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

An international cooperation whose objective is to facilitate the international coordination and collaboration necessary for the development and deployment of the tools, systems and organizational structures necessary to enable the international utilization of astronomical archives as an integrated and interoperating → Virtual Observatory. The IVOA, created in 2002, now comprises 20 Virtual Observatory programs from various countries and international organizations.

→ international; → virtual; → observatory; → alliance.

1) General: Having the efficacy without the material part; unreal but capable of being considered as real for some purpose.
2) Computers: Simulated by a computer (for reasons of experiment or convenience) of an entity that lacks some elements of total reality.

M.E., from M.L. virtualis, from L. virtus "manliness, excellence, potency, efficacy," from vir "man, human, husband, soldier," cf. Mid.Pers. vīr, wīr "man, hero;" Av. vīra- "man, human;" Skt. vīrá- "man, hero;" Lith. vyras "man, husband;" O.Ir. fer "man;" Goth. wair "man;" O.E. wer "man." In Roman philosophy, virtue became associated with virility and strength of character.

Virâgin from vir "intellect, mind, memory," variants bar, bir (Mid.Pers. vir, varm, vârom "mental faculty, memory, mind;" Av. vārəma, vārəm "according to one's wishes," from var- "to choose") + -âgin a suffix denoting "consisting of, similarity, possession."

In → analytical mechanics, any infinitesimal change in the configuration of a material system, consistent with any constraints acting on the system at a given instant. If the constraints are stationary (→ scleronomous), then the actual displacement of the system, in an infinitesimal length of time dt, coincides with one of its virtual displacements. In the case of time-dependent (→ rheonomous) constraints, the actual displacement of the system does not coincide with any of the virtual ones, since the conditions imposed by the constraints vary during the time dt.

→ virtual; → displacement.

Optics: An image formed inside an instrument at the point where diverging rays would cross if they were extended backward into the instrument. Such an image cannot be obtained on a screen placed at its apparent position, since the rays do not pass through that point. → real image.

→ virtual; → image.

An international initiative by the astronomical community to allow global electronic access to the available astronomical data archives of space and ground-based observatories. It also aims to enable data analysis techniques through a coordinating entity that provides common standards, wide-network bandwidth, and state-of-the-art analysis tools. The Virtual Observatory is also intended for re-using data for scientific objectives different from the original ones, in order to optimize the science return of astronomical observations. The Virtual Observatory's capabilities are enabled through the use of standard protocols for registering the existence and location of data and for requesting data that satisfies the user's interests. These standards are developed on an international basis through the → IVOA. The cornerstone of the Virtual Observatory is → interoperability.

→ virtual; → observatory.

A subatomic particle that, according to the uncertainty principle, comes into being out of energy fluctuations of the "vacuum" and lasts for extremely short periods of time. An electron-positron pair can exist only about 4 x 10^-21 seconds. The lifetime increases as the mass and energy involved decreases. Virtual particles are real and have measurable effects, but cannot be directly observed, according to the uncertainty principle. → vacuum polarization.

→ virtual; → particle

In → analytical mechanics, an element of work performed in a → virtual displacement by the → forces acting on all n particles of a → holonomic system with s degrees of freedom (→ degree of freedom).

→ virtual; → work.

In → analytical mechanics, a principle whereby it is necessary and sufficient for the equilibrium of any material system with ideal constraints that the sum of the elements of work, performed by the applied forces acting on the system in any virtual displacement, be equal to zero (if all constraints are bilateral) or less than zero (if some of the constraints are unilateral).

→ virtual; → work; → principle.