An Etymological Dictionary of Astronomy and Astrophysics

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”).

1. The act of measuring; a measured quantity.
   

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

See also: Verbal noun of → measure.

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

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

A person who repairs and maintains machinery, motors, etc. (Dictionary.com). Same as → mechanician.

See also: → mechanics.

1. Of, connected with, produced by → mechanics.
   

2. Like machines; automatic.

See also: → mechanic; → -al.

The energy that is possessed by an object due to its motion or due to its position. It is equal to the sum of the → kinetic energy and → potential energy.

See also: → mechanical; → energy.

1. The state of a → rigid body if, as viewed from an → inertial frame of rest: 1) the → linear acceleration of its → center of mass is zero, and 2) its → angular acceleration about any axis fixed in this reference frame is zero. The center of mass may be moving with constant velocity and the body may be rotating about a fixed axis with constant angular velocity.
   

   2. In → thermodynamics, the state of a system in which → pressure is the same every where with no other forces acting on the system except a uniform external pressure.

See also: → mechanical; → equilibrium.

Same as → Joule’s constant.

See also: → mechanical; → equivalent; → heat.

Any → mixing process that utilizes the → kinetic energy of relative → fluid motion.

See also: → mechanical; → mixing.

The → rate at which → work is done by a → force. In other words, → mechanical energy per unit time. Mechanical power is expressed in units of joules/sec (joules/s) or a watt (W) in the → mks system.

See also: → mechanical; → power.

1. Any system of elements that interact according to the laws of → mechanics (as distinguished from chemical, electrical, thermal, etc.).
   

2. A collection of → machines functioning together to achieve the transfer of motion.

See also: → mechanical; → system.

A process in which matter is shed into a → Keplerian disk from a star rotating at the → critical velocity. The disk is probably destroyed by the pressure exerted by the stellar radiation and finally matter is lost. Such a process seems to occur around → Be stars
which are stars rotating at or very near the critical limit (Meynet et al. 2007, arXiv:0709.2275).

See also: → mechanical; → wind.

1. In a mechanical manner; by a mechanism.
   

2. In a machine-like manner; without feeling.

See also: → mechanical; → -ly.

A person skilled in constructing, working, or repairing machines; mechanic; machinist (Dictionary.com).

See also: From → mechanic + -ian.

A branch of → physics that deals with motion and the → action of → forces on bodies.
Mechanics may be divided into three areas, → kinematics, → dynamics, and → statics.

Etymology (EN): From mechanic, from L. mechanicus, from Gk. mekhanikos “an engineer,” also “inventive,” literally “pertaining to machines” (adj.), from mekhane, → machine, + → -ics.

Etymology (PE): Mekânik, loan from Fr. Sâzokârik, from sokâr, → mechanism, + -ik, → -ic.

1. The structure or arrangement of parts of a machine or similar device, or of anything analogous.
   

2. The agency or means by which an effect is produced or a purpose is accomplished.

Etymology (EN): From Mod.L. mechanismus, from Gk. mekhane, → machine.

Etymology (PE): Sâzokâr, literally “making and working,” from sâz “apparatus; (musical) instrument,” from sâzidan, sâxtan “to build, make, fashion; to adapt, adjust, be fit” (from
Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”) + kâr “work,” from kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

Statistics: The middle value in a sample sorted into ascending order. If the sample contains an even number of values, the median is defined as the mean of the middle two.
Geometry: The line from a vertex of a triangle to the midpoint of the opposite side.

Etymology (EN): From M.Fr. médian, from L. medianus “of the middle,” from medius “middle;” akin to Pers. middle, → medium, → meddle;
from PIE *medhyo-, from base *me- “between;” → medium.

Etymology (PE): Miyâné from miyân, → middle, + -é nuance suffix.

The art or science of restoring or preserving health or due physical condition, as by means of drugs, surgical operations or appliances, or manipulations (Dictionary.com).

Etymology (EN): M.E. medicin, from O.Fr. medecine “art of healing, cure,” from L. medicina “the healing art; a remedy,” feminine of medicinus (adj.) “of a doctor,” from medicus “a physician;” PIE root *med- “to measure, limit, consider, advise,” → dimension; cf. Av. vi-mad- “physician;” Gk. medein “to rule;” L. meditari “think or reflect on, consider,” → meditation;
Ir. miduir “judge;” O.E. metan “to measure out.”

Etymology (PE): Pezeški, from pezešk “physician,” from
Mid.Pers. bizešk “physician,” bêšâz “medicinal;”
Av. bišaz- “to cure, heal;” cf. Skt. bhisaj- “healer, physician.”

To engage in thought or contemplation; reflect. → consider.

Etymology (EN): Back formation from → meditation.

Etymology (PE): Segâlidan “to meditate, consider, think,” from Mid.Pers. uskaridan “to think, consider, discuss,” from us-, → ex-, + kar- “to observe, to consider;” related to engâridan, negaristan, âgâridan, → consider.

The act of meditating. → consideration.

Etymology (EN): From L. meditatatus p.p. of meditari “to think over, reflect, consider,” from PIE root *med- “to measure, limit, consider, advise,” → mode.

Etymology (PE): Verbal noun of → meditate.

An intervening substance through which an effect is produced; surrounding objects, conditions, or influences; environment. → interstellar medium.

Etymology (EN): From L. medium, from neut. of adj. medius “middle,” from PIE *medhyo- “middle;” cf. Av.
maδəma- “middle,” as below.

Etymology (PE): Madim, from Av. maδəma- [adj.] “middle, being in the middle; middling, of a middling size or quality,” maiδim “in the midst of,” maiδyāna- “the middle;” cf. Skt. mádhya- “middle;” Gk. messos; Goth. midjis; O.E. midd “middle;” O.C.S. medzu “between,” Arm. mej “middle;” L. medium, as above.

To come upon; come into the presence of; encounter.

Etymology (EN): M.E. meten, from O.E. metan “to find, find out; encounter; obtain,” cognate with O.Frisian meta, O.Sax. motian “to meet,” Goth. gamotijan, ultimately from PIE root *mod- “to meet, assemble.”

