An Etymological Dictionary of Astronomy and Astrophysics

A star whose spectrum is dominated by the absorption bands of
→ titanium oxide (TiO) and → vanadium oxide (VO) and has many neutral metal lines. The → effective temperature of M dwarfs ranges from about 3850 to 2600 K. They are low mass stars with masses ranging from 0.6 times that of the Sun at spectral type M0 to less than 0.1 → solar masses. M dwarfs are very abundant, they account for about 70-80% of stars in the → Galactic disk. The nearest star to the Sun, → Proxima Centauri, is an M dwarf.

See also: M, letter of alphabet in the → Harvard classification; → dwarf.

A cool, red star of spectral type M with a surface temperatures of less than 3600 K. The spectra of M stars are dominated by molecular bands, especially those of TiO. Naked-eye examples are Betelgeuse and Antares.

See also: M, letter of alphabet, → star.

A type of → asteroid that consists of metallic → iron and displays high → albedo (0.10-0.18). M-type asteroids inhabit the middle part of the → asteroid belt.

See also: M for → metallic; → type; → asteroid.

An → open cluster in the constellation → Serpens associated with the → Eagle Nebula. It is usually mistakenly stated as being that nebula. Also called NGC 6611. First discovered by Philippe Loys de Chéseaux in 1745-6.

See also: Object bearing the number 16 in the → Messier catalog.

Half of the vertex angle of the → Mach cone
generated by a body in → supersonic flight.
It is given by sin μ = (a/v), where a is the speed of the sound waves and v the velocity of the object. In terms of Mach number: μ = asin (1/M). The Mach angle diminishes with the object velocity. For M = 1 it is 90°, for M = 2, it is 30°, and for M = 5, its is 11.5°.

See also: → Mach number; → angle.

The cone that confines the pressure disturbance created by a
→ supersonic object moving in a → compressible medium.

See also: → Mach number; → cone.

Same as → shock diamond.

See also: So named because Ernst Mach (1838-1916) was the first to record its existence.

The ratio of the speed of a moving object to the → sound speed in the medium through which the object is traveling.

See also: Named after the Austrian physicist Ernst Mach (1838-1916); → number.

The envelope of wave fronts created by a → supersonic source.

See also: → Mach number; → wave.

The local → inertial frame and the → inertia of any body results from the distribution of all matter in the Universe. This principle has been neither confirmed nor refuted.

See also: → Mach number; → number.

1. An apparatus consisting of interrelated parts with separate functions, used in the performance of some kind of work.
   

2. A device that transmits or modifies force or motion.

Etymology (EN): From M.Fr. machine “device, contrivance,” from L. machina “machine, fabric, device, trick,” from Doric Gk. makhana, Attic Gk. mekhane “device, means,” related to mekhos “means, expedient,” from PIE *maghana- “that which enables,” from base *magh- “to be able, have power” (cf. O.C.S. mogo “be able,” O.E. mæg “I can”).

Etymology (PE): Mâšin, loanword from Fr.

A collective term for objects that reside in the → halo of a galaxy (in particular → brown dwarfs) and which do not emit enough radiation to be detected from Earth. MACHOs can be spotted using the technique of → microlensing.

See also: Acronym from Massive Astrophysical Compact Halo Objects.

A → Taylor series that is expanded about the reference point zero.

See also: Named after Colin Maclaurin (1698-1746), a Scottish mathematician.

A combining form meaning “large, long, great, excessive,” used in the formation of compound words; opposite of → micro-.

Etymology (EN): From Gk. makros “long, large,” from PIE base *mak-/*mek- “long, thin” (cf. L. macer “lean, thin;” O.N. magr, O.E. mæger “lean, thin”).

Etymology (PE): Dorošt “large; rough, fierce,” from Mid.Pers. društ “harsh, coarse;” O.Pers. darš- “to dare,” daršam (adv.) “mightily;” Av. darš- “to dare,” darši-, daršita- “bold, strong;” cf. Skt. dhars- “to be bold, courageous, to attack,” dhrsita- “bold, daring;” Gk. thrasys “bold;” O.E. durran; E. dare.

The great world or Universe; the Universe considered as a whole (opposed to → microcosm).
A representation of a smaller unit or entity by a larger one, presumably of a similar structure.

See also: → macro-; → cosmos.

A stellar → explosion with energies between those of a → nova and a → supernova and observationally distinguished by being brighter than a typical nova (M_V ~ -8 mag) but fainter than a typical supernova (M_V ~ -19 mag) (Kulkarni 2005; arXiv:astro-ph/0510256).

See also: → macro-; → nova.

Of or relating to scales large enough to be visible to the naked eye or under low order of magnification. Compare → microscopic. → microscopic state.

See also: → macro-; → -scope + → -ic.

Same as → macrostate.

See also: → macroscopic; → state.

Statistical physics: A state of a physical system that is described in terms of the system’s overall or average properties at a macroscopic level (→ temperature, → pressure, → density, → internal energy, etc.). A macrostate will generally consist of many different → microstates. In defining a macrostate we ignore what is going on at the microscopic (atomic/molecular) level. The → probability of a certain macrostate is determined by how many microstates correspond to this macrostate. Therefore, the greater the number of microstates which lead to a particular macrostate, the greater the probability of observing that macrostate. Same as → macroscopic state. See also → entropy, → Boltzmann’s entropy formula, → multiplicity.

See also: → macro-; → state.

The broadening of a star’s → spectral lines due to → Doppler shifts from motions of different parts of the star’s atmosphere.

See also: → macro; → turbulence.

1. Of, relating to, or named from, Ferdinand Magellan (see below).
   

   2. Of or pertaining to characteristic of the → Magellanic Clouds.

See also: Named in honor of Ferdinand Magellan (c. 1480-1521), the Portuguese navigator, who undertook the first voyage around the world. The two Clouds were first described by Magellan’s chronicler Pigafetta, after leaving the Strait of Magellan in 1520; → -ic.

A filament of → neutral hydrogen which connects the → Small Magellanic Cloud and → Large Magellanic Cloud. The Magellanic Bridge appears to result from a → close encounter between these two galaxies some 200 million years ago.

See also: → Magellanic; → bridge.

Two irregular satellite galaxies of our own Galaxy which are visible from the Southern Hemisphere as misty patches in the night sky. → Large Magellanic Cloud; → Small Magellanic Cloud.

See also: → Magellanic; → cloud.

A class of low-mass galaxies with relatively rare features. In particular, these galaxies are characterized by a → stellar bar whose center is displaced from that of the disk and a one-armed spiral. The → Large Magellanic Cloud (LMC) is considered the prototype of this class of objects. However, despite a wealth of data, there is still a good deal of uncertainty concerning the nature of the LMC’s bar.

The majority of the observed Magellanic spirals in the nearby Universe share the LMC’s structure, in particular the evidence of an offset bar and a one-armed spiral structure. A good example of these systems is NGC 3906, which shows evidence of the bar offset from the photometric center of the galaxy by 1.2 kpc (Pardy et al., 2016, ApJ 827, 149).

See also: → Magellanic; → spiral; → galaxy.

A thin trail of gas stretching from the → Magellanic System toward our own Galaxy over about 150° on the sky, corresponding to hundreds of thousands of light-years. This gas consists primarily of → neutral hydrogen and is thought to have originated from the Large and Small Magellanic Clouds as a result of tidal interactions with the Milky Way. See, e.g., Fox et al. 2013, arxiv/1304.4240, and references therein.

See also: → Magellanic; → stream.

A system consisting of the → Magellanic Clouds, the → Magellanic Bridge, and the → Magellanic stream.

See also: → Magellanic Clouds; → system.

A → metal-poor, → irregular galaxy like the → Large Magellanic Cloud or the → Small Magellanic Cloud. Other examples are: NGC 4449, NGC 4214, and NGC 3109.

See also: → Magellanic; → type; → galaxy.

1a) The power of apparently influencing events by using mysterious or supernatural forces.

1b) Mysterious tricks, such as making things disappear and reappear, performed as entertainment.

2. Having or apparently having supernatural powers (OxfordDictionaries.com)

Etymology (EN): M.E. magik(e) “witchcraft,” from O.Fr. magique “magic, magical,” from L.L. magice “sorcery, magic,” from Gk. magike (tekhne “art”), from magos “one of the members of the learned and priestly class,” from O.Pers. magu-, possibly from PIE root *magh- “to be able, have power.”

Etymology (PE): Mid.Pers. yâtûk “wizard, sorcerer;” Av. yātu-

An n × n matrix in which every row, column, and diagonal add up to the same number.

See also: → magic; → square.

1. A civil officer charged with the administration of the law.
   

2. A minor judicial officer, as a justice of the peace or the judge of a police court, having jurisdiction to try minor criminal cases and to conduct preliminary examinations of persons charged with serious crimes (Dictionary.com).

Etymology (EN): M.E., from O.Fr. magistrat, from L. magistratus “a magistrate, public functionary,” from magistrare “to serve as a magistrate,” from magister, “chief, director,” → master.

Etymology (PE): Dâdyâr, from dâd, → justice,

• yâr, “assistant, helper,” → gravity assist.

1. Molten → rock material that occurs below Earth’s surface.
   

2. Molten material both below and on top of the surface of a → planet.

Etymology (EN): From L. magma “dregs of an ointment,” from Gk. magma “an ointment,” from root of massein “to knead, mold.”

Etymology (PE): Mâgma, loanword from Fr.

A large cavity within the Earth’s → crust containing → magma. When a → vent is opened to the surface, magma is extruded onto the surface as → lava.

See also: → magma; → chamber.

A metallic chemical element; symbol Mg. Atomic number 12; atomic weight 24.305; melting point about 648.8°C; boiling point about 1,090°C. The Scottish chemist Joseph Black recognized it as a separate element in 1755. In 1808, the English chemist Humphrey Davy obtained the impure metal and in 1831 the French pharmacist and chemist Antoine-Alexandre Brutus Bussy isolated the metal in the pure state.

Etymology (EN): The name originally used was magnium and was later changed to magnesium, which is derived from Magnesia, a district in the northeastern region of Greece called Thessalia.

An object that produces a magnetic field around itself.

Etymology (EN): From L. magnetum (nom. magnes) “lodestone,” from Gk. ho Magnes lithos “the Magnesian stone,” from Magnesia region in Thessaly where magnetized ore was obtained.

Etymology (PE): Âhanrobâ, literally “iron attracting, iron robbing,” from âhan→ iron + robâ agent noun of robudan, robâyidan “to attract, to grab, rob;” Av. urūpaiieinti “to cause racking pain(?);” cf. Skt. rop- “to suffer from abdominal pain,” rurupas “to cause violent pain,” ropaná- “causing racking pain,” rópi- “racking pain;” L. rumpere “to break;” O.E. reofan “to break, tear.”
Meqnâtis, from Ar., from Gk., as above.

A highly magnetized → neutron star with fields a thousand times stronger than those of → radio pulsars. There are two sub-classes of magnetars, → anomalous X-Ray pulsar (AXP)s and → soft gamma repeater (SGR)s, that were thought for many years to be separate and unrelated objects. In fact
SGRs and AXPs are both neutron stars possessing → magnetic fields of unprecedented strength of 10¹⁴ - 10¹⁶ G, and that show both steady X-ray pulsations as well as soft gamma-ray bursts. Their inferred steady X-ray luminosities are about one hundred times higher than their → spin-down
luminosities, requiring a source of power well beyond the magnetic dipole spin-down that powers → rotation-powered pulsar (RPP)s. New high-energy components discovered in the spectra of a number of AXPs and SGRs require non-thermal particle acceleration and look very similar to high-energy spectral components of young rotation-powered pulsars (A. K. Harding, 2013, Front. Phys. 8, 679).

See also: From magnet, contraction of → magnetic + -(s)tar, from → star.

Of or pertaining to a magnet or magnetism.

