An Etymological Dictionary of Astronomy and Astrophysics

The mathematical problem of studying the behavior (e.g., velocities, positions) of any number of objects moving under their mutual gravitational attraction for any time in the past or future. Same as the → many-body problem.

See also: → body; → problem.

The conductivity in a semiconductor caused by a flow of electrons, whereas p-type conductivity is caused by a flow of holes.

See also: n standing for → negative; → type; → conductivity.

The largest and the most luminous → H II region in the → Small Magellanic Cloud, which contains the prominent OB star cluster → NGC 346. N66 has a diameter of about 7 arcmin on the sky corresponding to about 120 pc for a distance of about 60 kpc. Compared with the → Orion Nebula, N66 has an Hα luminosity almost 60 times higher.

See also: Serial number in the Henize (1956) catalog, more precisely designated as LHA 115-N66.

→ D line.

See also: Na, → sodium; → D line.

Same as → Iota Orionis.

See also: From Ar. Nayyir al-Saif (نیر‌السیف) “the Bright One in the Sword,” from nayyir (نیر) “bright” + al-saif (السیف) “sword.”

The mathematical symbol for the → del operator.

See also: From Gk. nabla an ancient stringed instrument, triangular in shape and held like a harp, thus of the shape of ∇.

An → adaptive optics instrument at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) in service since 2001. It provides adaptive optics assisted imaging, imaging polarimetry, and → coronagraphy (only L) in the 1-5 μm range. It consists of two components: NAOS (Nasmyth Adaptive Optics System) and CONICA (COudé Near Infrared CAmera).

The adaptive optics system, NAOS, is equipped with both
→ <i><a class="linkVoir" href="/terms/visible/">visible</a></i> and → <i><a class="linkVoir" href="/terms/infrared/">infrared</a></i>→ <i><a class="linkVoir" href="/terms/wavefront-sensor/">wavefront sensor</a></i>s. 
It contains five → <i><a class="linkVoir" href="/terms/dichroic-filter/">dichroic filter</a></i>s 
which split the light from the telescope between CONICA and one of the
NAOS wavefront sensors. 

CONICA is the → <i><a class="linkVoir" href="/terms/infrared-camera/">infrared camera</a></i> and
→ <i><a class="linkVoir" href="/terms/spectrometer/">spectrometer</a></i> attached to NAOS and 
equipped with an Aladdin 1024 × 1024 pixel InSb array detector. It
possesses several wheels carrying masks/slits (including focal plane
coronagraphic masks), filters, polarizing elements,
→ <i><a class="linkVoir" href="/terms/grism/">grism</a></i>s and several
cameras allowing → <i><a class="linkVoir" href="/terms/diffraction-limited/">diffraction-limited</a></i>
sampling across the full wavelength range.

See also: NaCo, from NAOS (→ Nasmyth, → adaptive; → optics; → system) + CONICA (→ coude; → near-infrared; → camera).

The point on the celestial sphere
blocked from view by Earth and diametrically opposite to the → zenith (سرسو).

Etymology (EN): From M.L. nadir, from Ar. nazir “opposite to,” contraction of nazir as-samt (نظیر‌السمت), literally “opposite of the zenith,” from nazir “opposite” + samt “zenith.”

Etymology (PE): Pâsu, literally “direction of the foot,” from pâ “foot, step” (from
Mid.Pers. pâd, pây; Khotanese fad; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + su “direction, side” (from Mid.Pers. sôk “direction, side”).

1. A small metal spike with a broadened flat head, driven into wood to join things together or to serve as a hook.
   

   2. A horny covering on the upper surface of the tip of the finger and toe in humans and other primates (OxfordDictionaries.com).

Etymology (EN): M.E. nail(l), nayl(l), O.E. negel “metal pin,” nægl “fingernail;” cf. O.Norse nagl “fingernail,” nagli “metal nail;” O.H.G. nagel, M.Du. naghel, Du. nagel, Ger. Nagel “fingernail, small metal spike;” from PIE root *h3nog^h- “nail” (source also of Gk. onyx “claw, fingernail;” L. unguis (Fr. ongle, from diminutive L. ungula) “nail, claw;” Pers. nâxon, as below; Lithuanian naga “hoof,” nagutis “fingernail.”

Etymology (PE): 1) Mix, from Mid.Pers. mêx “peg, nail;” O.Pers. mayūxa- “doorknob;” cf. Skt. mayūkha- “peg for stretching the woof”).