Etymology (PE): Mavâzidan, from Sogd. mwz “to meet together, encounter” (Cheung 2007); ultimately from Proto-Ir. *ham-uaz-, from *ham- “together, → com-,”

• *uaz- “to move, carry, drive;” cf. Av. vaz- “to move, carry, drive (a chariot);” Pers. vazidan “to blow;” parvâz “→ flight.”

1. The act of coming together.
   

2. An → assembly or conference of persons for a specific purpose (Dictionary.com).

See also: → meet; → -ing.

1. Prefix, often meaning “large, great.”
   
2. Prefix meaning 10⁶.

See also: From Gk. megas “great, large, mighty,” from PIE *meg- “great;” cf. L. magnus, Goth. mikils, O.E. micel.

A unit of distance equal to a million → parsec (pc)s,
or 3.26 million → light-years.

See also: → mega-; → parsec.

Same as → Dyson sphere.

See also: → mega-; → structure.

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

See also: → mega-; → ton.

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

See also: From Ar. al-Maghriz(المغرز) “the root, base, bottom” (of the Bear’s tail).

A → giant star of → apparent visible magnitude 3.54 in the → Orion constellation.
λ Ori is of → spectral type O8 III, has a mass of about 28 → solar masses,
a size of about 10 → solar radii, and an → effective temperature of around 35,000 K. Meissa is a → soft X-ray source with a → luminosity of 10³² erg s^-1 and peak emission in the energy range of 0.2-0.3 keV, which is probably generated by the → stellar wind. Meissa is a member of the → Orion OB1 association.

It is in fact a → double star with a → companion at an angular separation of 4.41 arcseconds. This fainter component has a magnitude of 5.61, is a → main sequence star of → spectral type B0.5 V. There is an outlying component, Meissa C, which is an F-type main sequence F8 V star. This star in turn may have a very low mass companion that is probably a brown dwarf.

λ Ori, excites a fairly symmetric → H II region, Sh2-264 surrounding it. Many observational studies have found dark clouds external to this H II region.

See also: Arabic al-maysan (مَیسان) “bright.”

(v.int.) To become liquefied by warmth or heat, as ice, snow, butter, or metal. (v.tr.) To reduce to a liquid state by warmth or heat; to fuse.

Etymology (EN): M.E. melten, O.E. meltan “become liquid;”
cf. O.N. melta “to digest;” Gk. meldein “to melt,” L. mollis “soft, mild.”

Etymology (PE): Godâxtan, godâz- “to melt,” from Mid.Pers. vitâxtan, vitâcitan “to melt,” from Av. vi-taxti- “flowing away, melting,” from vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + tak- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
tacar- “course,” tacan “current, streaming;” Mod.Pers. tâz-, tâxtan “to run; to hasten; to assault,” tâzi “swift (greyhound),” tak “running, rush;”
Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
Khotanese ttajs- “to flow, to walk;” cf. Skt. tak- “to rush, to hurry,” takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.”

A severe accident in a nuclear reactor which is caused by a major failure in the reactor cooling circuit leading to the melting of the reactor core. Without coolant, the core would overheat so that the uranium fuel would melt. If the core continued to heat up, the steel walls of the core would also melt. If the molten core escaped from the containment housing of the reactor, large amounts of highly radioactive materials would be released into the atmosphere. The radioactive contamination of the air, water, and soil can have disastrous consequences for life.

Etymology (EN): → melt; down, M.E. doun, from O.E. dune “downward,” short for adune, ofdune, from a-, of “off, from” + dune “hill.”

Etymology (PE): Forugodâxt, from foru- “down, downward; below; beneath,” as in forurixtan, forunešastan, foruraftan (Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain”) + godâxt past stem of godâxtan, godazidan → melt.

The temperature at which a solid changes to a liquid.

See also: → melting; → point.

A person or a thing that is part of a group body; e.g. a star which belongs to a cluster.

Etymology (EN): From M.E., from O.Fr. membre, from L. membrum “limb, member of the body, part.”

Etymology (PE): Hamvand, literally “linked, joined together,” from ham- “together, with; same, equally, even” (Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-, Skt. sam-; also O.Pers./Av. hama- “one and the same,” Skt. sama-, Gk. homos-; originally identical with PIE numeral *sam- “one,” from *som-. The Av. ham- appears in various forms: han- (before gutturals, palatals, dentals) and also hem-, hen-)

• vand “joined, tied,” from
  bastan, vastan “to bind, shut” (O.Pers./Av. band- “to bind, fetter,” banda- “band, tie” (cf.
  Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind).

The state of being a member.

Etymology (EN): From → member + -ship a native E. suffix of nouns denoting condition, character, office, skill, etc., from M.E., O.E. -scipe; akin to shape.

Etymology (PE): hamvandi, from hamvand, → member,

• -i condition, character suffix.

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

See also: → membership; → function.

To commit to memory, learn by heart.

Etymology (EN): From memor-, → memory, + → -ize.

Etymology (PE): Barmidan, from barm, → memory; az bar kardan, literally “to do from memory,” from az preposition, bar, contraction of barm, kardan “to do, make,” → -ize.

1. The faculty of the mind to preserve and recall past sensations, thoughts, knowledge, etc.
   

2. In computers and recording instruments, any device into which information can be introduced and later extracted.

Etymology (EN): M.E. memorie, from L. memoria, from memor “mindful, remembering;” cf. Gk. mermera “care,” merimna “anxious thought, sorrow,” martyr “witness;” Pers. šomârdan “to count;” Mid.Pers. ôšmârtan, ôšmurtan “to reckon, calculate, enumerate, account for;” from Av. base (š)mar- “to have in mind, remember, recall,” pati-šmar- “to recall; to long for,” hišmar-;
Skt. smar- “to remember, become aware,” smarati “he remembers.”

Etymology (PE): Barm “memory,” variant bar (az bar kardan “to memorize”), bir, vir,
from Mid.Pers. varm “memory,” variants vir, vârom “mind, conscience;” from Av. vārəma, vārəm “according to one’s wishes,” from var- “to choose.”

