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

In probability and statistics, of a random variable, the summation or integration, over all values of the random variable, of the product of the value and its probability of occurrence. Also called → expectation, → expected value.

→ mathematical; → expectation.

A set of four fundamental equations that describe the electric and magnetic fields arising from varying electric charges and magnetic fields, electric currents, charge distributions, and how those fields change in time. In their vector differential form, these equations are:
i) ∇.E = ρ/ε₀ (→ Gauss's law for electricity),
ii) ∇.B = 0 (→ Gauss's law for magnetism),
iii) ∇ x E = -∂B/∂t (→ Faraday's law of induction),
iv) ∇ x B = μ₀J + μ₀ε₀∂E/∂t (→ Ampere's law), with c² = 1/(μ₀ε₀), where E is → electric intensity, B is → magnetic flux density, ρ is → charge density, ε₀ is → permittivity, μ₀ is → permeability, J is → current density, and c is → speed of light.

→ maxwell. It should be emphasized that the equations originally published by James Clerk Maxwell in 1873 (in A Treatise on Electricity and Magnetism) were 20 in number, had 20 variables, and were in scalar form. The German physicist Heinrich Rudolf Hertz (1857-1894) reduced them to 12 scalar equations (1884). It was the English mathematician/physicist Oliver Heaviside (1850-1925) who expressed Maxwell's equations in vector form using the notations of → gradient, → divergence, and → curl of a vector, thus simplifying them to the present 4 equations (1886). Before Einstein these equations were known as Maxwell-Heaviside-Hertz equations, Einstein (1940) popularized the name "Maxwell's Equations;" → equation.

The distribution law for kinetic energies (or, equivalently, speeds) of molecules of an ideal gas in equilibrium at a given temperature.

→ maxwell; → Boltzmann's constant; → distribution.

The average movement of a body along its orbit in one day, usually expressed in degrees.

→ mean; → diurnal; → motion.

The average angular velocity of a satellite in an elliptical orbit.

→ mean; → motion.

Same as → mean place.

→ mean; → position.

The act of meditating. → consideration.

From L. meditatatus p.p. of meditari "to think over, reflect, consider," from PIE root *med- "to measure, limit, consider, advise," → mode.

Verbal noun of → meditate.

One of several functions used in the → fuzzification and → defuzzification steps of a → fuzzy logic system to map the → nonfuzzy input values to → fuzzy linguistic terms and vice versa. A membership function is used to quantify a linguistic term.

→ membership; → function.

1) To refer briefly to; name, specify, or speak of (Dictionary.com).
2) A direct or incidental reference; a mentioning (Dictionary.com).

M.E. mencioun, from O.Fr. mencion "mention, memory, speech," from L. mentionem "a calling to mind, a speaking of," from root of Old L. minisci "to think," related to mens "mind," from PIE root *men- "to think;" cf. Pers. man, mân "thought, to think," → mind.

Ayât, from Mid.Pers. ayât, ayâd "remembrance, recollection, memory;" Mod.Pers. yâd.

The observation of a star when it crosses an observer's meridian.

→ meridian; → observation.

Of, pertaining to, or resembling a meridian.

→ meridian + → -al.

From nimruzân, → meridian, + -i adj. suffix.

The mass motion of material within a → rotating star generated by the star's departure from spherical symmetry. For a rotating star in which → centrifugal forces are not negligible, → radiative equilibrium and → hydrostatic equilibrium cannot be satisfied. In this condition energy transfer is accomplished by means of the physical motion of material. According to → von Zeipel theorem, the heating on an → equipotential surface is generally higher in the polar direction than in the equatorial direction, which drives a large scale circulation current rising at the pole and descending at the equator. As a consequence, → mixing of material takes place in the stellar interior; see also → Eddington-Sweet time scale. The meridional circulation plays an important role in the evolution of → massive stars. The circulation current was first suggested by Arthur S. Eddington in 1926 (The Internal Constitution of the Stars, Dover Pub. Inc., New York) and subsequently quantified by P. A. Sweet (1950, MNRAS 110, 548).

→ meridional; → circulation.

Meteo.: A flow between the poles, or between the equator and the poles. A positive value indicates flow away from the equator; a negative value, flow toward the equator.

→ meridional; → flow.

In the → solar dynamo model, a magnetic field that points from the north to south or south to north.

→ meridional; → magnetic; → field.

A plot representing the number of stars (or systems) per metallicity interval, usually expressed in [Fe/H] (abundance of → iron relative to → hydrogen).

→ metallicity; → distribution; → function.

The ionization of air molecules by the heat generated when a meteorite enters the atmosphere.

→ meteorite, → ionization.

Evaluation or measurement of one or more meteorological elements.

Meteorological, of or pertaining to → meteorology; → observation.

The linearization of the appropriate equations governing a system by the assumption of a steady state, with departures from that steady state limited to small perturbations. Also called perturbation method.

→ method; → small; → perturbation.

The solution of an equation or by proceeding from an initial approximation to a series of repeated trial solutions, each depending upon the immediately preceding approximation, in such a manner that the discrepancy between the newest estimated solution and the true solution is systematically reduced.

→ method; → successive; → approximation.

→ ideal magnetohydrodynamics.

→ magnetohydrodynamics (MHD); → condition.