2. Nâxon “nail;” Mid.Pers. nâxun “nail;” cf. Skt. nakha-; Russ. noga; E. nail, as above.

Not accompanied or supplemented by anything else.

→ naked eye, → naked singularity, → naked-eye star.

Etymology (EN): O.E. nacod “nude;” cf. M.Du. naket, Du. naakt, Ger. nackt, cognate with Pers. berehné, as below, , from PIE base *neg^w-no- “naked.”

Etymology (PE): Berehné, from Mid.Pers. brahnag (with secondary -r-) “naked;” Av. maγna- “naked;” cf. Skt. nagná- “naked;” Gk. gumnos “naked;” L. nudus “naked;” Arm. merk “naked;” Lith. nuogas “naked;”

The qualifier of an eye which is not assisted by any optical device, except for eyeglasses. Same as → unaided eye.

See also: → naked; → eye.

A singularity that is not surrounded by an event horizon. It will therefore be visible and communicable to the outside world.

See also: → naked; → singularity.

A star visible without a telescope. In principle, stars down to about sixth magnitude are visible to the naked eye under ideal conditions, but this depends on the individual, the location, and the conditions of the observation.

See also: → naked; → eye; → star.

A word or term by which somebody or something is commonly and distinctively known.

Etymology (EN): M.E., from O.E. nama; cf. O.H.G. namo, Ger. Name, Du. naam, Goth. namo “name;” cognate with Pers. nâm, as below.

Etymology (PE): Nâm “name;” Mid.Pers. nâm; O.Pers./Av. nâman-; cf. Skt. nama-;
Gk. onoma, onuma; L. nomen; PIE *nomen-.

A prefix denoting 10^-9.

See also: Combining form of Gk nanos “dwarf.”

The most stable carbon compound with an average particle size of about 5 → nanometers (50 Å). Nanodiamonds are synthesized by detonation of → T.N.T. and then gathering the soot that remains from the explosion.

See also: → nano-; → diamond.

Dust grains at → nanoscale sizes found in various astrophysical environments, such as → interstellar medium and → interplanetary space of our Solar System.

See also: → nano-; → dust.

A unit of length equal to 10^-9 m, or 10 Å.

See also: → nano-; → meter.

A solid particle of → nanoscale size; e.g. a → nanodust grain.

See also: → nano-; → particle.

The size range from approximately 1 → nanometer (nm) to 100 nm.

See also: → nano-; → scale.

The science and technology of producing and exploiting structures and systems at dimensions of roughly 1 to 100 → nanometers, that is those of single atoms and molecules. Materials with such minute structures possess mechanical, optical, chemical, magnetic or electronic properties not found in ordinary materials.

See also: → nano-; → technology.

A bright, blue star of V = 2.25 in the constellation → Puppis. It is a hot → supergiant of type O5, with a → surface temperature of 42,000 K, lying 1,400 → light-years away. Naos is one of the rarest → O-type stars visible to the → naked eye.

See also: Naos, from Gk. naos “ship,” since it used to be the Zeta star of Argo Navis (Jason’s vessel Argos) before becoming Zeta in Puppis.

Same as the → natural logarithm.

See also: After John Laird Napier (1550-1617), the Scottish mathematician who invented logarithm; → logarithm.

Of little breadth or → width.

Etymology (EN): M.E., from O.E. nearu; cf. O.S. naru “narrow,” Du. naar “unpleasant;” akin to Ger. Narbe “scar,” literally, “narrow mark.”

Etymology (PE): Bârik, from Mid.Pers. bârîk “narrow,” from O.Iranian *bāra-ya-ka-, from stem *bāra- “to cut;”
cf. Av. bāra- “edge, blade,” in ti&#382i-bāra- “sharp-edged,” from brī- “to cut, shave, shear;”
cf. Skt. bhrī- “to hurt, injure,” bhrinanti “they hurt;” O.C.S. briti “to shave;” PIE base *bhrē- “to cut, pierce.”

Optics: Of a filter, same as → interference filter.
Radio.: Encompassing a small frequency range, typically less than 300 Hz.

See also: → narrow; → band.

Photometry using narrow-band filters to isolate a particular spectroscopic line or molecular band.

See also: → narrow band; → photometry.

The outer region of a → quasar or an → active galactic nucleus displaying narrow spectral lines. Also called Type II AGN. See also → obscuring torus.

See also: → narrow; → line; → region.

1. Of, in, or relating to the → nose.
   

   2. Phonetics: A → nasal consonant.

See also: Adjective, from → nose.

