An Etymological Dictionary of Astronomy and Astrophysics
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A measure of the angular displacement per unit time. Of a particle traveling on a circular path or a rotating body, the ratio of the angle traversed to the amount of time it takes to traverse that angle: ω = dθ/dt. For a rigid body, all lines in it rotate through the same angle in the same time, and the angular velocity is the characteristic of the body as a whole. The angular velocity is related to the linear velocity by the equation v = rω, where r is the distance of the point from the rotation axis. → vector angular velocity.

→ angular; → velocity.

The departure from → isoplanicity.

→ an-; → isoplanicity.

The maximum phase of an → annular eclipse during which the Moon's entire disk is seen silhouetted against the Sun. Annularity is the period between second and → third contact during an annular eclipse. It can last from a fraction of a second to a maximum of 12 minutes 29 seconds (F. Espenak, NASA).

→ annular; → -ity.

Discrepant luminosity classes derived for the same → Am star when different criteria are used. Lore specifically, a luminosity criterion may indicate a → giant star, wheras another criterion indicates a → supergiant.

→ anomalous; → luminosity; → effect.

A measure of the degree to which a climate lacks effective moisture.

Aridity, noun from → arid.

Kamâbi, noun from amâbi, → arid.

Put in modern notation, if α and β are acute angles and if β <α, then sin α / sin β <α / β < tan α / tan β. Aristarchus probably used this inequality to show that the Sun is between 18 and 20 times as far from the Earth as the Moon is.

Aristarchus of Samos (c.310-c.230 BC); → inequality.

1) Roughness or unevenness of surface.
2) Physics: A microscopic projection on a metal surface resulting from normal surface finishing processes.
3) Geology: A site on a → fault surface where the two sides are held together by an area of higher strength than the areas surrounding it. When the stress on the fault exceeds the strength of the asperity, the asperity fails as an → earthquake.

M.E. asperite, from O.Fr. asperité "difficulty, painful situation," from L. asperitas, from asper "rough, harsh," of unknown origin.

From zaft "thick, gross, rude," (Steingass, Dehxodâ), + noun suffix -i.

Of or relating to association; state of being associative.

→ associative + → -ity.

For → stellar winds, same as → terminal velocity.

→ asymptotic; → velocity.

1) The power to determine, adjudicate, or otherwise settle issues or disputes; jurisdiction; the right to control, command, or determine (Dictionary.com).
2) A person or body of persons in whom authority is vested, as a governmental agency (Dictionary.com).

→ author; → -ity.

Dâtârgân, from dâtâr, → author, + -gân, on the model of xodâygân "a great lord."

The ratio of the displacement (Δx) of a particle, as it moves from point A to point B, to the corresponding time interval: v = Δx/Δt.

→ average; → velocity.

An abrupt decrease in the intensity of the continuum at the limit of the → Balmer series of hydrogen (at about 3650 Å), caused by the energy absorbed when electrons originally in the second → energy level are ionized. Same as → Balmer jump.

→ Balmer; → discontinuity.

1) A type of instability occurring within a rapidly → rotating star where non-axisymmetric motions can separate surfaces of constant pressure from → equipotential surfaces.
2) A hydrodynamic instability associated with a baroclinic layer of the atmosphere. It arises from temperature variation along the pressure surfaces. Baroclinic instability is associated with the vertical → shear of the mean flow, which is related to the horizontal temperature gradient by the thermal wind equation. Instabilities in a baroclinic region grow by converting potential energy associated with the mean horizontal temperature gradient into kinetic energy through ascending warm air and descending cold air (Rasmussen & Turner (eds.), Polar Lows, Cambridge Univ. Press, 2003).

→ baroclinic; → instability.

The state of stratification in a fluid in which surfaces of constant pressure do not coincide with those of constant density, but intersect. Where baroclinicity is zero, the fluid is → barotropic. Same as baroclinity.

→ baroclinic; → -ity.

A hydrodynamical instability that arises when the horizontal → shear gradient becomes very large. Barotropic instabilities grow by extracting kinetic energy from the mean flow field.

→ barotropic; → instability.

Any of a large number of inequality relations developed to study the → hidden variable hypothesis suggested in the → EPR paradox. Using Bell's inequalities, the → Aspect experiment showed that no local hidden variable theory can make predictions in agreement with those of quantum mechanics. If, in a measurement, the inequality is violated, the measurement is in agreement with the predictions of the quantum theory. If the equality is satisfied, it suggests that a classical, causal, and local model is adequate to explain the outcome of the measurements. See also → quantum entanglement.

John Stewart Bell (1928-1990); → inequality.

A hypothetical state of → infinite energy density representing an infinite → gravitational field and infinite → space-time curvature. The singularity arises from using Einstein's theory of → general relativity concerning gravity. We know, however, that when the density and heat become extremely large, quantum physics of gravity becomes important. Yet Einstein's equations ignore quantum effects. In other words, in certain extreme conditions, Einstein's equations do not apply.

→ Big Bang; → singularity.

The quality or state of being → bimodal.

→ bimodal; → -ity.

The quality or condition of being binary.

→ binary; → -ity.

The → variety of → plant and → animal → species in a particular → environment.

→ bio-; → diversity.