Etymology (EN): From → magnet + → -ic.

Etymology (PE): Meqnâtisi, meqnâti, from meqnâtis, → magnet; âhanrobâyik, from âhanrobâ, → magnet, + → -ik, → -ic.

The transport of the magnetic field by a fluid. It is given by the term ∇ x (v x B) in the → induction equation.

See also: → magnetic; → advection.

The imaginary straight line joining the two → poles of a → magnet.

See also: → magnetic; → meridian.

Any configuration of → magnetic fields used in the containment of a → plasma during controlled → thermonuclear reaction experiments.

See also: → magnetic; → bottle.

The process whereby a star which loses mass slows down under the action of its → magnetic field. The stellar material follows the → magnetic field lines extending well beyond the stellar surface. The material gain → angular momentum and the underlying object is slowed down. Magnetic braking is an efficient mechanism for removing angular momentum from the the rotating object. See also → disk locking.

See also: → magnetic; → braking.

The failure of numerical star formation calculations to produce rotationally supported → Keplerian disks because of the → magnetic braking effect, when → magnetic fields of strengths comparable to those observed in → molecular clouds are accounted for.

The formation and early evolution of disks is a long-standing fundamental problem in → star formation models. Early work in the field had concentrated on the simpler problem of disk formation from the → collapse of a rotating dense core in the absence of a magnetic field. However, dense star-forming cores are observed to be significantly magnetized. There is increasing theoretical evidence that disk formation is greatly modified, perhaps even suppressed, by a dynamically important magnetic field.

This has been found in analytic studies, axisymmetric numerical models and in 3D calculations using → ideal magnetohydrodynamics. By contrast, recent observations suggest the presence of massive, 50-100 AU disks and evidence for associated → outflows in the earliest (→ class 0) stages of star formation around both low and high mass stars.

Two primary solutions have been proposed: → turbulence and → non-ideal magnetohydrodynamics.

Calculations of the collapse of a massive 100 Msun core have shown that 100 AU scale disk formation in the presence of strong magnetic fields was indeed possible, with some argument over whether this is caused by turbulent reconnection or another mechanism. Studies, using simulations of collapsing 5 Msun cores, have found that turbulence diffuses the strong magnetic field out of the inner regions of the core, and that the non-zero → angular momentum of the turbulence causes a misalignment between the rotation axis and the magnetic field. Both of these effects reduce the magnetic braking, and allow a massive disk to form (Wurster et al. 2016, arxiv/1512.01597 and references therein).

See also: → magnetic; → braking; → catastrophe.

Same as → synchrotron radiation.

See also: → magnetic; → bremsstrahlung.

The phenomenon whereby the presence of a → magnetic field can make a portion of → compressible fluid less dense than its surroundings, so that it floats upward under the influence of gravity. This magnetic buoyancy is thought, in fact, to be the mechanism by which magnetic flux tubes rise through the Sun’s → convection zone and break at the surface in the form of → sunspots. The Sun’s rotation would have a major effect on the rate at which these magnetic flux tubes rise.
The rotation substantially lengthen the time taken for the flux tubes to reach the surface (D. J. Acheson, 1979, Nature 277, 41).

See also: → magnetic; → buoyancy.

A cataclysmic binary in which the white dwarf primary has a strong magnetic field that radically affects the accretion flow in the system. → polar

See also: → magnetic; → cataclysmic;
→ binary.

A transient ejection in the → solar wind having an enhanced field, a large and smooth change in field direction, and a low → proton temperature compared to the ambient proton temperature (L. F. Burlaga, 1995, Interplanetary Magnetohydrodynamics, Oxford Univ. Press, 89-114).

See also: → magnetic; → cloud.

→ compass.

See also: → magnetic; → compass.

Of magnetic field lines, the condition for them to be connected or the process whereby they become connected or connective.

See also: → magnetic;→ connectivity.

A physical constant relating mechanical and electromagnetic units of measurement. It has the value of 4π × 10^-7 henry per meter. Also called the permeability of free space, or → absolute permeability.

See also: → magnetic; → constant.

Thermal → convection modified by the presence of magnetic fields.

See also: → magnetic; → convection.

In terrestrial magnetism, the difference between → true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point,→ magnetic pole).

See also: → magnetic; → declination.

The process whereby the magnetic field tends to diffuse across the plasma and to smooth out any local inhomogeneities under the influence of a finite resistance in the plasma. For a stationary plasma the → induction equation becomes a pure → diffusion equation:

∂B/∂t = D_m∇²B, where

D_m = (μ₀σ₀)^-1 is the → magnetic diffusivity.

See also: → magnetic; → diffusion.

The → diffusion coefficient for a magnetic field. It is expressed as: η = 1/(μ₀σ), where μ₀ is the
→ magnetic permeability and σ the
→ conductivity.

See also: → magnetic; → diffusivity.

In terrestrial magnetism, the angle that a → magnetic needle makes with the horizontal plane at any specific location. The angle of magnetic dip at the → magnetic poles of Earth is 90°. Also called → inclination and → dip.

See also: → magnetic; → dip.

A system that generates a → magnetic field in which the field is considered to result from two opposite poles, as in the north and south poles of a magnet, much as an → electric field originates from a positive and a negative charge in an → electric dipole. A loop carrying an electric current also acts as a magnetic dipole.
Magnetic dipoles experience a torque in the presence of magnetic fields. → dipole moment; → magnetic moment.

See also: → magnetic; → dipole.

Same as → magnetic moment.

See also: → magnetic; → dipole; → moment.

Any of several microscopic areas in a → ferromagnetic material that
possesses a net → magnetic field, because electron spins are aligned in the same direction.

In the absence of an external magnetic field, the directions of the magnetization vectors of the separate domains do not coincide and the resultant magnetization of the whole body may be zero.

See also: → magnetic; → domain.

The energy stored in a magnetic field. It is the → work that must be done to establish a magnetic field in terms of the → magnetic induction. Magnetic energy varies as the square of the magnetic induction. It can be expressed in several other ways, for example in terms of the current and of the magnetic flux, or in terms of the current density and vector potential.

See also: → magnetic; → energy.

A field of force that is generated by electric currents, or, equivalently, a region in which magnetic forces can be observed.

See also: → magnetic; → field.

An imaginary line used for representing the strength and direction of a magnetic field. Charged particles move freely along magnetic field lines, but are inhibited by the magnetic force from moving across field lines.

See also: → magnetic; → field; → line.

Same as → magnetic intensity.

See also: → magnetic; → field; → intensity.

A measure of the quantity of magnetism or magnetic field. It is the number of lines of force passing normally through a given area. Magnetic flux is a scalar quantity defined as the surface integral of the → magnetic flux density. It is usually denoted by the Greek letter Φ and its SI unit is the → weber.

See also: → magnetic; → flux.

A vector quantity measuring the strength and direction of the magnetic field. It is the → magnetic flux per unit area of a magnetic field at right angles to the magnetic force. Magnetic flux density is expressed in → teslas. Also called → magnetic induction.

See also: → magnetic; → flux; → density.

A quantity that measures the extent to which the magnetic field lines wrap and coil around each other. It is closely related to field line topology. Magnetic helicity is defined by: H_M = ∫ A . B dV, where A is the vector potential of the magnetic field and the integration is over a volume V. → helicity; → kinetic helicity

See also: → magnetic; → helicity.

Same as → magnetic dip or → dip.

See also: → magnetic; → inclination.

1. Same as → magnetic flux density.
   

2. The production of a magnetic field in a piece of un-magnetized iron or other → ferromagnetic substance when a magnet is brought near it.
   The magnet causes the individual particles of iron, which act like tiny magnets, to line up so that the sample as a whole becomes magnetized.

See also: → magnetic; → induction.

Strength of a magnetic field at a point, denoted H. The force which could be exerted on unit north magnetic pole situated at that point. Measured in oersteds. Same as → magnetic field strength.

See also: → magnetic; → intensity.

A → stellar magnetic field associated with a → massive star. Magnetic fields are detected only for seven to ten percent of all studied massive → OB stars, and the magnetic field occurrence does not depend on the → spectral type. Because these magnetic fields seem to be stable over long time-scales and their strength does not seem to correlate with known stellar properties, it is assumed that they are of fossil origin (→ fossil magnetic field) and are frozen into the → radiative envelope of the stars. The fields are those of the birth → molecular clouds, partly trapped inside the → pre-main sequence star during the cloud → collapse phase, possibly further enhanced by a → dynamo effect in the early fully convective stellar phase. Typically, the polar field strength ranges from about a hundred → Gauss up to several kiloGauss. However, some weaker fields, below 100 G, have recently been detected.

The stellar magnetic field influences many different regions of the star with various effects. In the deep interior of the star, the field influences the internal → mixing of the star and this affects the size of the → convective overshooting region, changing the lifetime of the star by decreasing the amount of fuel for nuclear burning. Magnetic stars can also confine their → stellar winds, due to their strong magnetic fields, into a → magnetosphere, which slows down the → rotational velocity of the star. This → magnetic braking is an efficient mechanism for → angular momentum transport. At the stellar surface, the magnetic fields can create and sustain areas of chemical over- or under-abundances and/or large temperature differences, which are called spots (Buysschaert et al., 2016, astro-ph/1709.02619).

See also: → magnetic; → massive; → star.

A meridian passing through the Earth’s → magnetic poles.

See also: → magnetic; → meridian.

1. A measure of the strength of a magnet or current-carrying coil. In the case of a bar magnet it is obtained by multiplying the distance between the two magnetic poles by the average strength of the poles. Same as → magnetic dipole moment See also → dipole moment.
   

2. A measure of the magnetic flux set up by the gyration of an electric charge in a magnetic field.
   

3. In atomic and nuclear physics, → spin magnetic moment.

See also: → magnetic; → moment.

A hypothetical particle that carries a single → magnetic pole, in contrast to magnets which are north-south pole pairs.
These massive particles (billions of times heavier than the → proton) are required by grand unified theories(→ GUTs) to explain the actual matter content of the Universe, particularly the dominance of matter upon → antimatter. However, their existence contradicts → Gauss’s law for magnetism.

See also: → magnetic; → monopole.

A problem concerning the compatibility of grand unified theories (→ GUTs) with standard cosmology. If standard cosmology was combined with grand unified theories, far too many → magnetic monopoles would have been produced in the early Universe. The → inflation hypothesis aims at
explaining the observed scarcity of monopoles. The inflation has deceased their density by a huge factor.

See also: → magnetic; → monopole; → problem.

A slender → magnet suspended in a magnetic compass on a mounting with little friction; used to indicate the direction of the Earth’s → magnetic pole.

See also: → magnetic; → needle.

A point of the → magnetosphere where the Earth’s → magnetic field points vertically downward; in other words it has a 90° → magnetic dip toward the Earth’s surface. The magnetic north pole can also be defined as the point toward which the south pole of the → compass needle is directed. The magnetic north pole is different from the → geographic north pole. It is actually hundreds of kilometers south of the geographic north pole. However, this has not always been the case.

In the past 150 years it has moved more than 1,000 kilometers. Every 200,000 to 300,000 years the magnetic field of the Earth reverses direction, → magnetic reversal. Since the Earth’s magnetic field is not exactly symmetrical, the north and south magnetic poles are not → antipodal.

See also: → magnetic; → north; → pole.

A region of the → solar corona where the → magnetic field vanishes.

See also: → magnetic; → null; → point.

The ratio of the → magnetic induction, B, in the substance to the external magnetic field, H, causing the → induction: μ = B/H. It is measured in henry/meter and is known as absolute permeability. The relative permeability is equal to the ratio of absolute permeability to the permeability of the free space. Thus μ_r = μ/μ₀, where μ₀, the permeability of free space has the value 4π x 10^-7 henry/meter.

See also: → magnetic; → permeability.