The interior of the → nose. It is the structure which extends from the external opening, the → nostrils, to the → pharynx, where it joins the respiratory system.

See also: → nasal; → cavity.

A consonant produced with air escaping through the → nose. For this to happen the → soft palate is lowered and at the same time the mouth passage blocked at some point, so that all the air is pushed out of the nose. Examples of this sound include the English sounds [m], [n], and [ng].

See also: → nasal; → consonant.

A lunar → impact crater that is located in the southern part of the Moon’s near side. Latitude: 41.0°S, longitude: 0.2°E, diameter: 52 km, depth: 3.35 km.

See also: Named after the Iranian mathematician and astronomer Nasireddin Tusi (1201-1274), → Nasireddin couple.

A theorem put forward by the 13th century Persian mathematician and astronomer Nasireddin Tusi to generate linear motion from a combination of circular motions, and thus improve the geocentric model of Ptolemy. A circle of radius R rotates inside a circle of radius 2R. The smaller circle rotates at twice the speed of the larger one and in opposite direction. The initial tangent point will travel in linear motion back and forth along the diameter of the larger circle. A particular case of → hypocycloid curves.

See also: Named for Nasireddin Tusi (1201-1274), director of Marâgha observatory who created the Ilkhani zij; → couple.

In an altazimuth-mounted telescope, a focal point to one side of the tube, created by placing a third deflecting mirror in the optical path. This extra mirror directs the beam along the altitude axis, and through a hole in the supporting trunnions. Nasmyth focus has the advantage of remaining at a fixed position relative to the telescope wherever the instrument is pointed. Moreover, bulky or heavy instruments can be mounted there on a permanent platform, which rotates only in azimuth.

Etymology (EN): Named after the inventor James Nasmyth (1808-1890), who first used it in the 19-th century.

A large body of people united by common descent, history, culture, or language, inhabiting a particular state. → national, → international.

Etymology (EN): Nation, from O.Fr. nacion, from L. nationem (nom. natio) “nation, stock, race,” literally “that which has been born,” from natus, p.p. of nasci “to be born;” cognate with Pers. zâdan, → generate.

Etymology (PE): Nafân, from Av. nāfaēna- “family,” hama.nāfaēna- “of the same family,” related to nāfa- “near relationship, family, navel,” napāt- “grandson” (Mod.Pers. navé “grandson”); cf.
Mid.Pers. nâf “family,” nâfag “navel,” naft “descendant, " Parthian nap “family” (Mod.Pers. nâf “navel”); Sogd. nâf “country;” Sorani Kurd. natawa (Kurmanji Kurd. netewe) “family, race, nation,” from *nafata (with metathesis);
cf. Skt. nábhi- “nave, navel, central part; home, origin;” Gk. anepsios “cousin, sister’s son;” L. nepos “grandchild; descendant;” O.Lithuanian neputis “grandson; nephew;” O.H.G. nabalo “navel;” Ger. Nabel “navel;” O.E. nefa “grandson; descendant;”

• -ân suffix denoting family relationship, as in Bâbakân “family of Bâbak.”

Of, involving, or relating to a nation as a whole. → National Aeronautics and Space Administration (NASA).

See also: → nation; → -al.

A federal agency of the United States government founded in 1958 for civil aeronautical research and space exploration, superseding the National Advisory Committee for Aeronautics (NACA). Its goals include improving human understanding of the universe, the solar system, and Earth and establishing a permanent human presence in space. NASA is headquarted at Washington, D.C., and operates several research, development, and test facilities, as follows alphabetically: 1) Ames Research Center; 2) Dryden Flight Research Facility at Edwards, California, used for flight testing and as a landing site for the Space Shuttle; 3) Glenn Research Center at Cleveland, Ohio, concerned with aircraft and rocket propulsion; 4) Goddard Space Flight Center; 5) Jet Propulsion Laboratory; 6) Johnson Space center; 7) Kennedy Space Center; 8) Langley Research Center at Hampton, Virginia, which carries out research in aeronautics and space technology; 9) Marshall Space Flight Center; 10) the Space Telescope Science Institute; 11) Stennis Space Center, near Bay St Louis, Mississippi, for testing rocket engines; and 12) Wallops Flight Facility on Wallops Island, Virginia, which manages NASA’s sounding rocket and scientific balloon programs.

Etymology (EN): → national; → aeronautics; → space; administration, verbal noun of administer, from M.E. amynistre, from O.Fr. aministrer, from L. administrare “to serve, carry out, manage,” from → ad- “to” + ministrare “to serve,” from minister “servant, priest’s assistant,” from minus, minor “less,” hence “subordinate,” + comparative suffix *-teros.

