An Etymological Dictionary of Astronomy and Astrophysics
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A book of tables, usually covering a period of one calendar year, that lists the future positions of the Moon, planets, and other prominent celestial objects, together with other useful astronomical data.

M.E. almenak, from M.L. almanach, perhaps from late Gk. almenikhiaka "ephemeris," perhaps of Coptic origin.

Axtar sâlnâmé, from axtar, → star, + sâlnâmé "calendar," from sâl, → year, + nâmé "book."

The splitting of a spectral line into several components in the → Zeeman effect when the magnetic field is weak. The splitting is much more complex than in the normal effect. The number of components of the lines often considerably exceeds their number in the normal effect. Contrarily to the normal Zeeman effect, the anomalous effect cannot be explained by classical theory. The historically "anomalous" effect is accounted for by the inclusion of electron spin in the total angular moment. In fact the idea of electron spin was put forward (Uhlenbeck and Goudsmit, 1926) to explain the anomalous Zeeman effect.

→ anomalous; → Zeeman effect.

→ Boltzmann's constant.

→ Boltzmann's constant.

The factor e^-E/kT involved in the probability for atoms having an excitation energy E and temperature T, where k is Boltzmann's constant.

→ Boltzmann's constant; → factor.

The physical constant, noted by k, relating the mean → kinetic energy of → molecules in an → ideal gas to their → absolute temperature. It is given by the ratio of the → gas constant to → Avogadro's number. Its value is about 1.380 x 10^-16erg K^-1.

Named after the Austrian physicist Ludwig Boltzmann (1844-1906), who made important contributions to the theory of statistical mechanics; → constant.

In → statistical thermodynamics, a probability equation relating the → entropy S of an → ideal gas to the quantity Ω, which is the number of → microstates corresponding to a given → macrostate: S = k. ln Ω. Same as → Boltzmann's relation.

→ Boltzmann's constant; → entropy; → formula.

1) An equation that expresses the relative number (per unit volume) of → excited atoms in different states as a function of the temperature for a gas in → thermal equilibrium: N_u/N_l = (g_u/g_l) exp (-ΔE/kT_ex), where N_u and N_l are the upper level and lower level populations respectively, g_u and g_l the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h  → Planck's constant.

→ Boltzmann's constant; → equation.

A relation between the → entropy of a given → state of a → thermodynamic system and the → probability of the state: S = k . ln Ω where S is the entropy of the system, k is → Boltzmann's constant, and Ω the thermodynamic probability of the state. Boltzmann's relation connects → statistical mechanics and → thermodynamics. Ω is the number of possible → microstates of the system, and it represents the → randomness of the system. The relation also describes the statistical meaning of the → second law of thermodynamics. This expression has been carved above Boltzmann's name on his tombstone in Zentralfreihof in Vienna. Same as → Boltzmann's entropy formula.

→ Boltzmann's constant; → relation.

The act or process or an instance of dehumanizing.

Verbal noun of → dehumanize.

To deprive of → human qualities or personality.

→ de-; → dehumanize.

A major subdivision of Earth's internal structure, located beneath the → Earth's crust and above the central → core. On average, the mantle begins 35 km below the surface and ends at a depth of about 2,900 km. See also → upper mantle and → lower mantle, → asthenosphere, → lithosphere.

→ Earth; → mantle.

A kind of viscous → boundary layer in a rotating fluid system. Such a layer forms over a flat bottom that exerts a frictional → stress against the flow, bringing the velocity gradually to zero within the layer above the bottom. An Ekman layer occurs also on the fluid surface whenever there is a horizontal frictional stress, for example along ocean surface, when waters are subject to wind stress.

Named for Vagn Walfrid Ekman (1874-1954), Swedish oceanographer, who studied the phenomenon originally in his doctoral thesis (1902) and later developed it (1905, 1906); → layer.

A → dimensionless quantity that measures the strength of → viscous forces relative to the → Coriolis force in a rotating fluid. It is given by E_k = ν/(ΩH²), where ν is the → kinematic viscosity of the fluid, Ω is the → angular velocity, and H is the depth scale of the motion. The Ekman number is usually used in describing geophysical phenomena in the oceans and atmosphere. Typical geophysical flows, as well as laboratory experiments, yield very small Ekman numbers. For example, in the ocean at mid-latitudes, motions with a viscosity of 10^-2 m²/s are characterized by an Ekman number of about 10^-4.

→ Ekman layer; → number.

To flow out, issue, or proceed, as from a source or origin; come forth; originate; arise (Dictionary.com).

From L. emanatus, p.p. of emanare "flow out," figuratively "arise from, proceed from."

Runemudan "to appear, come out," literally "to show face," from ru "face," → surface, + nemudan "to show," → display.

An act or instance of emanating; something that emanates or is emanated.

→ emanate; → -tion.

A schematic representation, in quantum electrodynamics and quantum chromodynamics, of the way elementary particles like electrons and protons interact with each other by exchanging photons. Use of Feynman diagrams can greatly reduce the amount of computation involved in calculating a rate or cross section of a physical process.

After the American physicist Richard P. Feynman (1918-1988), Nobel prize 1965; → diagram.

A manifold with a → Riemannian metric that has → zero → curvature.

→ flat; → manifold.

A statistical finding about "normal" → spiral galaxies, whereby there is an upper limit on the mean central → surface brightness of disks. This value is constant for different spiral types, amounting to 21.65 ± 0.30 mag arcsec² in the B band.

Named after K. C. Freeman (1970, Ap.J. 160, 811); → law.

An equation that expresses energy conservation in an → expanding Universe. It is formally derived from → Einstein's field equations of → general relativity by requiring the Universe to be everywhere → homogeneous and → isotropic. It is expressed by H²(t) = (8πG)/(3c²)ε(t) - (kc²)/R²(t), where H(t) is the → Hubble parameter, G is the → gravitational constant, c is the → speed of light, ε(t) is the → energy density, k is the → curvature of space-time, and R(t) is the → cosmic scale factor. See also → Big Bang, → accelerating Universe. See also → Friedmann-Lemaitre Universe.

Named after the Russian mathematician and physical scientist Aleksandr Aleksandrovich Friedmann (1888-1925), who was the first to formulate an → expanding Universe based on Einstein's theory of → general relativity ; → equation.

One of the first → cosmological models to incorporate Einstein's → general relativity, predicting that → galaxies should be → receding from each other due to → cosmic expansion.

→ Friedmann equation; Georges Edouard Lemaître (1894-1966), a Belgian cosmologist and priest who proposed a first sketch of the → Big Bang theory; → universe.