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

Same as → viscometer.

→ viscosity; → -meter.

The property of a → fluid that resists the force tending to cause the fluid to flow. Viscosity may be thought of as the internal → friction of two fluid layers which flow parallel to each other at different speeds. The cause of viscosity is the transport of → momentum by the molecules from one layer to the other. Viscosity is given by η = φ.u.λ.ρ, where φ is a coefficient which depends on the nature of the interaction between the molecules, u is the average velocity of thermal motion of the molecules, λ is the → mean free path, and ρ the → density of the fluid. Also called → dynamic viscosity or → absolute viscosity. See also → kinematic viscosity.

Noun from → viscous; → -ity.

Having the property of → viscosity. See also: → viscous dissipation, → viscous decretion disk, → viscous fluid, → viscous force, → nonviscous.

M.E., from M.Fr. viscous, from L. viscosus "sticky," from viscum "anything sticky; mistletoe."

From vošk "a kind of sticky gum" + -sân suffix of similarity, from sân "way, manner."

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.

A fluid whose viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid.

→ viscous; → fluid.

The force per unit volume or per unit mass arising from the action of tangential stresses in a moving → viscous fluid.

→ viscous; → force.

1) General: The state or fact of being visible. → crescent Moon visibility, → Venus visibility.
2) Optics: → fringe visibility. See also: → visibility function, → visibility plane.
3) Meteo.: A measure of transparency of the atmosphere. The maximum horizontal distance at which objects can be identified. → turbidity.

→ visible + → -ity.

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

→ visibility; → function.

In → interferometry, the projection of a → baseline onto the plane normal to the source direction defining a vector in (u,v) space, measured in wavelength units.

→ visibility; → plane.

1) Capable of being seen by, or perceptible to, the human eye.
2) → visible light.

M.E., from O.Fr. visible, from L. visibilis "that may be seen," from visus, p.p. 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").

Diyâr "visible" in several dialects, e.g. štiyâni, Malâyeri, Širâzi, Tabari, related to 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."
Padidâr (→ phenomenon) and peydâ are both also related to dā(y)- (with prefixes).

That line where Earth and sky appear to meet, and the projection of this line upon the celestial sphere. Also known as the apparent horizon.

→ visible; → horizon.

The portion of the → electromagnetic radiation that can be seen by the human → eye. The → wavelengths extend from about 400 nm (violet) to 750 nm (red). The wavelengths of various colors of the visible spectrum are as follows: → violet: 390-455 nm; → blue: 455-492 nm; → green: 492-577; → yellow: 577-597; → orange: 597-622; → red: 622-780 nm.

→ visible; → light.

The portion of → electromagnetic spectrum corresponding to → visible light.

→ visible; → spectrum.

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").

1) Of or pertaining to seeing or sight.
2) → optical.

M.E., from L.L. visualis "of sight," from L. visus "sight," from visus, .pp. 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").

Didi, of or pertaining to 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"); didgâni, adj. of didgân, plural of didé, did "sight, eye; seen;" didâri, from didâr, from didan.

Same as → acuity of vision.

→ visual; → acuity.

A → binary system of stars whose components can be resolved telescopically and which have detectable orbital motion.

→ visual; → binary.

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

→ visual; → extinction

The → apparent magnitude of a celestial body in the color sensitivity region of the human eye at a wavelength of 5600 Å. Visual magnitude is now essentially synonymous with V magnitude, which is determined photometrically.

→ visual; → magnitude.