Etymology (PE): Sâzmân, → organization; fazâyi pertaining to fazâ, → space; Âmrikâ “United State of America.”

Loyalty and devotion to a nation; especially: a sense of national consciousness exalting one nation above all others and placing primary emphasis on promotion of its culture and interests as opposed to those of other nations or supranational groups (Merriam-Webster.com).

See also: → national; → -ism.

1. National status; specifically: a legal relationship involving allegiance on the part of an individual and usually protection on the part of the state b : membership in a particular nation.
   

2. Membership in a particular nation (Merriam-Webster.com).

See also: → national; → -ity.

The transfer of a major branch of industry or commerce from private to state ownership or control (OxfordDictionaries.com)

See also: → nationalize; → -tion.

1. To bring under the ownership or control of a nation, as industries and land (Dictionary.com).

See also: → national; → -ize.

Occurring in nature; not artificially prepared. → nature.

See also: Adj. of → nature.

Any frequency of small-amplitude oscillation for a system with a position of stable equilibrium and in the absence of external forces. In other words, the frequency of → free oscillation. Also called characteristic frequency.

See also: → natural; → frequency.

The → sciences, as → botany, → mineralogy, or → zoology,
dealing with the study of all objects in → nature: used especially in reference to the beginnings of these sciences in former times (Dictionary.com).

See also: → natural; → history.

Linguistics: A language that has evolved naturally as a means of communication among people, as opposed to → artificial language and → formal language (Dictionary.com).

See also: → natural; → language.

The broadening of any spectral line due to the fact that excited levels have mean lives, which, by virtue of the uncertainty principle, implies a spread in the energy values.

See also: → natural; → line;
→ broadening.

The logarithm in which the → base is the → irrational number e = 2.718281828…. Also called → Napierian logarithm. The natural logarithm is denoted ln, an abbreviation of logarithmus naturalis. Natural logarithms are related to → common logarithms through: ln x = (1/M) log x, with M = (1/ln 10) ≅ 0.434294.

See also: → natural; → logarithm.

Either a member of the set of positive integers 1, 2, 3, …, or the set of non-negative integers 0, 1, 2, 3, … There seems to be no general agreement about whether to include 0 in the set of natural numbers.

See also: → natural; → number.

Of a body or system, the period of → free oscillation.

See also: → natural; → period.

A resonance such that the period of the driving force is the same as the natural period of the system.

See also: → natural; → resonance.

A solar system → object that → revolves around a → primary body and is not man made.

See also: → natural; → satellite.

A science that deals with matter, energy, their interrelations and transformations,
In other words, natural sciences are concerned with physical processes observable in nature. They can be divided into physical and biological sciences.

See also: → natural; → science.

Physical units of measurement defined in terms of universal physical constants in such a manner that some chosen physical constants (e.g. the speed of light, Planck’s constant, Boltzmann’s constant, etc.) are equal to unity. The use of natural units allows these constants to be omitted from mathematical equations, leading to simpler calculations.

See also: → natural; → unit.

Uranium as found in nature. It contains 0.7% uranium-235, 99.3% uranium-238, and a trace of uranium-234 by weight.

See also: → natural; → uranium.

Philosophy: 1) The view of the world that takes account only of natural elements and forces, excluding the supernatural or spiritual.

2. The belief that all phenomena are covered by laws of science and that all teleological explanations are therefore without value (Dictionary.com).

See also: → natural; → -ism.

The natural world as it exists without human beings or civilization.

Etymology (EN): M.E. natur(e), from O.Fr. nature from L. natura “the things at the outset, as it was when brought into existence; conditions of birth; essence, natural qualities,” from natus “born,” p.p. of nasci “to be born,” from PIE *gen- “to give birth, beget,” cognate with Pers. zâdan “to give birth,” as below.

Etymology (PE): Zâstâr, literally “birth,” from zâst past stem of [Dehxodâ] zâstan, variant of zâdan “to bring forth, give birth;” (Lâsgardi, Sorxeyi, Aftari) nestor “barren, sterile” (Mid.Pers. zâtan; Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” cf. Skt. janati “begets, bears;”
L. gignere “to beget,” nasci “to be born,” as above, PIE base *gen- “to give birth, beget”) + suffix -âr (forming verbal nouns as in raftâr, kerdâr, goftâr, didâr, jostâr, and so on; or forming accusative nouns, as in gereftâr, koštâr, etc.).