1. The region of a magnet toward which the lines of magnetic force
   converge (south pole) or from which the lines of force diverge (north pole).
   

2. Either of the two points on the Earth’s surface where the magnetic lines of force converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other. → magnetic north pole, → magnetic south pole, → magnetic reversal.

See also: → magnetic; → pole.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of → kinematic viscosity to → magnetic diffusion. It is described by: P_r = σμ₀ν = ν/η, where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, ν is the kinematic viscosity of the fluid, and η is the → magnetic diffusivity.

See also: → magnetic; → Prandtl number.

The pressure exerted by a magnetic field on the material that contains the field.

See also: → magnetic; → pressure.

In atomic physics, a quantum number that denotes the energy levels available within a subshell. Designated by the letter m, it is one of a set of quantum numbers which describe the unique quantum state of an electron.

See also: → magnetic; → quantum;
→ number.

In a → plasma, a change of → magnetic connectivity of plasma elements due to the presence of a localized → diffusion region. It allows charged particles to move from one → magnetic field line to another. Magnetic reconnection is an important process transforming magnetic energy into heat or/and kinetic energy. Magnetic reconnection events occur in the Earth’s → magnetosphere. The process plays an important role in explosive phenomena in the Sun, such as → coronal mass ejections and
→ solar flares which heat the → solar corona.

See also: → magnetic; → re-; → connection.

The process by which a magnetic system relaxes to its minimum energy state over time.

See also: → magnetic; → relaxation.

A phenomenon exhibited by certain atoms whereby they absorb energy at specific (resonant) frequencies when subjected to alternating magnetic fields.

See also: → magnetic; → resonance.

A change in the Earth’s → magnetic field in which the → magnetic north pole is transformed into a → magnetic south pole and the magnetic south pole becomes a magnetic north pole. There are geological proofs indicating that the Earth’s magnetic field has undergone numerous reversals of → polarity in the past.

In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. At other times, for example during the → Cretaceous era, there have been much longer periods when no reversals occurred.

Over the past two centuries, Earth’s magnetic field has weakened by 15%. Risks of a weak magnetic field include more deaths from cancer due to increased radiation, electrical grid collapse from severe solar storms, climate change, and temporary ozone holes. See also → geomagnetic excursion.

See also: → magnetic; → reversal.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of
→ magnetic advection to → magnetic diffusion. It is given by:

R_m = σμ₀νLU₀,

where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, L is he characteristic length scale of the fluid flow, and
U₀ the characteristic velocity of the flow. A typical value for the Earth is R_m ~ 200.

See also: → magnetic; → Reynolds number.

In → plasma physics, a → quantity that describes the → resistance of a → charged particle to change its direction of motion under the influence of a perpendicular → magnetic field. Rigidity is defined as: R = r_LBc = (pc)/(Ze), where r_L is the → Larmor radius, B is
→ magnetic induction, c is the → speed of light, p is the → momentum of the particle, Z is → atomic number, and e the → electron charge. Since pc has the dimensions of energy and e the dimensions of charge, rigidity has the dimensions of → volts (a 10 GeV proton has a rigidity of 10 GV). In → cosmic ray studies, the energies of cosmic rays are often quoted in terms of their rigidities, rather than their energies per nucleon.

See also: → magnetic; → rigidity.

The → counterpart of the → magnetic north pole. It lies near the → geographic north pole.

See also: → magnetic; → south; → pole.

A process whereby the (internal) → magnetic field of a star modifies the → pulsations by lifting some of its degeneracy. Instead of just one pulsation frequency, a multiplet of frequencies is then observed. This effect was proposed as a possible explanation for the observed frequency pattern of → Beta Cephei. In practice, the magnetic splitting is difficult to observe, because of the very small expected frequency difference between the peaks. However, when unaccounted for, it may lead to a wrong mode identification. The current best candidate to detect magnetic splitting is → HD 43317, since this star displays two close frequency patterns (Buysschaert et al., 2017, astro-ph/1709.02619).

See also: → magnetic; → splitting.

A star whose → spectral lines show the → Zeeman effect. See also: → stellar magnetic field, → magnetic massive star, → Ap/Bp star.

See also: → magnetic; → star.

A temporary, worldwide disturbance of the Earth’s magnetic field by streams of charged particles from the Sun. Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normalcy, which may take several days.

See also: → magnetic; → storm.

A property of material defined by the ratio of the → magnetization to the → magnetic intensity. In other words, the magnetization per unit magnetic intensity.

See also: → magnetic; → susceptibility.

A continuous, flexible ribbon impregnated or coated with magnetic-sensitive material on which information (sound, images, data, etc.) may be recorded.

See also: → magnetic; → tape.

In → magnetohydrodynamic (MHD) treatment of → plasmas, that component of the → Lorentz force which is directed toward the centre of curvature of the → magnetic field lines and thus acts to straighten out the field lines.

The Lorentz force can be decomposed into two components orthogonal to the magnetic field:

j× B = (B . ∇) B / μ₀

• ∇ (B² / 2μ₀),

where j is the → current density, μ₀ is the → magnetic permeability of free space, and B is the → magnetic flux density. The left side term is the Lorentz force, the first term on the right side is the magnetic tension and the second term the → magnetic pressure.

See also: → magnetic; → tension.

A vector field A defined so that the → magnetic field  B is given by its → curl: B = ∇ x A.

See also: → magnetic; → vector; → potential.

Radiation emitted by a rotating magnet.

See also: → magnetic; → dipole;
→ radiation.

The study of magnetic phenomena, comprising magnetostatics and electromagnetism.

See also: → magnetic; → -ics.

The science of magnetic phenomena, including the fields and forces produced by magnets and, more generally, by moving electric charges.

See also: N.L. magnetismus; → magnet + → -ism.

An international collaboration devoted to the study of the origin and physics of → magnetic fields in → massive stars. The project uses several observatories and a large number of telescopes equipped with → spectropolarimetric and → asteroseismologic instruments, including → HARPS, → HARPSpol, and → ESPaDOnS (Wade et al., 2016, MNRAS 456, 2).

See also: → magnetism; → massive; → star.

1. General: The process of magnetizing or the state of being magnetized.
   

2. Electricity: The → magnetic moment per unit volume induced by an external magnetic field; measured in → amperes per meter.

See also: Verbal noun of → magnetize.

To make a magnet of, or impart the properties of a magnet to.

See also: From → magnet + → -ize.

Having been made magnetic; magnetic properties imparted to.

See also: Past participle of → magnetize.

A plasma containing a magnetic field which is strong enough to change the path of charged particles. It can be a → collisional plasma or → noncollisional plasma.

See also: → magnetized; → plasma.

Empty space influenced by very strong → magnetic field. Same as → birefringent vacuum.

See also: → magnetized; → vacuum.

The prefix form of → magnet, representing magnetic or magnetism
in compound words, such as → magnetogram, → magnetohydrodynamics.

See also: → magnet.

Combined study of the large-scale → magnetic field (→ magnetometry) and → stellar pulsations
(→ asteroseismology). Magneto-asteroseismology provides strong complementary diagnostics suitable for detailed stellar modeling and permits the determination of the → internal structure and conditions within → magnetic massive  → pulsators, for example the effect of magnetism on → mixing processes. More specifically, asteroseismology yields information on the → density, → composition, and → chemical mixing in multiple internal layers (depending on the number of studied frequencies). Additionally, when rotationally split pulsation modes are observed, the internal rotation profile can be retrieved. From magnetometry surface properties are determined, related to the → chemical composition, including → starspots, and the magnetic field, such as its geometry, obliquity, and strength. Magnetic studies also provide constraints about the → stellar wind geometry and the → circumstellar environment. Moreover, the stellar → rotation period period and the → angle of inclination toward the observer are also retrieved (Buysschaert et al., 2017, astro-ph/1709.02619).

See also: → magneto-; → asteroseismology.

The acceleration exerted on the plasma particles according to the → magnetocentrifugal model.

See also: → magnetocentrifugal model; → acceleration.

A → magnetohydrodynamic model devised to account for the → bipolar jets and → outflows observed around → protostars. Basically, a → poloidal magnetic field is frozen into a rotating → accretion disk. If the angle between the magnetic field lines threading the disk and the rotation axis of the disk is larger than 30°,
the plasma can be accelerated out of the accretion disk along the field lines. The field lines rotate at a constant → angular velocity, and as the gas moves outward along the field lines, it is accelerated by an increasing → centrifugal force (magnetocentrifugal acceleration). At some point, when the rotation velocity is about the same as the → Alfven velocity in the gas, the field lines get increasingly wound up by the inertia of the attached gas and a strong → toroidal magnetic field component is generated. The toroidal component is the main agent in collimating the flow into a direction along the → open magnetic field lines. The earliest version of the model was proposed by Blandford & Payne (1982, MNRAS 199, 883). It has two main versions: → X-wind and → disk wind models.
See also → magnetorotational instability.

See also: → magneto-; → centrifugal; → model.

A graphic representation of solar magnetic field strengths and polarity.

See also: From → magneto- + → -gram.

Of or relating to → magnetohydrodynamics.

See also: → magneto- + → hydrodynamic.

The dynamics of an ionized plasma in the non-relativistic, collisional case. In this description, charge oscillations and high frequency electromagnetic waves are neglected. It is an important field of astrophysics since plasma is one of the commonest forms of matter in the Universe, occurring in stars, planetary magnetospheres, and interplanetary and interstellar space.

See also: From → magneto- + → hydrodynamics.

Any of a variety of devices used to measure the strength and direction of a magnetic field.

See also: From → magneto- + → -meter.

The detection or measurement of a magnetic field, especially its strength and direction. See also → magnetometer.

See also: → magneto-; → -metry.

Fundamental constant, first calculated by Bohr, for the intrinsic magnetic moment of an electron. → Bohr magneton.

See also: From → magnet + → -on.

The boundary layer between a planet’s → magnetosphere and the → magnetic field of the → solar wind. It borders the → magnetosheath and is defined by the surface on which the pressure of the solar wind is balanced by that of the planet’s magnetic field. The front point of the Earth’s magnetopause, on the sun-ward side of the Earth, is about 10 terrestrial radii, on average. This point
can be closer or farther, because the magnetopause contracts or expands depending on the intensity of the solar wind.

Etymology (EN): From → magneto- + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): From meqnât-→ magnet + marz “frontier, border, boundary,” from Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); Ger. Mark; E. mark, margin.

An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport.
To put it simply, the → frozen magnetic field line
acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.

See also: → magneto-; → rotational; → instability.

The region between a planet’s magnetopause and the bow shock caused by the solar wind.

Etymology (EN): From → magneto- + sheath, from O.E. sceað, scæð, from P.Gmc. *skaithiz (cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide “scabbard”).

Etymology (PE): From meqnât-, → magnet, + niyâm “sheath,” from Proto-Iranian *nigāma-, from ni- “down; into,” → ni-, + gāma- “to go, to come” (Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
“step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”); cf. Skt. nigamá- “insertion, incorporation.”

The region around a celestial body in which the magnetic field of the body dominates the external magnetic field. Each planet with a magnetic field (Earth, Jupiter, Saturn, Uranus, and Neptune) has a magnetopause.
The Earth’s magnetosphere is a dynamic system that responds to solar variations. It prevents most of the charged particles carried in the → solar wind, from hitting the Earth.
Since the solar wind is → supersonic, a → bow shock is formed on the sunward side of the magnetosphere. The solar wind ahead is deflected at a boundary called → magnetopause. The region between the bow shock and the magnetopause is called the → magnetosheath. As the solar wind sweeps past the Earth, the terrestrial magnetic field lines are stretched out toward the night side to form a → magnetotail.

See also: From → magnet + → sphere.

The portion of a planet’s → magnetosphere which is pushed away from the Sun by the solar wind. Earth’s magnetosphere extends about 65,000 km on the day-side but more than 10 times further.

