (1787-1845), from Old Norse Vanadis, epithet of the goddess Freya, +
→ -ium.

A soft, ductile, silver-grey metal; symbol V. → Atomic number 23; → atomic weight 50.9415; → melting point about 1,890°C; → boiling point 3,380°C; → specific gravity about 6 at 20°C; and
→ valence +2,+3, +4, or +5. → It is used in various alloys to increase its shock resistance.

See also: Named 1830 by Swedish chemist Nils Gabriel Sefström (1787-1845), from Old Norse Vanadis, epithet of the goddess Freya, +
→ -ium.

1. A blade attached radially to a central axis, as in a windmill, propeller, revolving fin, or the like. → spider vane.
   

2. → wind vane.
   

3. The flat part of a bird’s → feather.

Etymology (EN): M.E., from O.E. fana “flag;” cognate with Ger. Fahne
“flag,” Gothic fana “piece of cloth.”

Etymology (PE): Parré, from parr, → feather.

1. A blade attached radially to a central axis, as in a windmill, propeller, revolving fin, or the like. → spider vane.
   

2. → wind vane.
   

3. The flat part of a bird’s → feather.

Etymology (EN): M.E., from O.E. fana “flag;” cognate with Ger. Fahne
“flag,” Gothic fana “piece of cloth.”

Etymology (PE): Parré, from parr, → feather.

1. To disappear from sight, especially quickly; become invisible.
   

2. To disappear by ceasing to exist; come to an end.
   

3. Math.: To become zero or to tend to zero.

Etymology (EN): M.E., from O.Fr. esvanir “disappear; cause to disappear,” from L. evanescere “disappear, pass away, die out,” from → ex- “out” + vanescere “to vanish,” from vanus “empty,” cognate with O.E. wanian “to lessen,” wan “deficient;” O.N. vanta “to lack;” L. vacare “to be empty,” vastus “empty, waste;” see below for Iranian cognates.

Etymology (PE): Venidan, from Kurd. ven “missing; away; absent,” cf. Pers. vang “empty, empty-handed, poor;” related to Av. ūna- “deficient, empty,” ū- “to be incomplete;” akin to Skt. vā- “to disappear, diminish,” ūná- “deficient;” L. vanus “empty,” as above.

1. To disappear from sight, especially quickly; become invisible.
   

2. To disappear by ceasing to exist; come to an end.
   

3. Math.: To become zero or to tend to zero.

Etymology (EN): M.E., from O.Fr. esvanir “disappear; cause to disappear,” from L. evanescere “disappear, pass away, die out,” from → ex- “out” + vanescere “to vanish,” from vanus “empty,” cognate with O.E. wanian “to lessen,” wan “deficient;” O.N. vanta “to lack;” L. vacare “to be empty,” vastus “empty, waste;” see below for Iranian cognates.

Etymology (PE): Venidan, from Kurd. ven “missing; away; absent,” cf. Pers. vang “empty, empty-handed, poor;” related to Av. ūna- “deficient, empty,” ū- “to be incomplete;” akin to Skt. vā- “to disappear, diminish,” ūná- “deficient;” L. vanus “empty,” as above.

1. To → conquer or subdue by superior force, as in battle.
   

   2. To defeat in any contest or conflict; be victorious over (Dictionary.com).

Etymology (EN): O.Fr. venquis-, extended stem of veintre “to defeat,” from L. vincere “to overcome, conquer.”

Etymology (PE): Vânidan, from Mid.Pers. vânidan “to vanquish, to conquer;” cf. Av. van- “to vanquish, to defeat;” Skt. van- “to win, conquer;” O.E. winnan, E. win, PIE *uen- “to win, conquer.”

1. To → conquer or subdue by superior force, as in battle.
   

   2. To defeat in any contest or conflict; be victorious over (Dictionary.com).

Etymology (EN): O.Fr. venquis-, extended stem of veintre “to defeat,” from L. vincere “to overcome, conquer.”