The amount of information which can be retained in a memory, usually expressed as the number of words which can be retained. For comparison of different memories this number is expressed in bits.

See also: → memory; → capacity.

See → periodic table.

See also: → periodic table.

A → lens with a crescent-shaped section; a → concavo-convex lens.

Etymology (EN): Mod.L. meniscus, from Gk. meniskos “lunar crescent,” diminutive of mene, → moon.

Etymology (PE): Mâhak, diminutive of mâh, → moon.
Kuž-kâv “convexo-concave,” → convex; → concave.

A multiple star of magnitude V = 1.90 which is situated in the → Auriga constellation at 81 → light-years away. Other main designations: HR 2088 and HD 40183. Although the third brightest star of the constellation, it bears “Beta” designation. Menkalinan is composed of two main components, which make up a → spectroscopic binary. The combined apparent magnitude varies over a period of 3.96 days between +1.89 and +1.94, as every 47.5 hours one of the stars partially eclipses the other. Both
are metallic-lined → subgiant stars of spectral type
A2 IV. Each is about 48 times more luminous than the Sun and has roughly the same mass and radius (2.6 times that of the Sun). There is a third component of magnitude 14.1, which is separated from the main pair by 13’’, corresponding to a projected distance of 330 → astronomical units.

See also: From Ar. Al Mankib dhi’l ‘Inan (منکب ذی العنان) “the Shoulder of the One Who Holds the Reins,” which is the rendition of the Gk. mythology character Auriga (Charioteer).

A → giant star of → apparent visual magnitude +2.06 located in the southern constellation → Centaurus. It has a → spectral type of K0 III and lies 61 → light-years away. Also called Haratan.

See also: Menkent, corruption of Ar. Mankib “shoulder,” short for Mankib al-Qanturis (منکب القنطورس ) “shoulder of Centaurus.”

The Table Mountain. A faint constellation near the south celestial pole, at 5h right ascension, 80° south declination. It contains part of the → Large Magellanic Cloud, and its brightest star
is of magnitude 5.1. Abbreviation Men; genitive Mensae.

Etymology (EN): First introduced by the French astronomer Nicolas Louis de Lacaille (1713-1762) under the name Mons Mensae, from L. mons “mountain” + mensa “table” to refer to Table Mountain in South Africa. Lacaille made important early observations of the southern sky from the Cape Town region.

Etymology (PE): Mizkuh, from miz “table,” originally “preparations for entertaining a guest; guest;” Mid.Pers. mêzd “offering, meal” + kuh→ mountain.

Of or pertaining to the → mind.

Etymology (EN): M.E., from M.Fr. mental, from L. mentalis “of the mind,” from mens (genitive mentis) “mind,” from PIE root *men- “to think.”

Etymology (PE): Menti, mentâl, adjectives of ment, → mind; → -al.

1. Mental capacity or endowment.
   

2. Mental capacity or endowment (Dictionary.com).

See also: → mental; → -ity.

1. To refer briefly to; name, specify, or speak of (Dictionary.com).
   

2. A direct or incidental reference; a mentioning (Dictionary.com).

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

Etymology (PE): Ayât, from Mid.Pers. ayât, ayâd “remembrance, recollection, memory;” Mod.Pers. yâd.

A blue → dwarf star of → spectral type A1 with an → apparent magnitude of 2.37 in the constellation → Ursa Major. It lies 79 → light-years away and has a → luminosity almost 60 times solar, and a mass about triple that of the Sun. Although Merak ranks fifth in brightness in the → Big Dipper, it received the Beta designation from Bayer, who lettered the Dipper’s stars from front to back.

See also: From Ar. al-Maraqq (المراق) “the soft parts of the belly, the loins.”

A 1.2 m semi-robotic telescope located at the Roque de los Muchachos Observatory on La Palma Island (Canary Islands, Spain). It is operated by the staff of the Institute of Astronomy, University of Leuven (Belgium).

The telescope uses two modern instruments HERMES: a → high-resolution optical → spectrograph, and MAIA, a three-armed camera equipped with large → charge-coupled device (CCD)s and optimized for more specific rapid variability studies.

The main science drivers of the research performed on the basis of Mercator data are related to a wide range of variable phenomena with a clear focus on stellar astrophysics, in particular the stellar internal structure by means of → asteroseismology.

See also: Named after the Flemish cartographer Gerardus Mercator (1512-1594), who studied and taught at the University of Leuven before moving to Duisburg (Germany)

1. The closest → planet to the → Sun and one of five planets visible with the naked eye. The → greatest elongation of Mercury is about 28°, making it visible at most about 112 minutes after sunset or before sunrise. It lies at a mean distance of about 0.39 → astronomical units from the Sun. Mercury is just 4,879 km in diameter, about 2.6 times smaller than the Earth. Its → orbital period is 87.97 Earth days. Mercury has a high → density, 5.4 g cm^-3, with only the Earth having a higher density among the planets. This is largely due to Mercury being composed mainly of heavy metals and rock.

One → solar day on Mercury lasts the equivalent of 176 Earth days while the sidereal day (the time for 1 rotation in relation to a fixed point) lasts 59 Earth days. Mercury is nearly → tidally locked to the Sun and over time this has slowed the rotation of the planet to almost match its orbit around the Sun. Mercury also has the highest orbital → eccentricity of all the planets with its distance from the Sun ranging from 46 to 70 million km. Mercury has just 38% the → gravity of Earth, this is too little to maintain an atmosphere against → solar winds, which blow it away.

The surface of Mercury which faces the Sun has
temperatures of up to 427°C, whilst on the alternate side this can be as low as -173°C. Mercury’s core has more iron than any other planet in the → solar system. This has to do with its formation and early life. If the planet formed quickly, increasing temperatures of the evolving Sun could have vaporized much of the existing surface, leaving only a thin shell.