See also: From → magneto- + → tail.

The factor by which the angular diameter of an object is apparently increased when viewed through an optical instrument to that of the object viewed by the unaided eye.

See also: Verbal noun of → magnify.

A thing or device that magnifies.

Etymology (EN): From → magnify + suffix -ir.

Etymology (PE): Bozognemâ, agent noun of bozorg nemudan, → magnify.

To increase the apparent size of, as a lens does.

Etymology (EN): From O.Fr. magnifier, from L. magnificare “esteem greatly, extol,” from magnificus “splendid,” from magnus “great” + root of facere “to make.”

Etymology (PE): From bozorg “large, magnificent, great” + nemudan “to show.” The first element from Mid.Pers. vazurg “great, big, high, lofty;” O.Pers. vazarka- “great;” Av. vazra- “club, mace” (Mod.Pers. gorz “mace”); cf. Skt. vájra- “(Indra’s) thunderbolt,” vaja- “strength, speed;” L. vigere “be lively, thrive,” velox “fast, lively,” vegere “to enliven,” vigil “watchful, awake;”
P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen “to be awake;” E. wake); PIE base *weg- “to be strong, be lively.” The second element nemudan from Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into,” → ni-, + māy- “to measure,” → display.
Bozorgidan infinitive from bozorg + -idan.

A lens or lens system that produces an enlarged virtual image of an object placed near its front focal point. According to Enoch (1998, SPIE vol. 3299, p. 424), the earliest lenses identified are from the IV/V Dynasties of Egypt,
dating back to about 4,500 years ago (e.g., the eyes of the Louvre statue Le scribe accroupi
and other examples located in the Cairo Museum). For more information see → burning sphere.

Etymology (EN): Magnifying, verbal adj. of → magnify; → glass.

Etymology (PE): Zarrebin, from zarré “a minute thing,” → particle,

• bin “seer; to see” (present stem of didan;
  Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
  Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see”).

The ratio between the focal lengths of the objective and ocular in a telescope.

See also: Magnifying, verbal adj. of → magnify; → power.

A measure of brightness in astronomy on a → logarithmic scale in which a difference of five magnitudes represents a difference of 100 times in brightness. In this scale the lower a magnitude, the brighter the object. The faintest magnitude reached by → unaided eye is 6.

Etymology (EN): From L. magnitudo “greatness, bulk, size,” from magnus “great,” cognate with Pers. meh “great, large” (Mid.Pers. meh, mas; Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest;” cf. Skt. mah-, mahant-; Gk. megas; PIE *meg- “great”) + -tudo, suffix forming abstract nouns from adjectives and participles.

Etymology (PE): Borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz), related to boland “high,” bâlâ “up, above, high, elevated, height,” berg “mountain, hill” (Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high”). Qadr, from Ar.

A scale for measuring and comparing the brightness of astronomical objects.

See also: → magnitude; → scale.

A survey in which the observed objects are bighter than a given → apparent magnitude.

See also: → magnitude; → limited; → volume.

The force exerted on a spinning object moving through a fluid medium in virtue of → Bernoulli’s theorem. The Magnus force can deviate a football from its path when a player strikes it so that it spins about an axis perpendicular to the flow of air around it.
As the spinning ball moves through the air, it will create a pressure difference between its two sides. The air travels faster relative to the centre of the ball where its periphery is moving in the same direction as the airflow. This reduces the pressure according Bernoulli’s theorem. The opposite effect happens on the other side of the ball, where the air travels slower relative to the centre of the ball. There is therefore an imbalance in the forces
that will curve the ball’s trajectory.

See also: Named after Heinrich Gustav Magnus (1802-1870), a German chemist and physicist; → force.

Any of various passerine birds of the genus Pica, especially Pica pica, having a black-and-white plumage, long tail, and a chattering call (Ditionary.com).

Etymology (EN): From Mag diminutive of Margaret, used to signify an excessively talkative person + pie the earlier name of the bird, from O.Fr. pie, from L. pica “magpie,” feminine of picus “woodpecker.”

Etymology (PE): Kaliž, variant of (Dehxodâ) kalâžé, kalâcé, qalivâš, qalivâj,
(Qäyen, Gonâbâd) kaliždak, (Xorâsâni) kelidjak, (Dari Yazd) kelociri, (Bardesir) kerâcik, related to kalâq + dimunitive suffix -iž, vaeiants -iz, -ak.

Chief in size, extent, or importance; leading; → principal.

Etymology (EN): From M.E. meyn, mayn “strength, power,”
from O.E. mægen “power, strength, force,” from P.Gmc. *maginam- “power,” from *mag- “to be able, have power.”

Etymology (PE): Farist, literally “foremost” (cf. Mid.Pers. frahist “main, principal, first, much”), from far-, Mid.Pers. fra-; O.Pers. fra- “forward, forth;” Av. frā “forth,” pouruua- “first”; cf.
Skt. pūrva- “first,” pra- “before, formerly;” Gk. pro; L. pro; O.E. fyrst “foremost,” superlative of fore, E. fore + -est superlative suffix, Mid.Pers. -ist, -išt-; Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost.

Same as → main lobe.

See also: → main; → beam.

The area between → Mars and → Jupiter where most of the → asteroids in our → solar system are found.

See also: → main; → belt.

In the n x n → matrix , the entities a₁₁, a₂₂, …, a_nn.

See also: → main; → diagonal.

The lobe in the reception pattern of a radio telescope
that includes the region of the maximum received power. Also called major lobe and main beam.

See also: → main; → lobe.

A thin strand of material encircling Jupiter;
the main component in → Jupiter’s ring system of three parts. The diffuse innermost boundary begins at approximately 123,000 km. The main ring’s outer radius is found to be at 128,940 km,

See also: → main; → ring.

An evolutionary stage in the life of a star when it generates its energy by the conversion of hydrogen to helium via → nuclear fusion in its core. Stars spend 90% of their life on the main sequence. On the → Hertzsprung-Russell diagram it appears as a track running from top left (high temperature, high luminosity, high mass) to lower right (low temperature, low luminosity, low mass). See also → zero age main sequence (ZAMS), → terminal age main sequence (TAMS).

See also: → main; → sequence.

The method of determining the distance to a star cluster by overlaying its main sequence on the theoretical zero-age main sequence and noting the difference between the cluster’s apparent magnitude and the zero-age main sequence’s absolute magnitude.

See also: → main sequence; → fitting.

The point on the → Hertzsprung-Russell diagram of a star cluster at which stars begin to leave the → main sequence and move toward the → red giant branch. The main-sequence turnoff is a measure of age. In general, the older a star cluster, the
fainter the main-sequence turnoff. Same as → turnoff point.

See also: → main sequence; → turnoff.

Greater in size, extent, or importance.

Etymology (EN): M.E. majour, from O.Fr., from
L. major, irregular comparative of magnus “large, great,” cognate with Pers. meh “large, great,” as below.

Etymology (PE): Mehin comparative and superlative of
meh “great, large”
(Mid.Pers. meh, mas; Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest;” cf. Skt. mah-, mahant-; Gk. megas; PIE *meg- “great”) + -in superlative suffix.

The greatest diameter of an ellipse; it passes through the two foci.

See also: → major; → axis.

The → merging of two spiral galaxies with roughly equal masses colliding at appropriate angles. The dynamical friction is so efficient that the galaxies merge after only a few perigalactic passages.

See also: → major; → merger.

A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.

See also: → major; → planète.

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

See also: → major; → premisse.

Logic: In a → syllogism, the → predicate of the → conclusion which occurs in the → major premise.

See also: → major; → term.

A function, or an element of a set, that dominates others or is greater than all others. In other words, for a function f defined on the interval I, the point M such that for each x on I, f(x)≤ M. See also → minorant.

Etymology (EN): From Fr. majorant, from majorer “to increase, raise,” from L. → major.

Etymology (PE): Mehân, from mehidan, from meh “great, large,” → major.

The greater number, part, or quantity of a whole.

See also: → major; → -ity.

The third largest known → dwarf planet after → Eris and → Pluto. Numbered 136472, and initially called 2005 FY_g, it
belongs to the → Kuiper belt in the solar system. Discovered in 2005, Makemake is roughly three-quarters of Pluto in size and orbits the Sun in about 310 years.

See also: Named after Makemake “the creator of humanity and god of fertility” in the mythology of the Rapanui, the native people of Easter Island.

Something put in a scale to complete a required weight.

Etymology (EN): → make; → weight.

Etymology (PE): Pârsang, → counterweight.

A → reflecting telescope incorporating a deeply curved → meniscus, → lens, which corrects the → optical aberrations of the spherical → primary mirror to give high-quality → images over a wide → field of view.

See also: Named for the Russian optical specialist Dmitri Maksutov (1896-1964), who developed the design; → telescope.

1. Belonging to the sex that typically has the capacity to produce gametes, especially spermatozoa which fertilize the eggs of a female.
   

   2. A person, plant, or animal capable of fertilizing.

Etymology (EN): M.E. male, from O.Fr. malle, masle, from L. masculus “masculine, a male,” → maculine.

Etymology (PE): Nar “male,” from Mid.Pers. nar, “male; manly;” Av. nar- “male, man,” nairya- “male, manly;” cf. Skt nara- “male, man.”

A selection effect in observational astronomy. If a sample of objects (galaxies, quasars, stars, etc.) is flux-limited, then the observer will see an increase in average luminosity with distance, because the less luminous sources at large distances will not be detected.

See also: Named after the Swedish astronomer Gunnar Malmquist (1893-1982); → bias.

A correction introduced into star counts distributed by apparent magnitude.

See also: → Malmquist bias; → correction.

If the light wave entering an → analyzer is → linearly polarized, the intensity of the wave emerging from the analyzer is I = k I₀ cos²φ,

where k is the coefficient of transmission of the analyzer, I₀ is the intensity of the incident light, and φ is the angle between the planes of → polarization of the incident light and the light emerging from the analyzer.

See also: Named after Etienne Louis Malus (1775-1812), French physicist who also discovered polarization by reflection at a glass surface (1808); → law.

A method for deriving the Earth’s size based on measuring a length of meridian between two points corresponding to the difference between the respective latitudes. The Abbasid caliph al-Ma’mun (ruling from 813 to 833 A.D.), appointed two teams of surveyors to this task. They departed from a place in the desert of Sinjad (nineteen farsangs from Mosul and forty-three from Samarra), heading north and south, respectively. They proceeded until they found that the height of the Sun at noon had increased (or decreased) by one degree compared to that for the starting point. Knowing the variation of the Sun’s → declination due to its apparent → annual motion, they could relate the length of the arc of meridian to the difference between the latitudes of the two places. They repeated the measurement a second time, and so found that the length of one degree of latitude is somewhat between 56 and 57 Arabic miles (Biruni, Tahdid). 360 times this number yielded the Earth’s circumference, and from it the radius was deduced.

See also: → Eratosthenes’ method, → Biruni’s method.

See also: The seventh Abbasid caliph Abu Ja’far Abdullâh al-Ma’mûn, son of Hârûn al-Rashîd (786-833 A.D.); → method.

1. An adult male person.
   

2. A member of the species Homo sapiens. See also → human, → anthropo-.

Etymology (EN): M.E., from O.E. man, mann “human being, person” (O.S., O.H.G. man, Ger. Mann, O.N. maðr, Goth. manna “man”), from PIE base *man-;
cf. Skt. mánu-, más- “man, person, husband;”
Av. manu- in proper noun Manus-ciθra- (Pers. Manucehr); O.C.S. moži, Russ. muž “man, male.”

Etymology (PE): (Mid.Pers./Mod.Pers.) mard “man,” mardom “mankind, people,” cognate with mordan “to die,” → death;
Sogd. martu, marti “man, human;” O.Pers. martiya-; Av. marəta- “mortal, man,” maša- “mortal;” cf. Skt. márta- “mortal, man;” Gk. emorten “died;” L. mortalis “subject to death;” PIE base *merto-, *morto-. Ensân, loan from Ar.

