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.

The mean distance which a particle moves between two successive collisions with other particles of the medium. Mean free path is inversely proportional to the number of particles per cm³ (n), and the collision → cross section (σ). In the case of a gas with molecules having a diameter of d, the cross section is equal to the area of a circle of radius d, i.e. σ = πd², and the mean free path is given by: l = 1/(nσ). Taking into account the relative velocity distribution of the colliding molecules, l = 1/(√2 . nσ). For a gas at one atmosphere pressure and room temperature, the average distance between molecules is roughly 3.5 × 10^-7 cm, that is some 35 times the diameter of a molecule. Taking the gas density n = 2.4 × 10¹⁴ molecules cm^-3, and a typical diameter d = 2 × 10^-8 cm for a molecule, the mean free path is 3.3 × 10^-5 cm. This means that the average distance between collisions is about 95 times the average distance between molecules.

→ mean; → free; → path.

Puyeš, verbal noun of puyidan "to run, trot; wander," from Mid.Pers. pôy-, pwd- "to run;" cf. Gk. speudein "to hasten;" Lith. spudinti; âzâd, → free; miyângin, → mean.

The average time interval between two successive → upper transits of the → mean equinox.

→ mean; → sidereal; → day.

The hour angle of the mean equinox for a given observer.

→ mean; → sidereal; → time.

1) If f(x) is a continuous function on the interval from a to b, then:
∫ f(x) dx = f(c)(b - a) (summed from a to b) for at least one point in that interval.
2) More generally, If f(x) and g(x) are continuous functions on the interval from a to b and g(x)≥ 0, then:
∫ f(x)g(x) dx = f(c) ∫ g(x) dx (both integrals summed from a to b).

→ mean; → value; → theorem.

1) A unit or standard of → measurement; the act or process of ascertaining the extent, dimensions, or quantity of something; measurement; the quantity obtained by such a process.
2) To use standard units to determine the magnitude, extent, size, etc. of something. The quantity obtained by such a process.

From O.Fr. mesurer, from L.L. mensurare "to measure," from L. mensura "a measuring, a thing to measure by," from mensus, p.p. of metiri "to measure," → meter.

1) Andâzé "measure," from Mid.Pers. andâzag, handâcak "measure," handâxtan, handâz- "to measure," Manichean Mid.Pers. hnds- "to measure," Proto-Iranian *hamdas-, from ham-, → com-, + *das- "to heap, amass;" cf. Ossetic dasun/dast "to heap up;" Arm. loanword dasel "to arrange (a crowd, people)," das "order, arrangement."
2) Andâzé gereftan, compound verb, literally "to take measure," from andâzé, as above, + gereftan "to take, seize" (Mid.Pers. griftan; Av./O.Pers. grab- "to take, seize;" cf. Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving;" M.L.G. grabben "to grab;" from P.Gmc. *grab; E. grab "to take or grasp suddenly;" PIE base *ghrebh- "to seize").

1) The act of measuring; a measured quantity.
2) The determination of the magnitude or amount of a quantity by comparison (direct or indirect) with the prototype standards of the system of units employed (IEEE Standard Dictionary of Electrical and Electronics Terms). → absolute measurement, → measurement uncertainty, → Roemer's measurement.

Verbal noun of → measure.

The interval within which lies the actually measured value of a physical quantity and the true value of the same physical quantity.

→ measurement; uncertainty, from negation prefix un- + → certainty.

Same as → Dyson sphere.

→ mega-; → structure.

One of the seven stars of the → Big Dipper, which links Ursa Major's tail to the Bear's hindquarters. Megrez is the dimmest of the Big Dipper stars at magnitude +3.3. It is an A3 dwarf, about 20 times more luminous that the Sun, lying 81 light-years away.

From Ar. al-Maghriz(المغرز) "the root, base, bottom" (of the Bear's tail).

A method used in → numerical simulations for studying the growth and development of galaxies and → dark matter halos. Within the currently accepted ΛCDM cosmology, dark matter halos merge from small → clumps to ever larger structures. This merging history can be traced in simulations and stored in the form of merger trees. Merger trees are necessary because a galaxy may have more than one → progenitor at an early time.

→ merger; → tree.

The layer of the atmosphere located between the → stratosphere and the → ionosphere, where temperature drops rapidly with increasing height. It extends between 17 to 80 kilometers above the Earth's surface.

→ meso-; → sphere.

The meteoroids distributed all along an → orbit and diffused somewhat around it.

→ meteoroid; → stream.

A method of fitting a curve to data points so as to minimize the sum of the squares of the distances of the points from the curve.

→ method; → least squares.

Any of the suffixes adopted by the International System of Units (→ SI units).
For numbers larger than 1: → deca- (10¹), → hecto- (10²), → kilo- (10³), → mega- (10⁶), → giga- (10⁹), → tera- (10¹²), → peta- (10¹⁵), → exa- (10¹⁸), → zetta- (10²¹), and → yotta- (10²⁴) .
For numbers smaller than 1: → deci- (10^-1), → centi- (10^-2), → milli- (10^-3), → micro- (10^-6), → nano- (10^-9), → pico- (10^-12), → femto- (10^-15), → atto- (10^-18), → zepto- (10^-21), and → yocto- (10^-24).

→ metric; → prefix.

The portion of the → electromagnetic radiation lying between the → near-infrared and the → far-infrared. This covers the wavelength range approximately from 8 to 30 → microns. See also: → infrared radiation, → submillimeter radiation.

→ mid-; → infrared.

The region lying between the → troposphere and the → thermosphere comprising the → stratosphere and the → mesosphere (Meteorology Glossary, American Meteorological Society).

→ middle; → atmosphere.

Same as → mid-infrared.

→ middle; → infrared.

A chemical experiment conducted in 1953 that aimed at checking Alexander Oparin's and J. B. S. Haldane's hypothesis that under putative conditions present in the atmosphere of the early Earth inorganic molecules would spontaneously form organic molecules. Miller and Urey filled a sterile flask with a mixture of water, ammonia, methane, and hydrogen. The mixture was heated to evaporate water to produce water vapor. High-voltage electric sparks were passed through the mixture to simulate lightning. After a week, contents were analyzed. Amino acids, the building blocks for proteins, were found.

Named after Stanley L. Miller (1930-2007) and Harold C. Urey (1893-1981); → experiment.

Logic: In a → categorical syllogism, the premise containing the → minor term.

→ major; → premisse.