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

A statement that one expects to be true, but for which one does not yet know a proof. Once the → proof is found, the conjecture becomes a → theorem.

→ mathematical; → conjecture.

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 → thought experiment meant to raise questions about the possibility of violating the → second law of thermodynamics. A wall separates two compartments filled with gas. A little "demon" sits by a tiny trap door in the wall. He is able to sort hot (faster) molecules from cold molecules without expending energy, thus bringing about a general decrease in → entropy and violating the second law of thermodynamics. The → paradox is explained by the fact that such a demon would still need to use energy to observe and sort the molecules. Thus the total entropy of the system still increases.

Named after James Clerk Maxwell (→ maxwell), who first thought of this experiment; → demon.

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.

A fictitious Moon that has the same average motion as the true Moon but that is not subject to any gravitational perturbations by other bodies.

→ mean; → moon.

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.

A unit of explosive force equal to one million metric tons of → T.N.T.. 1 megaton = 4.2 × 10²² → ergs = 4.2 × 10¹⁵ → joules.

→ mega-; → ton.

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 nuclear particle with a mass intermediate between that of a proton and an electron, which is believed to be responsible for the strong nuclear force. In contrast to the case of baryons or leptons, meson number is not conserved: like photons, mesons can be created or destroyed in arbitrary numbers. Their charge can be positive, negative, or zero.

From mes-, meso-, from Gk. mesos "middle," akin to L. medius, Pers. miyân, → medium, → middle, + -on a suffix used in the names of subatomic particles.

An environment (→ galaxy, → nebula) whose → metallicity is larger than that of the → Milky Way galaxy.

→ metal; → rich; → environment.