The branch of practical astronomy concerned with the determination of position and direction on sea by observation of celestial objects.

Etymology (EN): Nautical, from M.Fr. nautique, from L. nauticus “pertaining to ships or sailors,” from Gk. nautikos, from nautes “sailor,” from naus “ship,” from PIE *nau- “boat;” cf. Pers. nâv “ship;” O.Pers./Av. *nāv-, O.Pers. nāviyā- “fleet;” Skt. nau-, nava- “ship, boat;” → astronomy.

Etymology (PE): Axtaršenâsi→ astronomy; daryâ-navardik, relating to daryâ-navardi “sea navigation,” from daryâ “sea” (Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- “sea;” cf. Skt. jráyas- “expanse, space, flat surface”)

• navardi, noun of navardidan, navardan “to travel, walk, pass by and over” + -ik, → -ic.

One of the three twilight phases which is the period before sunrise and after sunset when the center of the Sun’s disk is between 6° and 12° below the horizon. This twilight phase is followed or preceded by → civil twilight. See also → astronomical twilight. In clear weather conditions, the horizon is faintly visible during this phase. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. This is why it is called nautical twilight.

See also: → nautical astronomy; → twilight.

One of a set of → differential equations that describes the motion of a → fluid as a function of → pressure, → density, total external force, and → viscosity. See also
→ Euler equation.

See also: Named after Claude-Louis Navier (1785-1836), a French engineer and physicist,
and George Gabriel Stokes, → stokes; → equation.

Same as → nautical astronomy.

Etymology (EN): Navigational, adj. of navigation, from L. navigationem (nom. navigatio), from navigatus, p.p. of navigare “to sail, steer a ship,” from navis “ship,” cognate with Pers. nâv “ship,” as below, + root of agere “to drive,” → act; → astronomy.

Etymology (PE): Axtaršenâsi→ astronomy; nâvrâni “navigation,” from nâv “ship;” O.Pers./Av. *nāv-, O.Pers. nāviyā- “fleet;” cf. Skt. nau-, nava- “ship, boat” + râni verbal noun of rândan “to drive, to cause to go,” causative of raftan “to go, walk, proceed” (present tense stem row-, Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

Tide which occurs during the → first quarter and → third quarter of the → Moon when the pull of the Sun is at → right angles to that of the Moon.

Etymology (EN): Neap, from M.E. neep, from O.E. nepflod “neap tide” + → tide.

Etymology (PE): Kehkešan “small tide,” from keh- “small, little,” → low, + kešand, → tide.

Close; to a point or place not far away.

Etymology (EN): O.E. near “closer, nearer,” comparative of neah, neh “nigh.”

Etymology (PE): Nazdik, from Mid.Pers. nazdik “near,” from nazd “close” (Mid.Pers. nazd, nazdik “near,” nazdist “first;” O.Pers. ašna- “close;” Av. nazdišta- “nearest, next,” nazdyo “nearer to,” nas- “to come near, approach, reach;” cf. Skt. nédīyas- “closer, very close,”
nas- “to approach, to reach”) + -ik, → -ic.

The longest wavelengths of the ultraviolet region, which are adjacent to the visible, from 200 to 350 nm.

See also: → near; → ultraviolet.

An → asteroid whose orbit lies partly between 0.983 and 1.3 → astronomical units from the Sun, so that it passes close to the Earth. Currently thousands of near-Earth asteroids are known, ranging in size up to about 30 km. Among them,
there are between 500 and 1,000 such asteroids larger than one km in diameter. They are divided into three subclasses: → Amor asteroids, → Apollo asteroids, and → Aten asteroids. See also → near-Earth object.

See also: → near; → earth; → asteroid.

An → asteroid, → comet, or large → meteoroid whose orbit brings it exceptionally close to the Earth, and which may therefore pose a collision danger. Most such objects are in orbits around the Sun with → perihelion distance less than 1.3 → astronomical units. See also → near-Earth asteroid.

See also: → near; → earth; → object.

That region of the → electromagnetic spectrum covering shorter infrared wavelengths. It contains the → infrared windows between about 0.8 and 8 → microns, but the longer wavelength limit is not well defined. See also: → infrared radiation, → mid-infrared, → far-infrared, → submillimeter radiation.

See also: → near; → infrared.

1. A cloud of gas and dust in the interstellar space. There are three general types: → emission nebulae,
   → reflection nebulae, and → dark nebulae.
   