2. (lower case): Metallic chemical element, also called quicksilver; symbol Hg (from L. hydrargyrum “liquid silver”). → Atomic number 80; → atomic weight 200.59; → melting point -38.842°C; → boiling point 356.58°C. Mercury was first recognized as a chemical element (in the modern sense) by the French chemist Antoine L. Lavoisier (1743-1794).

Etymology (EN): From L. Mercurius “Mercury,” the Roman god, originally a god of tradesmen and thieves, from merx “merchandise.”

Etymology (PE): 1) Mid.Pers. Tîr the name of the planet Mercury, O.Pers. proper noun *Tira-dāta- “given by Tir” (Hellenized Tiridates), Mid.Pers. Tîr.dât the name of three Parthian Kings; Av. Tīro.nakaθwa-.
2) Jivé, variant živé, from Mid.Pers. zivik, zivandag “alive, living,” from zivastan “to live,” zivižn “life;” O.Pers./Av. gay- “to live,” Av. gaya- “life,” gaeθâ- “being, world, mankind,” jivya-, jva- “aliving, alive;” cf. Skt. jiva- “alive, living;” Gk. bios “life;” L. vivus “living, alive,” vita “life;” O.E. cwic “alive;” E. quick; Lith. gyvas “living, alive;” PIE base *gweie- “to live.”
Simâb “liquid silver,” from sim “silver” (Mid.Pers. âsīm)

• âb, → water.

A narrow and elongated structure of glowing → sodium gas associated with Mercury. Mercury’s thin atmosphere contains small amounts of sodium that glow when excited by radiation from the Sun. Solar photons also liberate these molecules from Mercury’s surface and pushes them away. Because Mercury’s gravity is too weak to hold a permanent atmosphere, when atoms are evaporated from the planet’s surface, some of the atoms form a tail that points away from the Sun. In particular, the yellow glow from sodium → D line is relatively bright. First predicted in the 1980s, the tail was first discovered in 2001 (A.E. Potter et al., 2001). Many tail details were revealed in multiple observations by NASA’s robotic → MESSENGER spacecraft that orbited Mercury between 2011 and 2015.

See also: → Mercury; → tail.

1. (v.intr.) To become combined, united, swallowed up, or absorbed; lose identity by uniting or blending.
   

2. (v.tr.) To cause to combine or coalesce. To combine, blend, or unite gradually so as to blur the individuality or individual identity of.
   Related terms: → fuse, → coalesce. See also → merger, → mergeburst, → merger process, → merger tree, → merging, → merging galaxy, → minor merger, → mixed merger, → wet merger.

Etymology (EN): From L. mergere “to dip, immerse,” probably rhotacized from *mezgo, and cognate with Skt. majj- “to dive, to sink,” majjati “dives under;” Lith. mazgoju “to wash.”

Etymology (PE): Taškidan, taškândan, from Gilaki tašk “tie, knot;” Tabari tešk “knot”

• -idan infinitive suffix.

A hypothetical → transient event undergone by a
→ star due to its violent → merging with another star in a → close binary star. The release of → orbital energy causes the → envelope of the star to heat up and → inflate, causing the star to brighten considerably. Mergebursts are predicted to rival or exceed the brightest classical → novae in luminosity, but to be much cooler and redder than classical novae, and to become slowly hotter and bluer as they age.

See also: → merge; → burst.

1. Any combination of two or more bodies into a single body. In particular, the formation of a galaxy from the collision of two or more separate galaxies.
   
2. An act or instance of merging.

Etymology (EN): From → merge + -er (as in waiver).

Etymology (PE): Tašké; tašk, nouns from taškidan, → merge.

The process of collision between galaxies which leads to a single galaxy.

See also: → merger; → process.

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

See also: → merger; → tree.

1. (n.) The act of joining together as one, such as galaxy → merger.
   
2. (adj.) That merges.

See also: Noun from → merge.

Two or more galaxies that collide and merge into one galaxy.

See also: → merging; → galaxy.

1. Geography: An imaginary line on the Earth’s surface joining the north and south poles at right angles to the equator. See also → local meridian, → prime meridian.
   

2. Astron.: An imaginary great circle on the → celestial sphere that passes through its poles
   and the observer’s → zenith.

Etymology (EN): M.E., from O.Fr. meridien, from L. meridianus “of noon, southern,” from meridies “noon, south,” from meridie “at noon,” altered by dissimilation from *medi die, locative of medius “mid-” + dies “day.”

Etymology (PE): Nimruzân, coined by Pers. astronomer (A.D. 973-1048) in his at-Tafhim, from nim “mid-, half” (Mid.Pers. nêm, nêmag “half;” Av. naēma- “half;” cf. Skt. néma- “half”)

• ruz, → day, + -ân suffix denoting time and place.

Same as hour angle.

See also: → meridian; → angle.

A telescope with a graduated vertical scale, used to measure the declinations of heavenly bodies and sometimes to determine the time of meridian transits.

See also: → meridian; → circle.

An instrument designed to observe objects when they cross the meridian.

See also: → meridian; → observation.

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

See also: → meridian; → observation.

The moment when a celestial object crosses an observer’s meridian. Same as meridian transit.

See also: → meridian; → passage.

Of, pertaining to, or resembling a meridian.

Etymology (EN): → meridian + → -al.

Etymology (PE): From nimruzân, → meridian, + -i adj. suffix.

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

See also: → meridional; → circulation.

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

See also: → meridional; → flow.

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

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

One of the stars in the Pleiades with a visual magnitude 4.17. It is a giant of spectral type B6 lying at a distance of about 1400 light-years.

See also: Merope was one of the seven Pleiades, daughters of the Titan Atlas.

A → prime number of the form 2^p - 1, where p is a prime. As of February 2013, 48 Mersenne primes are known. The largest known prime number is 2^57,885,161 - 1. Each prime gives rise to an even → perfect number.

See also: Marin Mersenne (1588-1648), French theologian, philosopher, mathematician and music theorist; → prime.

A star of visual magnitude 4.8 lying 204 light-years away in the constellation → Aries. It is in fact a triple star system.