To direct or control the use of; to exercise executive, administrative, and supervisory direction of.

Etymology (EN): Probably from It. maneggiare “to handle, train (a horse),”
from L. manus “hand.”

Etymology (PE): Gonârdan, from Mid.Pers vinârtan, variant vinâristan “to organize, arrange, put in order,” from vi- “apart, away from” (Av. vi- “apart, away from, out;” O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”)

• âristan, ârâstan “to arrange, adorn;” O.Pers. râs- “to be right, straight, true,” râsta- “straight, true” (Mod.Pers. râst “straight, true”), râd- “to prepare,”
  Av. râz- “to direct, put in line, set,” Av. razan- “order,” Gk. oregein “to stretch out,” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight,” Skt. rji- “to make straight or right, arrange, decorate,” PIE base *reg- “move in a straight line.”

The act or manner of managing; handling, direction, or control.

See also: Verbal noun of → manage.

A person who manages; a person who has controls or directs an institution,
a team, a division, or part it.

See also: Agent noun of → manage.

A set of points in the complex plane, the boundary of which forms a fractal with varying shapes at different magnifications. Mathematically, it is the set of all C values for which the iteration z_n+1 = z_n² + C, starting from z₀ = 0, does not diverge to infinity.

See also: Discovered by Benoît Mandelbrot (1924-) a Polish-born French mathematician, best known as the “father of fractal geometry;”
→ set.

A movement or action to accomplish a change of position.

See also: From Fr. manoeuvre “manipulation, maneuver,” from O.Fr. manovre “manual work,” from M.L. manuopera, from manuoperare “work with the hands,” from L. manu operari, from manu ablative of manus “hand” + operari “to work,” → operate.

Metallic chemical element; symbol Mn. Atomic number 25; atomic weight 54.938; melting point about 1,244°C; boiling point about 1,962°C.

Etymology (EN): The name derives from the Latin magnes for “magnet” since pyrolusite (MnO₂) has magnetic properties. It was discovered by the Swedish pharmacist and chemist Carl-Wilhelm Scheele in 1774.

Etymology (PE): Manganez, loan from Fr.

1. Readily perceived by the eye or the understanding; evident; obvious; apparent; plain.
   

2. To make clear or evident to the eye or the understanding; show plainly (Dictionary.com).

Etymology (EN): M.E., from O.Fr. manifest “evident, palpable,” or from L. manifestus “plainly apprehensible, clear, apparent, evident;” “proved by direct evidence;” “caught in the act,” probably from manus “hand,” + -festus “struck;
(able to be) seized.”

Etymology (PE): From Torbat-Heydariye-yi nemusâr “evident, conspicuous, visible,” from nemu-, nemudan “to show, display”
from Mid.Pers. nimūdan, from ne- “down; into;” O.Pers./Av. ni- “down; below; into,” → ni-, + mu- (as in âz-mu-dan, â-mu-dan, far-mu-dan, pey-mu-dan, etc.); Av. mā(y)- “to measure,” → display, + -sâr a suffix of state, position, similarity.

1. An act of manifesting.
   

2. The state of being manifested.
   

3. Outward or perceptible indication; materialization.
   

4. A public demonstration, as for political effect (Dictionary.com).

See also: Verbal noun of → manifest; → -tion.

A → topological space in which every point has a → neighborhood which resembles → Euclidean space (Rⁿ), but in which the global structure may be different. An example of a one-dimensional manifold would be a circle;
if you zoom around a point the circle looks locally like a line (R¹). An example of a two-dimensional manifold would be a sphere; a small portion looks locally like a plane (R²). See also → flat manifold.

Etymology (EN): O.E. monigfald (Anglian), manigfeald (W.Saxon) “varied in appearance,” from manig “many” + -feald “fold.”

Etymology (PE): Baslâ, from bas “many, much” (Mid.Pers. vas “many, much;” O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish”) + lâ “fold.”

The fractional or the decimal part of a → common logarithm. For example, log₁₀ 4000 = 3.602, where the → characteristic is 3 and the mantissa 0.602.

Etymology (EN): From L. mantis “makeweight, addition,” of unknown origin. Introduced by Henry Briggs (1561-1630).

Etymology (PE): → makeweight.

1. General: Something that covers, envelops, or conceals. → grain mantle; → plasma mantle.
   

2. Geology: → Earth’s mantle.

Etymology (EN): O.E. mentel “loose, sleeveless cloak,” from L. mantellum “cloak,” perhaps from a Celtic source.

Etymology (PE): 1) Rupuš “over-garment, cloak,” from ru “surface, face; aspect; appearance” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”) + puš “covering, mantle,” from pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).
2) Gušté, from gušt “flesh, meat, pulp of fruit;” Mid.Pers. gôšt “meat;” Av. gah- “to eat;” cf. Skt. ghas- “to eat, devour,” ghásati “eats” + nuance suffix -é.

The mathematical problem of solving the equations of motions of any number of bodies which interact gravitationally. More specifically, to find their positions and velocities at any point in the future or the past, given their present positions, masses, and velocities.

See also: Many, from M.E. mani, meni, O.E. monig, manig; → body; → problem.

1a) A representation usually on a flat surface of an area of the Earth or a portion of the sky, showing them in their respective forms, sizes, and relationships.

1b) Math.: Same as → mapping.

2. To make a map of; show or establish the features of, details of.

Etymology (EN): Shortening of M.E. mapemounde “map of the world,” from M.L. mappa mundi “map of the world,” first element from L. mappa “napkin, cloth” (said to be of Punic origin) + L. mundi “of the world,” from mundus “universe, world.”

Etymology (PE): Naqšé “map,” from naqš “painting, embroidering, carving,” variant of negâštan, negâridan “to paint,” negâr “picture, figure,” → graph.

The theory and method of transforming the features, geometry, and topology on a sphere surface (in particular the spherical Earth) onto a plane.

See also: → map; → projection.

Any tree of the genus Acer. The maple leaf is an emblem of Canada.

Etymology (EN): M.E. mapel, O.E. mapul-, related to O.N. möpurr, O.S. mapulder, M.L.G. mapeldorn.

Etymology (PE): Afrâ, of Tabari origin.

1. The process of producing a map.
   

2. Math.: The operation of establishing → relations between two → sets in which one element of the second set is assigned to each element of the first set, as the expression y = x². Same as → function.

Etymology (EN): Verbal noun from → map + → -ing.

Etymology (PE): 1) Naqšé bardâri;, → map.

2. Hamtâyeš, verbal noun of hamtâyidan literally “folding together, relating units together,” from ham- “together,” → com-, + tâ
   “fold, plait, ply; piece, part; single, a single unit of a pair,” also a multiplicative suffix; Mid.Pers. tâg “piece, part,” + infinitive suffix -idan.

A mature female horse or other equine animal.

Etymology (EN): M.E., variant of mere, O.E. m(i)ere feminine of mearh “horse,” (cognates: O.Sax. meriha, O.Norse merr, Du. merrie, O.H.G. meriha, Ger. Mähre “mare”), probably of Gaulish origin (cf. Irish and Gaelic marc, Welsh march, Breton marh “horse”).

Etymology (PE): Mâdiyân, from mâdé “female,” → feminine.

Of or pertaining to the sea; produced by the sea.

Etymology (EN): From M.E. maryne, from M.Fr. marin, from O.Fr. marin “of the sea, maritime,” from L. marinus “of the sea,” from mare “sea, the sea, seawater,” from PIE *mori- “body of water, lake.”

Etymology (PE): Daryâyi “of, or pertaining to the sea,” from daryâ, → sea.

1. A visible impression or trace on something, as a line, cut, dent, stain, or bruise
   (Dictionary.com).
   

2. To put a mark or marks on.

Etymology (EN): M.E., O.E. mearc, merc “boundary, sign, limit, mark” (cf. O.N. merki “boundary, sign,” mörk “forest” (which often marked a frontier); O.Fr. merke, Goth. marka “boundary, frontier,” Du. merk “mark, brand,” Ger. Mark “boundary, boundary land”), from PIE *merg- “edge, boundary, border;” cf. Pers. marz,
→ frontier.

Etymology (PE): Dâj, variants dâq “brand, marking; hot,” Hamedâni daj “in harvest, the sign placed on a wheat pile indicating not to be touched,” dežan “acid, pungent;” Mid.Pers. dâq, dâk “hot,” dažitan “to burn, scorch,” dažišn “burning;” Av. dag-, daž- “to burn;” cf. Skt. dah- “to burn;” L. fovere “to warm, heat; " Arm. dažan “violent, wild;” Lith. degu “to burn;” O.E. fefor; E. fever. PIE base *dhegh- “to burn.”

A blue star of visual magnitude 2.49, the brightest in the constellation → Pegasus. Markab is a relatively hot star of → spectral type B9, with a total luminosity about 200 times that of the Sun, a surface temperature of about 11,000 K, and
a radius 4.3 times solar lying 140 light-years away.

See also: Markab seems to be a corruption of Mankab in the original Ar. name of this star Mankib al-faras (منکب‌الفرس) “the horse’s shoulder,” from mankib “shoulder” + faras “horse,” referring to Pegasus in Gk. mythology.

A galaxy with abnormally strong emission in the ultraviolet continuum and
broad emission lines arising in a bright, semi-stellar nucleus.

See also: Named after B. E. Markarian (1913-1985), an Armenian astronomer who made a catalog of such galaxies (1967-81); → galaxy.

A string of a dozen or so galaxies in the central region of the → Vigo cluster. The chain lies to the right of the cluster’s dominant galaxy M87 and extends over nearly 2° on the sky. The chain’s brightest galaxies are the lenticulars M84 and M86. At least seven galaxies in the chain appear to move coherently, although others appear to be superposed by chance.

See also: → Markarian; → chain.

Two → interacting galaxies, NGC 4438 and NGC 4435, located in → Markarian’s chain of galaxies in the → Virgo cluster of galaxies. About 50 million → light-years away, the two galaxies are about 100,000 light-years apart. Gravitational → tidal forces from the → close encounter have ripped away at their stars, gas, and dust. The more massive NGC 4438 kept much of the material ripped out in the collision, while material from the smaller NGC 4435 was more easily lost.

See also: → Markarian galaxy; → eye.

1. An object used to indicate a position, place, or route.
   

   2. A distinctive feature or characteristic indicative of a particular quality or condition.
      

   3. Genetics: An allele used to identify a chromosome or to locate other genes on a genetic map (OxfordDictionaries.com).
      

   4. → biomarker.

See also: → mark; → -er.

A → stochastic process, based on the classical → random walk concept, in which the probabilities of occurrence of various future states depend only on the previous state of the system and not on any of earlier states. Also called Markov process and Markovian principle.

See also: Named after Andrey Andreyevich Markov (1856-1922), a Russian mathematician, who introduced this model in 1906; → chain.

A method for sampling from → probability distributions
using → Markov chains. MCMC methods are widely used in data modeling for → Bayesian inference and numerical integration in physics, chemistry, biology, statistics, and computer science.

See also: → Markov chain; → Monte Carlo Method.

Fourth planet from Sun and the seventh largest. Mass 6.42 × 10²⁶ g (0.11 Earth’s), radius 3397 km. Mean distance from Sun 1.52 → astronomical units. → Sidereal period 687 days, → synodic period 779.9 days. Surface temperature 248 K., → rotation period, or → sol, 24h37m22s.6. Mars’ → obliquity is currently 25.19 degrees, but has changed dramatically over billions of years since solar system formation. Atmosphere more than 90% CO₂, traces of O₂, CO, H₂O. Two tiny satellites (→ Phobos and → Deimos), both of which are locked in → synchronous rotation with Mars.

Etymology (EN): Late M.E., from L. Mars the Roman god of war, Ares in Gk. mythology.