Etymology (PE): Vânidan, from Mid.Pers. vânidan “to vanquish, to conquer;” cf. Av. van- “to vanquish, to defeat;” Skt. van- “to win, conquer;” O.E. winnan, E. win, PIE *uen- “to win, conquer.”

The gaseous form of substances that are normally in liquid or solid form.

Etymology (EN): M.E. vapour, L. vapor “exhalation, steam, heat,” of unknown origin.

Etymology (PE): Boxâr, loan from Ar. buxâr.

The gaseous form of substances that are normally in liquid or solid form.

Etymology (EN): M.E. vapour, L. vapor “exhalation, steam, heat,” of unknown origin.

Etymology (PE): Boxâr, loan from Ar. buxâr.

The equilibrium pressure of a vapor above its condensed phases (liquid or solid). In other words, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. The vapor pressure varies with the temperature. As the temperature increases its the vapor pressure also increases, and conversely. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the → boiling point. For example, at 20 °C, water would boil at a pressure of about 0.023 atm, or about a fortieth of atmospheric pressure.

See also: → vapor; → pressure.

The equilibrium pressure of a vapor above its condensed phases (liquid or solid). In other words, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. The vapor pressure varies with the temperature. As the temperature increases its the vapor pressure also increases, and conversely. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the → boiling point. For example, at 20 °C, water would boil at a pressure of about 0.023 atm, or about a fortieth of atmospheric pressure.

See also: → vapor; → pressure.

The act of vaporizing. The state of being vaporized.
The rapid change of water into steam.

See also: Verbal noun of → vaporize.

The act of vaporizing. The state of being vaporized.
The rapid change of water into steam.

See also: Verbal noun of → vaporize.

1. To become converted into vapor.
   
2. To cause to change into a vapor.

See also: → vapor + → -ize.

1. To become converted into vapor.
   
2. To cause to change into a vapor.

See also: → vapor + → -ize.

The quality, state, or degree of being variable or changeable.

See also: Quality noun from → variable.

The quality, state, or degree of being variable or changeable.

See also: Quality noun from → variable.

A measure of variability of a star (such as Welch-Stetson variability index) which combines the information from two passbands assuming that changes in the star’s luminosity occur nearly simultaneously at all optical wavelengths.

See also: → variability; → index.

A measure of variability of a star (such as Welch-Stetson variability index) which combines the information from two passbands assuming that changes in the star’s luminosity occur nearly simultaneously at all optical wavelengths.

See also: → variability; → index.

Something that may or does vary. → variable star.
Math.: A symbol, usually a letter, which in a mathematical or logic expression represents any member of a set of possible values or possible objects.

Etymology (EN): M.E., from O.Fr. variable, from L. variabilis “changeable,” from variare “to change,” → vary.

Etymology (PE): Vartandé agent noun from vartidan “to change,” → vary.

Something that may or does vary. → variable star.
Math.: A symbol, usually a letter, which in a mathematical or logic expression represents any member of a set of possible values or possible objects.

Etymology (EN): M.E., from O.Fr. variable, from L. variabilis “changeable,” from variare “to change,” → vary.

Etymology (PE): Vartandé agent noun from vartidan “to change,” → vary.

A star whose luminosity changes over periods of time; there are many reasons and many types. → cataclysmic variable; → Cepheid;
→ pulsating star; → nova; → long-period variable; → short-period variable; → Luminous Blue Variable (LBV); etc.

See also: → variable; → star.

A star whose luminosity changes over periods of time; there are many reasons and many types. → cataclysmic variable; → Cepheid;
→ pulsating star; → nova; → long-period variable; → short-period variable; → Luminous Blue Variable (LBV); etc.

See also: → variable; → star.

A set of conventions used for naming → variable stars. Stars with existing → Bayer designations are not given new designations. Alternatively, the letters R through Z are used followed by the Latin genitive of the name of the hosting constellation. Otherwise, two letters of alphabet are used (334 combinations) with the Latin genitive of the name of the constellation. Finally,
the letter V (variable) is used followed by numbers 335, 336, and so on. Some examples are: → P Cygni, → T Tauri, → FU Orionis, → EX Lupi, and → V2052 Oph.