See also: The origin of Mesarthim (or Mesartim) is a matter of controversy. Some scholars have related it to the Ar. methartim (مثرطم) “very fat (animal),” but the connection is not obvious although the words are apparently similar. The original Ar. name was Šaratayn (الشرطین) “the two marks” denoting the current β and γ stars in Aries. It was also the name of the lunar mansion of which these two stars were members. Johann Bayer (1572-1625) erroneously related Šaratayn to the Hebrew Sartai, a current term in the astrological literature of his time.
Subsequently, others figured that Sartai was related to Hebrew Mesartim “servants.” The Latin transliteration and alteration Mesarthim found much
success in establishing itself as the proper name for star γ Arietis.

1. One of the open spaces between the cords or ropes of a net.
   

2. Any knit, woven, or knotted fabric of open texture.
   

3. An interwoven or intertwined structure; network.
   

4. Electricity: A set of branches that forms a closed path in a network so that removal of a branch results in an open path. (Dictionary.com).

Etymology (EN): M.E. mesche “open space in a net,” apparently from O.E. max “net,” earlier mæscre (cf. Dan. maske, Sw. maska, M.Du. maessce, Du. maas “mesh,” O.H.G. masca, Ger. Masche “mesh”).

Etymology (PE): Bâncé “aperture, opening, window” in (Kermânšâhi) Kurd., ultimately from Proto-Ir. *banaka-, from *baH-
“to shine,” cf. Av. bāmya- “light, bright;” Pers. bâm “morning, dawn; splendor, light,” Ossetic bon “day,” probably related to bezel “opening, aperture,” in several dialects of the Fârs province (Lâr, Gerâš, Xonj, Fišvar), → morning.

A combining form meaning “middle,” used in the formation of compound words; e.g.
→ meson; → mesosphere.

Etymology (EN): From Gk. mesos “middle, in the middle;” akin to L. medius, Pers. miyân, → middle; → medium.

Etymology (PE): Meso-, loan from Gk.

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

See also: From mes-, meso-, from Gk. mesos “middle,” akin to L. medius, Pers. miyân, → medium, → middle,

• -on a suffix used in the names of subatomic particles.

A class of → meteorites that is → brecciated→ stony-iron with nearly equal amounts of → metal and → silicates.

See also: → meso-; → siderite.

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

See also: → meso-; → sphere.

1. General: A piece of information (written, spoken, or by signals).
   

2. Computer science: A defined amount of information, coded or not, between machines or machines and men. → information theory.

Etymology (EN): M.E., from O.Fr. message “message, news, embassy,” from M.L. missaticum, from L. missus “a sending away, dispatching,” from mittere “to send,” → mission.

Etymology (PE): Payâm, variants payqâm, peyqâm, Mid.Pers. paygâm, ultimately from *patigam-, literally “arriving, newcomer,” from *pati- “to, toward, at, in,” → ad hoc, + *gam- “to come,” → heliosheath.

One who brings → messages.

Etymology (EN): From M.E. messengere, messingere, messangere, from O.Fr. messanger, a variant of messagier, from → message + → -er.

Etymology (PE): Payâmbar “messenger,” from payâm, → message,

• bar “carrier,” from bordan “carry,” → vector.

A NASA robotic spacecraft that orbited the planet → Mercury for more than four years, between 2011 and 2015. Among its accomplishments, the mission determined Mercury’s surface composition, revealed its geological history, discovered details about its internal magnetic field, and verified its polar deposits are dominantly water-ice. → Mercury’s tail.

See also: Messenger, short for “MErcury Surface, Space ENvironment, GEochemistry, and Ranging”, a reference to the → messenger god Mercury from Roman mythology; → spacecraft.

A catalog of more than 100 nebulous-appearing astronomical objects, initially established to avoid confusion with comets. These objects are now well known to be among the brightest and most striking gaseous nebulae, star clusters, and galaxies. The designations of the catalog
are still used in identification; e.g. M1 is the Crab Nebula (in Taurus).

See also: In honor of the French astronomer Charles Messier (1730-1817), who compiled the list between 1760 and 1784 in order to avoid confusion with comets; → catalog.

Any of the nebulous-looking → astronomical objects listed in the → Messier catalog.

See also: → Messier catalog; → object.

The first quantitative model showing that the energy of → white dwarfs is the leftover heat from the star’s past nuclear fusion that leaks slowly into space. In this analytic model constructed by Mestel (1952), a white dwarf consists of two layers. The inner layer, which contains most of the mass, is assumed to be → isothermal because of efficient thermal conductivity by the → degenerate electrons. Moreover, it is supposed that the electrons do not contribute significantly to the → heat capacity. The heat capacity comes entirely from the ions, which are assumed to behave as a classical → ideal gas. The thin non-degenerate outer layer forms an insulating
blanket and controls the rate at which the energy from the ion reservoir is leaked out into space. The specific rate is controlled by the radiative opacity at the boundary between these two layers, and is assumed to obey → Kramers’ opacity law. The Mestel theory shows that the cooling rate of a white dwarf is proportional to its temperature (hotter white dwarfs cool faster), and gives a relationship between the luminosity (L) of the white dwarf and the cooling time: t ∝ L^-5/7. More recent models take into account some or all of the following processes neglected in the Mestel theory: neutrino cooling (important for L > 10^-1.5 L_sun), latent heat of crystallization release (important for L < 10^-4 L_sun), nuclear energy generation via proton-proton burning (important when M_H ≥ 10^-4 M_*), and gravitational energy release from surface layers. The Mestel theory is a very good approximation of more recent calculations. For a review of the Mestel theory see Van Horn (1971, IAU Symp. 42, 97; W. J. Luyten, Editor), Wood (1990, J. Roy. Astro. Soc. Canada 84, 150), and Kepler and Brdaley (1995, Baltic Astron. 4, 166).

See also: Named after Leon Mestel (1927-), British astrophysicist, who put forward this theory in 1952 (MNRAS, 112, 583); → theory.

Reduced growth or stagnation undergone by → cold dark matter perturbations during the period when the → early Universe was → radiation-dominated. The photons cannot collapse, and by their pressure prevent the matter to do so, when radiation dominates. Matter pertubation (collapse) remains frozen until the density equality between radiation and matter.