Etymology (PE): Bahrâm, from Mid.Pers. Vahrâm, from Vahrân “god of victory,” from Av. vərəθraγna- “victory, breaking the defence, the god of victory.” The first element vərəθra- “shield, defensive power,” cf. Skt. vrtrá- “defence, name of a demon slain by Indra,” Arm. vahagan name of a god (loanword from Iranian). The second element γna-, from Av., also O.Pers., jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”); cf. Mod.Pers. zadan, zan- “to strike, beat;” Mid.Pers. zatan, žatan;
Skt. han- “to strike, beat” (hantar- “smiter, killer”);
Gk. theinein “to strike,” phonos “murder;” L. fendere “to strike, push;” Gmc. *gundjo “war, battle;” PIE *g^when- “to strike, kill.”

The → zero point of elevation on Mars. It is the elevation at which the atmosphere pressure is 6.1 millibars, or 610 → Pascals. Atmosphere pressure has to be used because Mars has no ocean, and “sea level” cannot be used like on Earth. More formally, the datum is a fourth-order, fourth-degree surface of equal → gravitational potential (determined from the Viking orbiter spacecraft) such that the pressure of the atmosphere is 6.1 millibars (source: Lunar and Planetary Institute, USRA).

See also: → Mars; → datum.

A member of the family of → asteroids located at either of the stable → Lagrangian points (L4 or L5) of the orbit of → Mars.

See also: → Mars; → Trojan asteroid.

A special calendar for time reckoning on Mars in which the year consists of 668 → sols (687 Earth days) and each sol has 24h 39m 35s. For convenience, sols are divided into a 24-hour clock. Each landed Mars mission keeps track of local solar time at its landing site, which depends upon the lander’s longitude of Mars.

The Martian year begins when Mars arrives at the → vernal point of the orbit in its northward journey; in other words, when the solar longitude Ls is 0°. Mars’ other seasons begin when Ls = 90° at → summer solstice, 180° at → autumn equinox, and 270° at → winter solstice. The year counts begin at Mars Year 1, at the northern → vernal equinox of April 11, 1955. The second half of that year was marked by a major dust storm (→ Mars’ dust storm). February 7, 2021 marked the start of Year 36 on Mars. Year 37 will start on Dec. 26, 2022, and Year 38 on Nov. 12, 2024.

On Earth, spring, summer, autumn, and winter are all similar in length, because Earth’s orbit is nearly circular (→ eccentricity = 0.0167), so it moves at nearly constant speed around the Sun. By contrast, Mars’ elliptical orbit (→ eccentricity = 0.0934) makes its distance from the Sun change with time, and also makes it speed up and slow down in its orbit. Mars is at → aphelion (249,200,000 km from Sun) at Ls = 70°, near the northern summer solstice, and at → perihelion (206,700,000 km) at Ls = 250°, near the southern summer solstice. The Mars dust storm season begins just after perihelion at around Ls = 260°.

See also: → Mars; → calendar.

A violent atmospheric disturbance on Mars marked by high amounts of dust, especially during spring and summer seasons of the planet southern hemisphere. The elongated orbit of Mars has several important consequences. During southern spring and summer, Mars travels near its → perihelion, while its southern pole is tilted toward the Sun. Therefore, its surface receives much more heat . The atmosphere’s temperature near surface raises and since the upper layers of the atmosphere are cold, warm air moves up and takes dust particles upward. Each several years big storms occur and cover significant portions of the planet such that dust stays in the atmosphere for several weeks or months. See also → Mars’ calendar.

See also: → Mars; → dust; → storm.

A quake on the → planet Mars, probably caused by some phenomena other than → tectonic plate motions.
Unlike Earth, Mars seems to lack tectonic plates. Therefore, its quakes are thought to arise from the slow cooling of the planet over time, which causes the → crust to contract and develop fractures. These quakes can also come from the impact of → meteorites and possibly the movement of → magma deep below the surface. On April 6, 2019, the instrument called Seismic Experiment for Interior Structure (SEIS) on NASA’s Mars → InSight Mission lander recorded quakes that appear to have come from inside the planet, the first time ever a likely marsquake.

See also: → Mars; → quake.

Of, relating to, or like the planet → Mars.

See also: M.E. marcien, from L. Marti(us) of, belonging to → Mars + -an a suffix of adjectives.

A piece of rock that was ejected from the Martian surface into space by the impact of an asteroid or comet, and landed on Earth. So far about 100 Martian meteorites have been collected. These meteorites have elemental and isotopic compositions that match those of the Martian crust as measured by NASA’s Mars exploration missions.

See also: → Martian; → meteorite.

A slender, cloudy projection sometimes seen to extend from the surface of → Mars to very high altitudes. Noted and confirmed by amateur astronomers on photos of Mars in March 2012, possibly similar plumes have been found on archived images as far back as 1997. The plumes reach 200 km up, which seems too high for them to be related to wind-blown surface dust. Since one plume lasted for more than 10 days, it seemed too long lasting to be related to → aurora. The origin of this phenomenon is not yet known.

See also: → Martian; → plume.

A region on the surface of the → Moon where the → gravitational attraction is slightly higher than normal due to the presence of dense rock.

See also: Short for mass concentration; → mass; → concentration.

1. Having qualities appropriate to or usually associated with a man.
   

2. Of, relating to, or constituting the gender that ordinarily includes most words or grammatical forms referring to males (Merriam-Webster.com).

Etymology (EN): M.E. masculin, from O.Fr. masculin “of the male sex,” from L. masculinus “male, of masculine gender,” from masculus “male, masculine; worthy of a man,” diminutive of mas “male person, male,” of unknown origin.

Etymology (PE): Narin, from nar, → male.

1. A source of very intense, narrow-band, coherent microwave radiation involving → stimulated emission, as in the → laser.
   

2. A device that generates such radiation.
   

3. In astronomy, maser emission detected from a number of molecules and associated with several environments: the vicinity of newly forming stars and → H II regions (OH, water, SiO, and methanol masers); the circumstellar shells of evolved stars, i.e. red giants and supergiants (OH, water, and SiO masers); the shocked regions where supernova remnants are expanding into an adjacent molecular cloud (OH masers); and the nuclei and jets of active galaxies (OH and water masers). The hydroxyl radical (OH) was the first interstellar maser detected (Weinreb et al. 1963).

See also: Maser stands for Microwave Amplification by Stimulated Emission of Radiation; → laser.

An emission arising from the → maser process.

See also: → maser; → emission.

1. Something that serves to cover or conceal.
   Electronics: A pattern used to control the configuration of conducting material deposited or etched onto a semiconductor chip.
   
2. Electronics: To override one signal with a stronger one.

Etymology (EN): From M.Fr. masque “covering to hide or guard the face,” from It. maschera, from M.L. masca “mask, specter, nightmare,” of uncertain origin.

Etymology (PE): 1) Mâsk, loan from Fr., as above; 2) with verb zadan “to make, to do,” originally “to strike, beat; to do; to play an instrument” (Mid.Pers. zatan, žatan; O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”); cf.
Skt. han- “to strike, beat” (hantar- “smiter, killer”);
Gk. theinein “to strike;” L. fendere “to strike, push;” Gmc. *gundjo “war, battle;” PIE *g^when- “to strike, kill”).

1. A method of improving → spatial resolution of images. → pupil masking; → unsharp masking.
   

2. Computers: The process of specifying a number of values that allow extracting desired information from a set of characters or bits while suppressing the undesired information.

See also: Verbal noun of → mask.

1. A measure of the amount of material in an object, defined either by the inertial properties of the object or by its gravitational influence on other bodies. See also → inertial mass, → gravitational mass.
   

2. A considerable assemblage, number, or quantity.

Etymology (EN): From O.Fr. masse “lump,” from L. massa “kneaded dough, lump,” from Gk. maza “barley cake, lump, mass, ball,” related to massein “to knead.”

Etymology (PE): Jerm, from Ar. jirm.
Qond “assembled, collected; a crowd,” related to gondé “coarse, thick; big;” Mid.Pers. gund “troop, group, gathering;” loaned into Arm. gund and Ar. jund.
Tudé “heap, stack, tumulus;” cf. Kurd. tavda “all, total;” Tati tâya “heap, mass;” Sogd. tuδē “heap, mass.”

Perhaps related to PIE *teuta- “people, tribe;” cf. Lith. tauta, Oscan touto, O.Irish tuath, Goth. þiuda, O.E. þeod “people, folk, race.”
Anbuh “numerous, abundant,” from Proto-Iranian *ham-buH- “to come together,” from ham- “together,” → com- + *buH- “to be , become,” Av. ham.bauu- “to come together, unite,” from ham- as above + bauu-, bu- “to be, become,” O.Pers. bav- “to be, become,” Mod.Pers. budan “to be,” Skt. bhavati “becomes, happens,” PIE base *bheu-, *bhu- “to grow, become;” cf. Gk. phu- “become,” P.Gmc. *beo-, *beu-, E. be.

A measure of the rate of absorption of radiation, expressed as the linear absorption coefficient divided by the density of the medium through which radiation is passing.

See also: → mass; → absorption;
→ coefficient.

The difference between the rest mass of an atomic nucleus (made up of protons and neutrons) and the sum of the masses of its individual protons and neutrons. The mass difference is equal to the released binding energy. Also called mass deficiency

See also: → mass; → defect.

The mass per unit area of the ring material, integrated through the thickness of the ring. Sometimes called → surface density (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → mass; → density.

1. For → massive stars and → supergiants, the difference between the → spectroscopic mass and the → evolutionary mass. Early studies found that the spectroscopic mass was systematically less than the evolutionary mass by as much as a factor of 2 for supergiants. Improvements in the stellar atmosphere models (taking into account → line blanketing) have decreased or eliminated the size of the discrepancy for Galactic stars. There is still a mass discrepancy for the hottest → O stars in the → Magellanic Clouds
   (See, e.g. Massey et al. 2009, ApJ 692, 618).
   

2. For a → cluster of galaxies, the apparent difference between the mass of the cluster obtained by using the → virial theorem, and the mass inferred from the total luminosities of the member galaxies.

See also: → mass; → discrepancy.

The energy (E) associated with a mass (m), as specified by the → mass-energy equivalence  E = mc², where c is the → speed of light. For a moving body the total energy of the particle is expressed by: E² = m²c⁴ + p²c², where m is → rest mass and p → momentum.

See also: → mass; → energy.

An event in the history of life on Earth in which large numbers of species
(sometimes more than 90% of some species) vanish in a relatively short period of time. In spite of controversy, it is generally recognized that there have been at least six major mass extinctions. These occurred in the late Cambrian (500 million years ago), in the late Ordovician (440 million years ago), in the late Devonian (365 million years ago), at the end of the Permian (245 million years ago), in the late Triassic (208 million years ago), and at the end of the Cretaceous (65 million years ago).

See also: → mass; → extinction.

The mass of a fluid that passes a specified unit area in a unit amount of time.

See also: → mass; → flow.

An → equation expressing the → atomic mass of a → nuclide as a function of its → mass number and the → atomic mass unit.

See also: → mass; → formula.

The fractional amount (by mass) of a given → chemical element or → nuclide in a given → chemical composition.

In chemical composition studies of astrophysical objects
the mass fractions of → hydrogen, → helium, and all the remaining chemical elements are usually denoted by the parameter X, Y, and Z, respectively. Their sum is defined as X + Y + Z = 1. The parameter Z is usually referred to as → heavy elements or → metals.

See also: → mass; → fraction.

1. The number of a class of objects as a function of their mass. → initial mass function (IMF); → present-day mass function (PDMF).
   

2. A numerical relation between the masses of the two components of a → spectroscopic binary that provides information on the relative masses of the two stars when the spectral lines of only one component can be seen. If M_p is the mass of primary (whose spectrum is known), M_s is the mass of secondary, and i the → angle of inclination of the orbit, the mass function is given by:
   (M_s³. sin³i) / (M_p + M_s)².

