An Etymological Dictionary of Astronomy and Astrophysics
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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.

Same as → acuity of vision.

→ visual; → acuity.

A logarithmic scale, ranging from 1 to 8, used to measure the intensity of volcano eruptions. The VEI is based on several factors: the degree of fragmentation of the volcanic products released by the eruption, the amounts of sulfur-rich gases that form stratospheric aerosols, the volume of the eruptions, their duration, and the height is reached. The largest eruptions (8) produce an amount of bulk volume of ejected → tephra of ~ 1,000 km³.

→ volcanic; → explosivity; → index.

In fluid mechanics, a measure of the rate of rotational spin in a fluid. Mathematically, vorticity is a vector field defined as the curl of the velocity field: ω = ∇ x v.
Meteo.: The rotation of air around a vertical axis.

From L. vortic-, from → vortex + → -ity.

Gerdšârigi, from gerdšâr→ vortex + -igi, → -ity.

The principle admitted in → quantum mechanics that all particles have a wave-like nature and that waves have a particle aspect. The wave-particle duality is of fundamental importance in obtaining a realistic picture of the → elementary particles.
See also: → particle nature, → wave nature, → de Broglie hypothesis, → Davisson-Germer experiment.

→ wave; → particle; → duality.

A constant the value of which is characteristic for any given → tensor density.

→ weight; → tensor; → density.

The final kinetic energy of the → stellar wind expressed by: (1/2)Mdot.v_∞² = (1/2)(v_∞/c)L For an O6 star, L ~ 3 x 10⁵Lsun and v_∞ ~ 2000 km s^-1, which give a wind luminosity of ~ 1 x 10³⁷ erg s^-1, about 1% of the → stellar luminosity. See also → photon tiring limit.

→ wind; → luminosity.

The speed at which the → stellar wind is forced away from the star. Wind velocities of → hot stars are directly measured from → P Cygni profiles, which indicate velocities from several hundred to several thousand km s^-1. See also → escape velocity, → terminal velocity, → velocity law, → radiation-driven wind, → CAK model.

→ wind; → velocity.

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ C_J. The surface that limits this region, defined by U = C_J, is called the zero-velocity surface.

→ zero; → velocity; → surface.