See also: Péter Mészáros, 1974, A&A 37, 225; → effect.

A prefix appearing in loanwords from Gk., with the meanings

1. “after, behind;” 2) “changed, altered;” 3) “higher, beyond.” → metagalaxy; metaphysics; → metastable.

Etymology (EN): From Gk. meta (prepositin) “in the midst of, among, with, after,” originally me-ta (Mycenaean Greek), from PIE *me- “in the middle” (cf. Goth. miþ, O.E. mið “with, together with, among,” E. with).

Etymology (PE): Matâ-, from Av. matay-, mati- “protrusion of mountain range,” framanyente “to be protruding, jutting;”
from PIE base *men- “to stand out, to project;” cf. L. mons (genitive montis) “mountain,” minere “to project, jut, threaten” (other related terms: mouth, prominent, amount, etc.).

Specifically defined data elements that describe how and when a particular set of data was collected, and how it is formatted. Metadata is used to organize, manipulate, and work with data when it is not necessary or desired to actually deal with the data itself. The reason is that the metadata is usually far smaller and easier to work with than the data that it represents.

See also: → meta-; → data.

An obsolete term which once denoted the entire system of galaxies including the Milky Way.

See also: → meta-; → galaxy.

1. Chemistry: An → element in which the highest occupied energy band (→ conduction band) is only partially filled with electrons.
   
2. Astrophysics: Conventionally, any element heavier than → helium. The term “metal,” as used for this concept, is in fact inappropriate. Same as → heavy element. See also → metallicity.

Etymology (EN): From O.Fr. metal, from L. metallum “metal, mine, quarry, what is got by mining,” from Gk. metallon “metal, ore,” originally “mine, quarry, pit,” probably from metalleuein “to mine, to quarry,” of unknown origin, but related somehow to metallan “to seek after.”

Etymology (PE): Felez “metal,” loanword from Ar. filizz.

The quality of being metal deficient, e.g. → metal-deficient galaxy.

Etymology (EN): → metal; deficiency from L. deficientem (nominative deficiens), pr.p. of deficere “to desert, fail,” from → de- “down, away” + facere “to do, perform” + -ency a noun suffix, equivalent to -ence.

Etymology (PE): Kamfelezi, from kam “little, few; deficient, wanting; scarce” (Mid.Pers. kam “little, small, few,” O.Pers./Av. kamna- “small, few,” related to
keh “small, little, slender” (related to kâstan, kâhidan “to decrease, lessen, diminish,” from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen;” Av. kasu- “small, little;” Proto-Iranian *kas- “to be small, diminish, lessen”) + felez→ metal + -i suffix denoting state.

A galaxy whose → metallicity is smaller than that of the → Milky Way galaxy.

See also: Adj. from → metal deficiency; → galaxy.

Describing an → astronomical object in which the → chemical abundances of → heavy elements are lower than that of a reference value (usually solar or of Milky Way).

See also: → metal; → poor.

Same as → metal-deficient galaxy.

See also: → metal; → poor; → galaxy.

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

See also: → metal; → rich; → environment.

A star whose → metal content is higher than the → solar metallicity. The stars that harbor → extrasolar planets tend to be considerably more metal-rich than the average → Population I star in the Galactic neighborhood. See also → super-metal-rich star.

See also: → metal; → rich; → star.

Any language that is used to describe a language. See also → object language.

See also: → meta-; → language.

Of, relating to, or consisting of metal.

See also: → metal; → -ic.

A kind of → degenerate matter resulting from hydrogen gas when it is sufficiently compressed to undergo a phase change to liquid or solid state. Metallic hydrogen is thought to be present in compressed
astronomical objects, such as the interiors of the solar system planets Jupiter and Saturn. Above the core of these planets
(at a temperature of 10,000 degrees and a pressure of 3 million bars)
the electrons are squeezed out of the hydrogen atoms and the fluid starts to conduct like a metal.

See also: → metallic; → hydrogen.

In a star, nebula, or galaxy, the proportion of the material that is made up of → metals, that is elements heavier than → helium. It is generally denoted by Z. The term “metallicity” is a misnomer used in astrophysics.

1. In practice, the metallicity of stars is usually expressed by the number ratio of → iron atoms to → hydrogen atoms per unit volume, with respect to the solar values:
   [Fe/H] = log₁₀(N_Fe/N_H)_star - log₁₀(N_Fe/N_H)_Sun, where
   N_Fe and N_H are the numbers of iron and hydrogen atoms per unit volume. In fact it is taken to be equal to the iron → abundance with respect to the solar value.
   The solar logarithmic iron abundance is 7.50 ± 0.04 (Asplund et al. 2009, ARAA 47, 481), with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. See also → solar metallicity.
   

2. Nebular metallicity is often defined as the relative abundance of → oxygen:
   (N_O/N_H)_neb/(N_O/N_H)_Sun, where N_O and N_H represent the numbers of oxygen and hydrogen atoms per unit volume.

See also: From metallic, from → metal + → -ity.

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

See also: → metallicity; → distribution; → function.

The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars.
Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.

See also: → metallicity; → gradient.

An excited state in an atom, which is at the origin of the spectral lines called → forbidden lines.
The time duration of the excited state being relatively long, under laboratory conditions the atom cannot pass directly to the ground state by emitting radiation. In the extremely rarefied interstellar medium, however, such highly improbable transitions do occur.

See also: → meta-; → stable; → state.

A streak of light caused when a → meteoroid enters Earth’s → atmosphere and becomes incandescent, mostly from → friction with the air at high speed. Meteors are also referred to as shooting stars. Very bright meteors are called → fireball or → bolide.

Most of visible meteors arise from particles ranging in size from about that of a small pebble down to a grain of sand, and generally weigh less than 1-2 grams. The brilliant flash of light from a meteor is mainly caused by the → meteoroid’s high level of → kinetic energy as it collides with the atmosphere at high speeds (11-72 km/s).