See also: → variable; → star; → designation.

A set of conventions used for naming → variable stars. Stars with existing → Bayer designations are not given new designations. Alternatively, the letters R through Z are used followed by the Latin genitive of the name of the hosting constellation. Otherwise, two letters of alphabet are used (334 combinations) with the Latin genitive of the name of the constellation. Finally,
the letter V (variable) is used followed by numbers 335, 336, and so on. Some examples are: → P Cygni, → T Tauri, → FU Orionis, → EX Lupi, and → V2052 Oph.

See also: → variable; → star; → designation.

A measure of the scatter of the values of a random variable (X) about its mean (μ). Var(X) = E(X -μ)². The positive square root of the variance is called the → standard deviation.

Etymology (EN): M.E., from O.Fr. variance, from L. variantia, from variare “to change,” → vary.

Etymology (PE): Vartâyi quality noun from vartâ agent noun from vartidan, → vary.

A measure of the scatter of the values of a random variable (X) about its mean (μ). Var(X) = E(X -μ)². The positive square root of the variance is called the → standard deviation.

Etymology (EN): M.E., from O.Fr. variance, from L. variantia, from variare “to change,” → vary.

Etymology (PE): Vartâyi quality noun from vartâ agent noun from vartidan, → vary.

1. Liable to or displaying variation.
   

2. Something that differs from a standard or type. For example, a different form of the same word.

See also: → vary.

1. Liable to or displaying variation.
   

2. Something that differs from a standard or type. For example, a different form of the same word.

See also: → vary.

A quantity which may take any of the values of a specified set with a specified relative frequency or probability. The variate is therefore often known as a → random variable.

Etymology (EN): L. variatus, p.p. of variare, → vary,

• → -ate.

Etymology (PE): → variable; → random.

A quantity which may take any of the values of a specified set with a specified relative frequency or probability. The variate is therefore often known as a → random variable.

Etymology (EN): L. variatus, p.p. of variare, → vary,

• → -ate.

Etymology (PE): → variable; → random.

1. General: An instance of changing, or something that changes.
   

2. Astro.: The periodic inequality in the Moon’s motion that results from the combined gravitational attraction of the Earth and the Sun. Its period is half the synodic month, that is 14.77 days, and the maximum longitude displacement is 39'29’’.9.
   

See also:

→ calculus of variations, → annual variation, → secular variation.

Etymology (EN): M.E., from O.Fr. variation, from L. variationem (nominative variatio) “difference, change,” from variatus, p.p. of variare “to change,” → vary.

Etymology (PE): Varteš, verbal noun from vartidan, → vary.

1. General: An instance of changing, or something that changes.
   

2. Astro.: The periodic inequality in the Moon’s motion that results from the combined gravitational attraction of the Earth and the Sun. Its period is half the synodic month, that is 14.77 days, and the maximum longitude displacement is 39'29’’.9.
   

See also:

→ calculus of variations, → annual variation, → secular variation.

Etymology (EN): M.E., from O.Fr. variation, from L. variationem (nominative variatio) “difference, change,” from variatus, p.p. of variare “to change,” → vary.

Etymology (PE): Varteš, verbal noun from vartidan, → vary.

Of or describing a → variation.

See also: → variation; → -al.

Of or describing a → variation.

See also: → variation; → -al.

Any of the physical principles that indicate in what way the actual motion of a state of a mechanical system differs from all of its kinematically possible motions or states. Variational principles that express this difference for the motion or state of a system in each given instant of time are called → differential. These principles are equally applicable to both → holonomic and → nonholonomic systems. Variational principles that establish the difference between the actual motion of a system during a finite time interval and all of its kinematically possible motions are said to be → integral. Integral variational principles are valid only for holonomic systems. The main differential variational principles are: the → virtual work principle and
→ d’Alembert’s principle.