See also: → mass; → function.

The outpouring of particles and gas from a star, occurring at varying rates and by a variety of processes throughout a star’s lifetime. → Bipolar flows are believed to be due to mass loss by forming → protostars, while → massive stars lose their mass through powerful → stellar winds.

See also: → mass; → loss.

The rate with which the → mass loss process takes place, usually expressed in → solar mass per year. → radiation-driven mass loss. The mass loss rate and the → terminal velocity are anti-correlated, since the → wind momentum is constant, → bi-stability jump.

See also: → mass loss; → rate.

The total number of → protons and → neutrons in the → atomic nucleus (symbol A). The mass number is written either after the → chemical element name or as a superscript to the left of an element’s symbol. For example, the most common isotope of oxygen is oxygen-16, or ¹⁶O, which has 8 protons and 8 neutrons.

See also: → mass; → number.

The flowing out of mass through various processes from an object,
for example in a star forming region or in a close binary.

See also: → mass; → outflow.

A consequence of the → dynamical relaxation process in a gravitationally → bound system, such as a → star cluster or a → globular cluster, where massive and low-mass members occupy different volumes. Massive members sink toward the center, while less massive members tend to move farther away from the center.

See also: → mass; → segregation.

The portion of the isotope shift which results from the difference between the nuclear masses of different isotopes.

See also: → mass; → shift.

An analytical technique for identification of chemical structures, determination of mixtures, and quantitative elemental analysis, in which ions are separated according to the mass/charge ratio and detected by a suitable detector.

See also: → mass; → spectrometry.

A spectrum of charged particles, arranged in order of mass or mass-to-charge ratios. → mass spectrometry.

See also: → mass; → spectrum.

The process in which the evolved member of a close binary system passes gaseous material to its companion star.

See also: → mass; → transfer.

In fluid mechanics, the motion of a given amount of material carried by a fluid from one point to another.

See also: → mass; → transfer.

The principle of interconversion of mass and energy, described by the → mass-energy relation.

See also: → mass; → energy; → equivalence.

The famous equation proposed by Einstein as a consequence of his special theory of relativity describing the equivalence of mass and energy: E = mc², where E is energy, m is the equivalent amount of mass, and c is the velocity of light.

See also: → mass; → energy; → relation.

The ratio of the mass of a system, expressed in solar masses, to its visual luminosity, expressed in solar luminosities.

The Milky Way Galaxy has a mass-luminosity ratio in its inner regions of about 10, whereas a rich cluster of galaxies such as the Coma Cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter.

See also: → mass; → luminosity; → ratio.

A relationship between luminosity and mass for stars that are on the main sequence, specifying how bright a star of a given mass will be. Averaged over the whole main sequence, it has been found that L = M^3.5, where both L and M are in solar units. This means, for example, that if the mass is doubled, the luminosity increases more than 10-fold.

See also: → mass; → luminosity; → relation.

A correlation between the → stellar mass (or → luminosity) and the → gas metallicity of → star-forming galaxies (Lequeux et al. 1979) according to which massive galaxies have higher gas metallicities. Several large galaxy surveys, such as the → Sloan Digital Sky Survey (SDSS), have confirmed that galaxies at all → redshifts with higher stellar masses retain more metals than galaxies with lower stellar masses.

Besides the dependence on stellar mass, other studies have found further dependences of gas metallicity on other physical properties at a given mass, such as → specific star formation rate, → star formation rate, and stellar age. These higher dimensional relations could provide additional constraints to the processes that regulate the metal enrichment in galaxies. In addition to gas metallicity, also the → stellar metallicity of galaxies is found to correlate with the stellar mass, suggesting the mass-metallicity relation already existed at early epochs of galaxy evolution (Lian et al., 2017, MNRAS 474, 1143, and references therein).

See also: → mass; → metallicity; → relation.

The relation between the → stellar mass and the physical size of a galaxy. Studies show that the sizes increase with stellar mass, but that the relation weakens with increasing → redshift. Separating galaxies by their → star formation rate, model simulations show that → passive galaxies are typically smaller than → active galaxies at a fixed stellar mass. These trends are consistent with those found in observations; the level of agreement between the predicted and observed size-mass relations is of the order of 0.1 dex for redshifts < 1 and 0.2-0.3 dex from redshift 1 to 2. Known also as the → luminosity-size relation
(Furlong et al., 2016, MNRAS 465, 722, and references therein).

See also: → mass; → size; → relation.

Consisting of or forming a large mass.

Etymology (EN): From M.Fr. massif (feminine massive) “bulky, solid,” from O.Fr. masse “lump.”

Etymology (PE): Porjerm, from por “full, much, very, too much,” (Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru-, from
par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;” Gk. polus “many,” plethos “great number, multitude;” O.E. full)

• jerm, → mass.

A black hole with a mass between millions and billions of solar masses residing in galactic nuclei. The mass of this type of black holes represents about 0.2% of the bulge mass. When matter is swallowed by the black hole, this gives rise to the tremendous energetic phenomena observed in quasars and active galactic nuclei.

See also: → massive; → black hole.

A → close binary system composed of two → massive stars.

See also: → massive; → close binary star.

Spheroidal distribution of dark matter surrounding a galaxy.

See also: → massive; → halo.

A star whose mass is larger than approximately 10 → solar masses. The → spectral types of massive stars range from about B3 (→ B star) to O2 (→ O star)
and include → Wolf-Rayet stars as well as → Luminous Blue Variables. Massive stars are very rare;
for each star of 20 solar masses there are some 100,000 stars of 1 solar mass. Despite this rarity, they play a key role in astrophysics. They are major sites of → nucleosynthesis beyond oxygen and,
therefore, are mainly responsible for the → chemical evolution of galaxies. Due to their high ultraviolet flux and powerful → stellar winds,
they bring about interesting phenomena in the → interstellar medium, like → H II regions, → turbulence, → shocks, → bubbles, and so on. Massive stars are progenitors of → supernovae (→ type Ia, → type Ic and → type II), → neutron stars, and → black holes. The formation processes of massive stars is still an unresolved problem. For massive stars the → accretion time scale is larger than the → Kelvin-Helmholtz time scale. This means that massive stars reach the → main sequence while → accretion is still going on.

See also: → massive; → star.

1. A person with the ability or power to use, control, or dispose of something.
   

2. An employer of workers or servants.
   

3. The male head of a household (Dictionary.com).

Etymology (EN): M.E. maistre, maister, from O.E. magister, from L. magister “chief, head, director, teacher,” ultimately from PIE root *meg- “great,” cf. Pers. meh-, as below.

Etymology (PE): (Aftari) Mastar “elder; larger,” (Dari Kermân) mastar “leader, guide,” variants (Aftari, Tafreši) mester “elder; great,” massar “large, great, high,” from (Nâini, Sangesari, Dari Yazd, Kermâni) mas “great, large,” variant of meh “great, large, principal,” cognate with L. magister “chief, head, director, teacher;” → Big Bang, + comparative suffix -tar.

A game or contest in which two or more contestants or teams oppose each other (Dictionary.com).

Etymology (EN): Originally “one of a pair, an equal;” O.E mæcca, “companion, mate, one of a pair, wife, husband, an equal,” from gemæcca; cf. O.S. gimaco “fellow, equal,” O.H.G. gimah “comfort, ease,” M.H.G. gemach “comfortable, quiet,” Ger. gemach “easy, leisurely.”

Etymology (PE): Kâd, from Mid.Pers. kâdag “game,” Sogd. kâtak “game, play;” cf. Kurd. (Sorani) kâya “game,” Zazaki kây, Abyâneyi, Anâraki, Nâini kâye, Qohrudi kâda, Shamerzâdi ke, Zefrehi kê “game, play;” Av. kā- “to take pleasure, desire;” Skt. kā- “to desire, wish.”

The body of the → planispheric astrolabe which is a thin circular plate, with a hole in the center. It has a thicker, raised, and graduated edge, called the → limb. The hollow of the mater holds the → tympanum and the rotating → rete. The upper part of the mater carries a jointed ring, called the → throne. By slipping one’s thumb into the ring, one raises the instrument so that its weight and symmetrical design keeps it perpendicular to the ground. On the back of the mater are engraved several circular scales (online museo galileo, VirtualMuseum).

See also: From L. mater, → mother.

1. (adj.) Formed or consisting of matter.
   

2. (n.) The substance or matter from which something is or can be made, or also items needed for doing or creating something.

Etymology (EN): From L.L. materialis (adj.) “of or belonging to matter,” from L. materia, → matter, + → -al.

Etymology (PE): Mâdig, from mâd, mâddé, → matter, + -ig, → -ic.

Belief that physical matter is the only reality and that everything, including thought, feeling, mind, and will, can be explained in terms of matter and physical phenomena.

Etymology (EN): N.L. materialismus; → material + -ism.

Etymology (PE): Mâddebâvari, from mâddé, → matter, +
bâvari, from bâvar “belief” (Mid.Pers. wâbar “beleif;” Proto-Iranian *uar- “to choose; to convince; to believe;” cf. Av. var- “to choose; to convince” varəna-, varana- “conviction, faith;” O.Pers. v(a)r- “to choose; to convince;” Skt. vr- “to choose,” vara- “choosing”).

The state or quality of being material.

See also: → material; → -ity.

The act or process of materializing.

See also: Verbal noun of → materialize.

1a) To come into material form. To take shape.

1b) To form material particles from energy, as in → pair production.

2. To give material form to.

See also: → material; → -ize.

Of, relating to, or of the nature of mathematics.

See also: → mathematics; → -al.

Same as → mathematical elegance.

See also: → mathematical; → beauty.

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.

See also: → mathematical; → conjecture.

A mathematical solution or demonstration when it yields
a result in a surprising way (e.g., from apparently unrelated theorems), is short, and is based on fundamental concepts. According to Henri Poincaré, what gives the feeling of elegance “is the harmony of the different parts, their symmetry, and their happy adjustment; it is, in a word, all that introduces order, all that gives them unity, that enables us to obtain a clear comprehension of the whole as well as of the parts. … Elegance may result from the feeling of surprise caused by the un-looked-for occurrence together of objects not habitually associated. … Briefly stated, the sentiment of mathematical elegance is nothing but the satisfaction due to some conformity between the solution we wish to discover and the necessities of our mind” (Henri Poincaré, Science and Method, 1908). According to Bertrand Russell,
“Mathematics, rightly viewed, possesses not only truth, but supreme beauty – a beauty cold and austere, like that of sculpture, without appeal to any part of our weaker nature, without the gorgeous trappings of painting or music, yet sublimely pure, and capable of a stern perfection such as only the greatest art can show” (Bertrand Russell, A History of Western Philosophy, 1945).

See also: → mathematical; → elegance.

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.

See also: → mathematical; → expectation.

An → abstract object dealt with in mathematics that has a definition, obeys certain properties, and can be the target of certain operations. It is often built out of other, already defined objects. Some examples are → numbers, → functions, → triangles, martices (→ matrix), → groups, and entities such as → vector spaces, and → infinite series.

See also: → mathematical; → object.

An expert or specialist in → mathematics.

Etymology (EN): M.E. mathematicion, from M.Fr. mathematicien, from mathematique, from L. mathematicus, → mathematics.

Etymology (PE): Mazdâhikdân, from mazdâhik, → mathematics,

• -dân “knower,” present stem of dânestan “to know,” → science.

A broad-ranging field of knowledge dealing with the systematic treatment of magnitude, relationships between figures and forms, and relations between quantities expressed symbolically.