The increase in the number of meteors visible toward the end of the night results from the fact that the Earth rotates about its axis in the same direction as it orbits the Sun. This means that the leading edge (morning side) of the Earth encounters more meteoroids than the trailing edge (evening side).

In general, 2 to 3 times as many meteors can be seen in the hour or so just before morning twilight, than can be seen in the early evening. Moreover, the numbers of random, or sporadic, meteors vary from season to season, due to the tilt of the Earth on its axis and other factors. See also → meteor shower.

Etymology (EN): From M.Fr. meteore, from M.L. meteorum (nom. meteora), from Gk. ta meteora “the celestial phenomena,” pl. of meteoron, literally “thing high up,” neuter of meteoros (adj.) “high up,” from → meta- “over, beyond” + -aoros “lifted, hovering in air,” related to aeirein “to raise.”

Etymology (PE): Šahâb, from Ar. Šihâb.

A → meteorite impact crater located about 55 km east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Meteor Crater is about 1,200 m in diameter and some 170 m deep. It is thought to have formed between 20,000 to 50,000 years ago, by the impact of a small → asteroid about 25 m in diameter. Same as → Barringer Crater.

See also: → meteor; → crater.

The reflection of → radio waves from transmitters located on the ground by a → meteor or by the corresponding trail left behind. When a meteor enters the Earth’s upper atmosphere it excites the air molecules, producing a streak of light and leaving a trail of ionization behind it tens of kilometers long. This ionized trail occurs typically at a height of about 85 to 105 km, and may persist for less than 1 second up to several minutes.

See also: → meteor; → echo.

An increased number of → meteors all appearing to → diverge from the direction of a single point, called → radiant.

Meteor showers occur → annually on the same dates, when the Earth crosses through a → meteoroid stream. Meteor showers are named after the → constellation in which the radiant is located. For example, the → Perseids’s radiant lies near the top of the constellation Perseus.

Most meteor showers are caused by → comets. As a comet orbits the Sun it sheds an icy, dusty → debris stream along its orbit. When the Earth’s orbit intersects the dust trail, more meteors are seen as the cometary debris encounters our planet’s → atmosphere.

In the case of the → Geminids and → Quadrantids, those meteor showers come from the debris scattered by orbiting → asteroids.

Typical meteor showers show 15 to 100 meteors per hour at their peak. On very rare occasions, during a → meteor storm, thousands of meteors fall per hour.

Prominent meteor showers are:

→ Quadrantids, → Lyrids, → Eta Aquariids, → Delta Aquariids, → Perseids, → Orionids, → Taurids, → Leonids, → Geminids, → Ursids, → Alpha Capricornids.

See also: → meteor; → shower.

An extremely intense → meteor shower, in which hundreds or even many thousands of → meteors per hour may be observed. During the great → Leonids meteor storm of 1833 an estimated number of about 150,000 meteors fell per hour.

See also: → meteor; → storm.

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

See also: → meteorite, → ionization.

An object of → extraterrestrial origin that survives entry through the atmosphere to reach the Earth’s surface. → Meteors become meteorites if they reach the ground. See also → stony meteorite, → iron meteorite, → stony-iron meteorite, → chondrite, → micrometeorite , → achondrite, → CAI meteorite, → differentiated meteorite, → undifferentiated meteorite, → Hoba meteorite, → Jilin meteorite, → Martian meteorite, → meteorite flux.

Etymology (EN): From → meteor + -ite a suffix of nouns.

Etymology (PE): Šaxâné “metor,” may be from šaxudan, šaxânidan “to scratch, to thrust, to assail,” as the meteor light scratches the dark night.
Šahâbsang, from šahâb, → meteor, + sang, → stone.
Âsmânsang, from âsmân, → sky, + sang, → stone.

The total mass of extraterrestrial objects that land on Earth during a given time period. The meteorite flux is currently estimated to be about 10⁷ to 10⁹ kg yr^-1. Much of this material is dust-sized objects called → micrometeorites.

See also: → meteorite; → flux.

Of or pertaining to a → meteorite.

See also: From → meteorite + → -ic.

The abundance of a chemical element as derived from meteorites. Meteoritic abundances measured from carbonaceous → CI chondrites are believed to represent → protosolar abundances.

See also: → meteoritic; → abundance.

A striking of a meteorite against another body, especially the solar system planets or satellites.

See also: → meteoritic; → impact.

The science or study of meteorites.

See also: → meteorite + → -ics.

A solid object in → interplanetary space before it reaches the Earth’s atmosphere. Meteoroids are of → silicate and/or → metallic matter having a size from tiniest grains up to that of the smallest → asteroids.

See also: → meteor; → -oid.

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

See also: → meteoroid; → stream.

A relatively dense collection of meteoroids at certain spots along some → meteoroid streams.

See also: → meteoroid; → swarm.

Evaluation or measurement of one or more meteorological elements.

See also: Meteorological, of or pertaining to → meteorology;
→ observation.

A scientific establishment dedicated to making precise and detailed meteorological observations and to studying and forecasting atmospheric phenomena by means of special equipments.

See also: Meteorological, of or pertaining to → meteorology;
→ observatory.

The study of the physics, chemistry, and dynamics of the Earth’s atmosphere, including the related effects at the air-earth boundary over both land and the oceans.

Etymology (EN): From Gk. meteorologia “discussion of celestial phenomena,” from meteoron→ meteor + -logia, &rarr-logy.

Etymology (PE): Havâšenâsi, from havâ “weather, air,” → air.

The → SI unit of length; symbol m. It is defined by taking the fixed numerical value of the → speed of light in vacuum, c, to be 299 792 458 when expressed in the unit m/s, where the → second is defined in terms of Δν_Cs.

Etymology (EN): From Fr. mètre, from Gk. metron “measure,” from PIE base *me- “to measure” (cf. O.Pers., Av. mā- “to measure;” Skt. mati “measures;”
L. metri “to measure”).

Etymology (PE): Metr, loan from Fr.

A colorless, odorless, inflammable gas gas of formula CH₄; the simplest hydrocarbon.