See also: → variational; → principle.

Any of the physical principles that indicate in what way the actual motion of a state of a mechanical system differs from all of its kinematically possible motions or states. Variational principles that express this difference for the motion or state of a system in each given instant of time are called → differential. These principles are equally applicable to both → holonomic and → nonholonomic systems. Variational principles that establish the difference between the actual motion of a system during a finite time interval and all of its kinematically possible motions are said to be → integral. Integral variational principles are valid only for holonomic systems. The main differential variational principles are: the → virtual work principle and
→ d’Alembert’s principle.

See also: → variational; → principle.

1. The quality or state of having different forms or types.
   

2. A number or collection of different things especially of a particular class.
   

3. Something differing from others of the same general kind.
   

4. Any of various groups of plants or animals ranking below a species (Merriam-Webster.com).

Etymology (EN): M.Fr. variété, from L. varietatem (nominative varietas) “difference, diversity; a kind, variety, species, sort,” from varius, → various.

Etymology (PE): Vartiné, from vartin, → various, + noun/nuance suffix -é.

1. The quality or state of having different forms or types.
   

2. A number or collection of different things especially of a particular class.
   

3. Something differing from others of the same general kind.
   

4. Any of various groups of plants or animals ranking below a species (Merriam-Webster.com).

Etymology (EN): M.Fr. variété, from L. varietatem (nominative varietas) “difference, diversity; a kind, variety, species, sort,” from varius, → various.

Etymology (PE): Vartiné, from vartin, → various, + noun/nuance suffix -é.

The → moment of the resultant of a → coplanar system of → concurrent forces about any center is equal to the algebraic sum of the moments of the component forces about that center.

See also: Named after Pierre Varignon (1654-1722), a French mathematician, who outlined the fundamentals of statics in his book Projet d’une nouvelle mécanique (1687).

The → moment of the resultant of a → coplanar system of → concurrent forces about any center is equal to the algebraic sum of the moments of the component forces about that center.

See also: Named after Pierre Varignon (1654-1722), a French mathematician, who outlined the fundamentals of statics in his book Projet d’une nouvelle mécanique (1687).

1. Of different kinds, as two or more things; differing one from another.
   

2. Marked by or exhibiting variety or diversity (Dictionary.com).

Etymology (EN): M.Fr. varieux and directly from L. varius “changing, different, diverse,” → vary.

Etymology (PE): Vartin, from vart “change,” present stem of vartidan, → vary, + adj. suffix -in.

1. Of different kinds, as two or more things; differing one from another.
   

2. Marked by or exhibiting variety or diversity (Dictionary.com).

Etymology (EN): M.Fr. varieux and directly from L. varius “changing, different, diverse,” → vary.

Etymology (PE): Vartin, from vart “change,” present stem of vartidan, → vary, + adj. suffix -in.

1. To undergo change in form, substance, appearance, etc.
   
2. To change or alter. → covariance.

Etymology (EN): M.E. varien, from O.Fr. varier, from L. variare “change, alter, make different,” from varius “variegated, different, spotted.”

Etymology (PE): Vartidan “to change,” from Mid.Pers. vartitan “to change, turn” (Mod.Pers. gardidan “to turn, to change”); Av. varət- “to turn, revolve;” cf. Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;”
O.H.G. werden “to become;” PIE base *wer- “to turn, bend.”

1. To undergo change in form, substance, appearance, etc.
   
2. To change or alter. → covariance.

Etymology (EN): M.E. varien, from O.Fr. varier, from L. variare “change, alter, make different,” from varius “variegated, different, spotted.”

Etymology (PE): Vartidan “to change,” from Mid.Pers. vartitan “to change, turn” (Mod.Pers. gardidan “to turn, to change”); Av. varət- “to turn, revolve;” cf. Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;”
O.H.G. werden “to become;” PIE base *wer- “to turn, bend.”