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

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 act of verifying. The state of being verified.
The process of research and examination required to establish correctness, authenticity, or validity.

Verbal noun of → verify.

A particular form or variant of something.
An account of something, given from a specific point of view.

M.E., from M.Fr. version, from M.L. versionem (nominative versio) "a turning," from p.p. stem of L. vertere "to turn;" cognate with Pers. gardidan "to change," as below.

Gardâk, present stem of gardidan "to change, to turn" (Mid.Pers. vartitan "to change, to turn;" 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") + -âk noun suffix.

1) The act of vibrating. The state of being vibrated.
2) A continuing → periodic  → oscillation relative to a fixed reference point or → equilibrium state.

Verbal noun from → vibrate.

Of or pertaining to → vibration. → vibrational mode.

→ vibration; → -al.

The energy due to the vibration of the molecules making up atoms (→ molecular vibration). A molecule in space can have energies in various forms: → rotational energy, vibrational energy, or electronic energy. These energies of molecules are → quantized and a particular molecule can exist in different rotational and vibrational → energy levels. The molecules can move from one level to another level only by a jump involving a finite amount of energy. → Quantum mechanics predicts that any molecule can never have zero vibrational energy, that is atoms can never be completely at rest relative to each other. The harmonically oscillating molecules can undergo vibrational changes determined by simple selection rules obtained from → Schrödinger equation.

→ vibrational; → energy.

The frequency at which the atoms in a molecule vibrate. The frequencies of → molecular vibrations in diatomic molecules are in the order of 10^-12 to 10^-14 Hz. In such molecules, the only → vibrational mode available is along the bond. More complicated molecules have many types of vibration and stretching modes.

→ vibrational; → frequency.

Any of the ways in which a → molecule vibrates. Each vibrational mode has a different → frequency frequency. The number of vibrational modes of a molecule is determined by the number of atoms in it. The number of vibrational modes for a non-linear molecule is 3N - 6, where N is the number of atoms making up the molecule. For a linear molecule it is 3N - 5.

→ vibrational; → mode.

A transition between two → quantized  → levels of a → molecule that have different vibrational energies.

→ vibrational; → transition.

A slight change in the → energy level of a → molecule due to → vibrational transition and/or → rotational transition.

→ vibrational; → rotational; → transition.

The act of violating. The state of being violated. → parity violation

Verbal noun from → violate.

A process in which a dynamical system made up of many objects (star cluster, galaxy cluster) rapidly relaxes from a chaotic initial state to a quasi-equilibrium.

→ violent; → relaxation.

The concept of star formation pertaining to a variety of systems (OB associations, giant H II regions, H II galaxies, massive star clusters, etc.) that are believed to have formed large numbers of stars in a very short time.

→ violent; → star formation.

In thermodynamics, a generalized → equation of state obtained when the → compression factor Z is expanded in terms of a power series, e.g.: Z = 1 + B(T) / V_m + C(T) / V_m² + ...

→ virial; → equation of state.

The process whereby a system of gravitationally interacting particles attains stability. The comparable mass components interact with each other, but the whole system does not expand or collapse. Virialization occurs when the → potential energy is twice the negative → kinetic energy: - W_vir = 2 K_vir (→ virial theorem). In the case of a → galaxy cluster, when the cluster is virialized the merging process and the collapse of matter have finished and the formation process of the galaxy cluster is considered to be done. A cluster has formed by → hierarchical clustering. Virialized clusters, in other words finished clusters, can be found by looking at their radius and density. A cluster is virialized when it satisfies the condition: R_vir ~ R_max/2, where R_vir is the radius when the cluster is virialized and R_max is the radius when the collapse starts. From this condition it follows that the object is 8 times denser at virialization than when the collapse started.

Verbal noun of → virialize.

A model for explaining several observational features of → circumstellar disks around → Be stars. According to this model, the central star provides → angular momentum to the disk at the innermost radius, and then it is redistributed over the whole disk via → viscosity. Thus, in isolated Be stars, the equatorial disk can spread out to a large distance as long as the star can give angular momentum to the disk. The VDD model, first introduced by Lee et al. (1991, MNRAS, 250, 432) and further developed by several other researchers, is now widely accepted as the best physical model for describing the circumstellar disks of Be stars. Among the growing evidence supporting the VDD model is the confirmation that the disks rotate in a Keplerian way (→ Keplerian orbit), allowing for the identification of viscosity as the mechanism that makes the disk grow (see, e.g., Klement et al., 2015, A&A 584, A85).

→ viscous; → decretion; → disk.

A degradation of → mechanical energy that is irreversibly converted to → thermal energy due to → viscous forces in the → fluid. Viscous dissipation occurs in → turbulent flows.

→ viscous; → dissipation.

The → Fourier transform of a source's → brightness distribution, weighted by the characteristics of the → interferometer's antennas.

→ visibility; → function.

The act or power of sensing with the eyes. → averted vision; → acuity of vision.

M.E., from O.Fr. vision, from L. visionem (nominative visio) "act of seeing, sight, thing seen," from p.p. stem of videre "to see," cognate with Pers. bin, present stem of didan "to see" (Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" cf. Skt. veda "I know;" Gk. oida "I know," idein "to see;" PIE base *weid- "to know, to see").

Negâh, "look," from Mid.Pers. nikâh "look, glance, observation;" Proto-Iranian *ni-kas- "to look down," from ni- "down, below," → ni- (PIE), + *kas- "to look, appear;" cf. Av. nikā-, nikāta- (in the name of the 15-th nask) "that which is observed," ākas- "to look;" Mid.Pers. âkâh, Mod.Pers. âgâh "aware, knowing;" Skt. kāś- "to become visible, appear;" Ossetic kast/kaesyn "to look;" did, from didan "to see" (Mid.Pers. ditan "to see, regard, catch sight of, contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)- "to see," didāti "sees;" cf. Skt. dhī- "to perceive, think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka "have seen").

The → extinction in the visual range of the electromagnetic radiation.

→ visual; → extinction