See also: From meth- a combining form representing methyl + -ane a suffix used in names of hydrocarbons of the methane or paraffin series.

Alcohol, also known as methyl alcohol, formula CH₃OH, formed in small quantities in the oxidation of methane. → methanol maser.

See also: From → methane + -ol a suffix used in the names of chemical derivatives.

A maser source in which excited methanol molecules (CH₃OH) produce → maser emission. Methanol masers are signposts of the early stages of star formation, many being associated with sources that have not developed an → H II region. There are more than 20 different methanol transitions that have been observed. They are divided into two categories: Class I, excited by collisions, and class II, excited by infrared radiation. The most important class I masers are at a frequency of 44.1 GHz, while he most important class II masers are at a frequency of 6.7 GHz.

See also: → methanol; → maser.

A manner or mode of procedure, especially an orderly, logical, or systematic way of instruction, inquiry, investigation, experiment, and so on.

Etymology (EN): From M.Fr. méthode, from L. methodus “way of teaching or going,” from Gk. methodus “scientific inquiry, method of inquiry,” originally “following after,” from → meta- “after” + hodos “way.”

Etymology (PE): Raveš “mthod,” originally “going, walking,” from row “going,” present stem of raftan “to go, walk;” Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack” + -eš a suffix of verbal nouns.

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

See also: → method; → least squares.

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

See also: → method; → small;
→ perturbation.

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

See also: → method; → successive;
→ approximation.

→ HD 140283.

See also: Name given to → HD 140283 by the popular press due to its very old age. Methuselah is a biblical patriarch supposed to have lived
969 years (Genesis 5:21-27). The name Methuselah, or the phrase “old as Methuselah,” is commonly used to refer to any living thing reaching great age.

A → methane molecule lacking one → hydrogen atom: CH₃. → methyl chloride.

See also: Back formation from Fr. méthylène, → methylene.

A → chemical compound (CH₃Cl), which is the most abundant → organohalogen in the Earth’s atmosphere. It has both natural and synthetic origins. Also called chloromethane. Low levels of methyl chloride occur naturally in the environment. Methyl chloride is formed in the oceans by natural processes (e.g., marine phytoplankton) and from biomass burning in grasslands and forested areas (e.g., forest fires); it has been detected at low levels in air all over the world. Other sources of exposure to methyl chloride include cigarette smoke, polystyrene insulation, and aerosol propellants; home burning of wood, coal, or certain plastics.

High levels may occur at chemical plants where it is made or used. Acute (short-term) exposure to high concentrations of methyl chloride in humans has caused severe neurological effects. Methyl chloride has also caused effects on the heart rate, blood pressure, liver, and kidneys in humans (United States Environmental Agency, EPA).

See also: → methyl; → chloride.

The innermost moon of → Jupiter. Also known as Jupiter XVI. It was discovered in 1979 in images taken by Voyager 1. Its mass is about 3.6 × 10¹⁶ kg and its dimensions 60 × 40 × 34 km. Its mean distance from Jupiter is 128 000 km and its → orbital period is 0.29 Earth days, which is faster than Jupiter’s rotation period. Metis is one of the → Shepherd moons of Jupiter.

See also: Named in 1983 after the first wife of Zeus.

A time interval lasting 235 → lunations, or about 19 → tropical years (235 = 19 x 12 + 7), after which → lunar phases recur on the same days of the year.

See also: Named after Meton of Athens, a Gk. mathematician, astronomer, geometer, and engineer who used it in 432 B.C., but it was known to the Babylonians by around 500 B.C. and to the Chinese around 600 B.C.; → cycle.

1. A mathematical → expression consisting of an → array of → components which are needed for calculating → infinitesimally small → distances between two → points in some geometrical → space. More simply put, the → function used to define a distance between two points in a → metric space. Also called → distance function.
   

2. In → general relativity the metric describes the → space-time geometry and gives the interval between two neighboring → events.
   

3. Pertaining to the meter or to the → metric system.

See also: From → meter + → -ic.

Any of the suffixes adopted by the International System of Units (→ SI units).

For numbers larger than 1:

→ deca- (10¹), → hecto- (10²), → kilo- (10³), → mega- (10⁶), → giga- (10⁹), → tera- (10¹²), → peta- (10¹⁵), → exa- (10¹⁸), → zetta- (10²¹), and → yotta- (10²⁴) .

For numbers smaller than 1: → deci- (10^-1), → centi- (10^-2), → milli- (10^-3), → micro- (10^-6), → nano- (10^-9), → pico- (10^-12), → femto- (10^-15), → atto- (10^-18), → zepto- (10^-21), and → yocto- (10^-24).

See also: → metric; → prefix.

An set of points such that the distance between every pair of points is defined by a → distance function with
the following properties: 1) the distance from the first point to the second equals zero if and only if the points are the same, 2) the distance from the first point to the second equals the distance from the second to the first, and 3) the sum of the distance from the first point to the second and the distance from the second point to a third exceeds or equals the distance from the first to the third.

In mathematical language, the properties, for a nonempty set X, can be expressed as:

1. d(x,y)≥ 0 and d(x,y) = 0 if and only if x = y.
   
2. d(x, y) = d(y,x) for all x, y ∈ X.
   
3. d(x,z)≤ d(x,y) + d(y,z) for all x, y, and z ∈ X. Also called → triangle inequality.

See also: → metric; → space.

A standard system of measurement using decimal units, in which the units of length, time, and mass are meter, second, and kilogram respectively.

See also: → metric; → system.

The abstract tensor operation which is computed in a particular → reference frame using the → metric components. The metric tensor defines magnitude and direction of vectors about a point.

See also: → metric; → tensor.

A physical → measurement unit in the → metric system.

See also: → metric; → unit.

The science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.

Etymology (EN): From metro-, a combining form meaning “measure,” → meter,

• → -logy.

Etymology (PE): Andâze-šenâsi, from andâzé, → measure,

• -šenâsi, → -logy.

Mega (million) → electron volt. A unit of → energy used to describe the total energy carried by a → particle or → photon.

Etymology (EN): → mega- + → electron volt.