Etymology (EN): M.E. mathematic, from L. mathematica (ars), from Gk. mathematike (tekhne) “mathematical science,” from mathema (gen. mathematos) “science, knowledge,”
(+ -ike, → -ics),
related to manthein “to learn, to know” from PIE base *men- “to think,” (cf. Av. mazdāh- “memory,” as below, Lith. mandras “wide-awake,” O.C.S. madru “wise, sage,” Goth. mundonsis “to look at,” Ger. munter “awake, lively”).

Etymology (PE): Mazdâhik, from Av. mazdāh- “memory,” mazdā- “wisdom,” mazdāθa- “what must be borne in mind;” from PIE base *men- “to think,” as above; cf.
Skt. medhā- “mental power, wisdom, intelligence;” Gk. manthein, mathematike, as above.
Riyâzi, loan from Ar. riyâZî, riyâZîyat.

1. An orderly array of numbers, algebraic symbols, or mathematical
   functions, especially when such arrays are added and multiplied according to certain rule; e.g. → Jordan matrix.
   

2. The fine grained material found in between the → chondrules, fragments and metal grains found inside → stony meteorites.

Etymology (EN): From O.Fr. matrice, from L. matrix “female animal kept for breeding,” in L.L. “womb, source, origin,” from mater, → mother.

Etymology (PE): Mâtris, loan from Fr., as above.

The treatment of matrices whose entries are functions.

See also: → matrix; → calculus.

For a → square matrix whose → determinant is not zero, the unique matrix A^-1 satisfying the relation
AA^-1 = A^-1A = I, where I is the → identity matrix.

See also: → matrix; → inverse.

1. Physical or corporeal substance in general, whether solid, liquid, or gaseous, especially as distinguished from incorporeal substance, as spirit or mind, or from qualities, actions, and the like.
   

2. Whatever has size and shape, is solid and tangible, takes up space.
   

3. Anything that contains mass. → material.

Etymology (EN): M.E. mater(e), materie, from O.Fr. mat(i)ere, materie, from L. materia “substance from which something is made,” also “hard inner wood of a tree,” from mater, → mother, PIE base *mater-, see below.

Etymology (PE): Mâddé, variant mâyé “substance, essence; quantity, amount;”
Mid.Pers. mâtak/mâdak “substance, the essential element of anything; materials” (Sogd. patmâδé “matter, substance”), from mât, mâd “mother; substance” (see E. matter, as above), from O.Pers./Av. mātar- “mother;” cf. Ossetic mad/madae “mother;” Khotanese mâta “mother;” Skt. mātár- “mother;” Gk. meter, mater; L. mater (Fr. mère, Sp. madre);
O.E. môdor from P.Gmc. *mothær (O.S. modar, Dan. moder, Du. moeder, Ger. Mutter); Lith. mote “wife.”
Note: Ar. mâddat is borrowed from Mid.Pers. mâdak, as above, and Arabicized through association with madda “to extend.”

A critical change in the history of the Universe, which occurred after the radiation era, when the density of energy contained within matter exceeded the density of energy contained within radiation. This transition started about 5000 years after the Big Bang, when the temperature had fallen to 3 x 10⁴ K. Later, 380 000 years after the Big Bang, when the temperature was 3000 K, matter and radiation were no longer coupled together and the Universe became transparent.

See also: → matter; → era.

A Universe in which the matter energy density (Ω_m ≈ 1) provides most of the total energy density. According to the → Big Bang model, in the early history of the → Universe a → radiation-dominated phase preceded the matter-dominated phase. This phase is characterized by R/R₀ ∝ t^2/3, where R is the → cosmic scale factor and t is time.

See also: → matter; → dominate; → Universe.

A period from about 1645 to 1715 when the number of → sunspots was unusually low. This → solar activity minimum is attested also through the increased content of carbon 14 in tree rings in that period. The reason is that the cosmic rays which produce ¹⁴C reach the Earth in a greater number when there is weak solar activity (see also → radiocarbon dating). The Maunder minimum occurred during a period of cooling of the Earth, called the → Little Ice Age. The Maunder minimum is one of a number of periods of low solar activity, including the → Dalton minimum, the → Sporer minimum, the → Wolf minimum, and the → Oort minimum.

See also: After the British astronomer Edward Walter Maunder (1851-1928) who,
along with Gustav Spörer of Germany, first called attention to this phenomenon; → minimum.

The greatest value attained (or attainable) by a function; the opposite of minimum.

Etymology (EN): From L. maximum, neuter of maximus “greatest,” superlative of magnus “great, large” cognate with Pers. meh “great, large” (Mid.Pers. mah, mas; Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest;” cf. Skt. mah-, mahant-; Gk. megas; PIE *meg- “great”).

Etymology (PE): Bišiné, from biš “much, more; great” (from Mid.Pers. veš “more, longer; more frequently,” related to vas “many, much” (Mod.Pers. bas);
O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish”) + -in superlative suffix + -é nuance suffix.

The density of pure water occurring at 3.98 °C, which is 1.0000 g cm^-3, or 1000 kg m^-3. Water when cooled down contracts normally until the temperature is 3.98 °C, after which it expands. Because the maximum density of water occurs at about 4 °C, water becomes increasingly lighter at 3 °C, 2 °C, 1 °C, and 0 °C (→ freezing point). The density of liquid water at 0 °C is greater than the density of frozen water at the same temperature. Thus water is heavier as a liquid than as a solid, and this is why ice floats on water.

When a mass of water cools below 4 °C, the density decreases and allows water to rise to the surface, where freezing occurs. The layer of ice formed on the surface does not sink and it acts as a thermal isolator, thus protecting the biological environment beneath it.

This property of water liquid is very unusual; molecules pack more closely than in the crystal structure of ice. The reason is that → hydrogen bonds between liquid water are not stable, they are continuously broken and new bonds are created. In the crystal structure of ice molecules have a fixed pattern creating empty space between molecules.

See also: → maximum; → density; → water.

A deconvolution algorithm which functions by minimizing a smoothness function in an image. The MEM seeks to extract as much information from a measurement as is justified by the data’s signal-to-noise ratio.

See also: → maximum; → entropy;
→ method.

Of a → supernova, → peak luminosity.

See also: → maximum; → light.

A statistical procedure based on choosing the value of the unknown parameter under which the probability of obtaining an observed sample is highest.

See also: → maximum; → likelihood.

The unit of → magnetic flux. The flux through 1 square cm normal to a magnetic field of 1 → gauss. It is equal to 10^-8 → weber (Wb)s.

See also: After James Clerk Maxwell (1831-1879), British outstanding physicist, who made fundamental contributions to electromagnetic theory and the kinetic theory of gases.

A type of → Wheatstone bridge used for measuring → inductance in terms of → resistance and → capacitance.

See also: → maxwell; → bridge.

A division in Saturn’s ring in the outer part of the C ring. It is about 87500 km from Saturn’s center and is 500 km wide. The gap was discovered in 1980 by Voyager 1.

Etymology (EN): Not discovered by J. C. Maxwell, but named in his honor; → maxwell; → gap.

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

See also: → maxwell; → Boltzmann’s constant; → distribution.

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.

See also: 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.

See also: → 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.

Every part of a deformable electric circuit tends to move in such a direction as to enclose the maximum magnetic flux.

See also: → maxwell; → rule.

A complex calendar created by the ancient central American Mayas which uses three different dating systems in parallel: Long Count, Tzolkin, and Haab. Only Haab has a direct relationship with the length of the year. It is a solar → vague year consisting of 18 months of 20 days each, and an additional period of 5 → epagomenal days.
Tzolkin is a calendar of 13 x 20 = 260 days running within Haab and is used for ritual purposes. A date is usually described by specifying its position in both the Tzolkin and Haab calendars. The least common multiple of the two calendars, called the Calendar Round, has 18,980 days, representing a cycle of 73 sacred years, or 52 vague years. The Long Count is
the number of days since the start of the Maya era. There is disagreement about the beginning date of the Long Count. Most authorities agree, however, that the Long Count started in 3114 B.C., with several possible dates.

See also: Maya, proper name; → calendar.

1a) General: A quantity having a value intermediate between the values of other quantities; an average.

1b) → arithmetic mean.

1c) → geometric mean.

1d) → harmonic mean.

1e) → weighted mean.

1f) → root mean square.

2. To have as its sense or signification; signify.

Etymology (EN): 1) From O.Fr. meien, from L. medianus “of or that is in the middle,” → median.

2. Verb of → meaning.

Etymology (PE): 1) Miyângin “the middle; middle-sized; the middle pearl in a string,” from miyân, → middle, + -gin a suffix forming adjectives of possession.

2. → meaning.

The angle between the periapsis of an orbit and the position of a hypothetical body that orbits in the same period as the real one but at a constant mean angular velocity.

See also: → mean; → anomaly.

That point on the → celestial sphere at which an object would be seen from the solar system → barycenter affected by the → e-terms → aberration.

See also: → catalog;
→ mean; → place.

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

See also: → mean; → diurnal; → motion.

An element of an adopted reference orbit that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations.

See also: → mean; → element.

The orientation the Earth’s equator would have if the nutation was subtracted.

See also: → mean; → equator.

A fictitious equinox whose position is that of the vernal equinox at a particular epoch with the effect of nutation removed.

See also: → mean; → equinox.

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.

Etymology (EN): → mean; → free; → path.

Etymology (PE): 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 amount of time an unstable radioisotope exists before it decays, It is equal to 1.44 times the half-life.

See also: → mean; → life.

The total atomic or molecular weight divided by the total number of particles. For instance, the mean molecular weight of a plasma of pure ionized ⁴He would be 4 (the atomic mass number) divided by 3, the total number of particles (1 nucleus plus 2 electrons), i.e. 4/3.

See also: → mean; → molecular;
→ weight.

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.

See also: → mean; → moon.

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

See also: → mean; → motion.

The parallax, derived by means of statistical studies of brightness and motions, for a large group of stars whose individual parallaxes cannot be measured.

See also: → mean; → parallax.

An object’s celestial position as determined for a given mean equator and equinox. → mean position.

See also: → mean; → place.

The direction of the Earth’s axis at a particular epoch if the nutation is ignored.

See also: → mean; → pole.

Same as → mean place.

See also: → mean; → position.

The shape of a pulsar’s pulse as determined by averaging several pulses.

See also: → mean; → profile.

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

See also: → mean; → sidereal; → day.

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

See also: → mean; → sidereal;
→ time.

The average length of the apparent solar day. In other words, the interval between successive transits of the mean Sun for a given observer.

See also: → mean; → solar; → day.

The time since the mean Sun crossed the meridian with 12 hours added to make the day begin at midnight.

See also: → mean; → solar; → time.

A plot of the mass-to-charge ratio of elementary particles, sorted by their isotopic mass.

See also: → mean; → spectrum.

A hypothetical Sun that moves along the ecliptic at a uniform rate equal to the average motion of the real Sun.

See also: → mean; → sun.

In → syllogism, the term which is common to both → premises and is excluded from the → conclusion.

See also: → mean; → term.

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).

See also: → mean; → value; → theorem.

The sense or significance of a word, sentence, symbol, etc. The study dealing with meanings is called → semantics. See also → semiotics.

Etymology (EN): M.E., from mean; O.E. mænan “to mean, intend, signify” (cf. O.Fris. mena “to signify,” O.S. menian “to intend, signify,” M.Du. menen, Du. meenen, Ger. meinen “think, suppose”), related to Pers. maneš “disposition, temperament,” mênidan “to think, consider,” → idea; + → -ing.

Etymology (PE): Cemâr, from cem or cim “meaning, signification;” Mid.Pers. cim “meaning, reason, cause;” ultimately from Proto-Ir. *cahmāt “wherefore?” cf. Skt. kasmāt “why, where from? whence?,” kim “what? how? why?” + âr short form of âvar present stem of âvardan “to cause or produce; to bring,” → production, as in bonâr, → cause, used also as a nuance suffix;
see also the verb → mean.

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.

Etymology (EN): 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.

Etymology (PE): 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”).