2. A celestial body appearing nebulous or fuzzy when seen with the telescope. Formerly, galaxies, which appeared nebular but are constituted of billions of stars, were not distinguished from truly nebular objects, made of gas and dust.

Etymology (EN): From L. nebula “mist,” nimbus “rainstorm, rain cloud;” cognate with Av. napta- “moist,” nabās-câ- “cloud,” nabah- “sky;” Pers. nam “moisture;” cf. Gk. nephos “cloud, mass of clouds,” nephele “cloud;” Skt. nábhas- “moisture, cloud, mist;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” PIE base *nebh- “cloud, vapor, fog, moist, sky.”

Etymology (PE): Miq “nebula” (used by Tusi, in Pers. translation of Sufi’s “Book of Fixed Stars”),
variants meh “fog,” mož, Tabari miyâ, Lori/Laki (kara) mozy, Ossetic mig/megæ,
from Mid.Pers. mēq “cloud, mist;” Av. mēγa- “cloud;” cf. Skt. meghá- “cloud, overcast weather;” Gk. omikhle “mist;” Lith. miglà “mist, haze;” PIE base *mighlā- “cloud.”

Of or relating to or resembling a → nebula.

See also: → nebula + → -ar.

The part of a nebular object’s → spectrum that is created by → free-free emission.

See also: → nebular; → continuum.

The hypothesis first put forward in the 18-th century that the solar system formed from a primeval nebula around the Sun. Same as the → Kant-Laplace hypothesis.

See also: → nebular; → hypothesis.

A → forbidden line that is found in the spectra of → interstellar  → ionized gas. The nebular lines are emitted by several
atomic species (e.g. O, O⁺, O⁺⁺, N⁺, S⁺⁺) and correspond to the → transition from the electronic → metastable state ¹D to the → ground state  ³P. Examples are the doubly ionized oxygen lines [O III] at 4959 and 5007 Å (→ [O III] doublet) and the ionized nitrogen doublet [N II] at 6548 and 6583 Å. See also → auroral line; → transauroral line.

See also: → nebular; → line.

A type of eruptive variable star, mainly young FU Orionis and T Tauri types, associated with nebulosity.

See also: → nebular; → variable.

A hypothetical element, the existence of which was postulated in the nineteenth century to account for unidentified emission lines
(e.g. at 3727 and 5007 Å) in the spectra of some luminous nebulae. It was also believed that this element had a small atomic weight. However, the advances of chemistry and physics showed that all the light elements were known and there was no place for this elusive element. Those unidentified lines have now been shown to come from known elements, but they are not usually observable under laboratory conditions. → forbidden lines.

See also: From nebul(a), → nebula, + -ium L. neuter suffix.

1. A nebulous form, shape, or mass.
   

2. The state or condition of being nebulous.
   

3. A fuzzy celestial object, constituted of gas and dust, generally part of a larger → nebula.

See also: → nebulous; → -ity.

1. Hazy, vague, indistinct, or confused.
   

2. Cloudy or cloudlike.
   

3. Of or resembling a nebula or nebulae; nebular (Dictionary.com).

See also: → nebula; → -ous.

1. Being essential or indispensable.
   

2. Logic, Math.: A condition which must hold for a result to be true, but which does not guarantee it to be true. → if and only if.

Etymology (EN): M.E. necessaire, from L. necessarius “unavoidable,” , from necesse “unavoidable, indispensable,” from ne- “not,” → un-, + cedere “to withdraw, go away, yield,” → precession.

Etymology (PE): Bâyesté, p.p. of bây-, bâyestan “to be necessary,” from Mid.Pers. abây-, abâyistan “to be necessary” (abâyišn “necessity,” abâyišnig “necessary”), from Proto-Ir. *upa-aya- “to reach,” from upa-, → hypo-, + ay- “to go, to come,” → precession.

If event A must occur for event B to occur, then it is said that A is → necessary for B. If event A may cause B but there could be some other cause as well, then it is said that A is sufficient to cause B. See also → if and only if (iff).

See also: → necessary; → and; → sufficient; → condition.

Logic: A → proposition if its → denial is self-contradictory. Also called “logical truth” and “non-contingent truth.”

See also: → necessary; → truth.

1. The fact of being necessary or indispensable.
   

2. Something necessary or indispensable.

See also: → necessary; → -ity.

The part of a person’s or animal’s → body that connecting the → head to the rest of the body.

Etymology (EN): M.E. nekke, from O.E. hnecca, cognate with Du. nek “the nape of the neck;” Ger. Nacken, O.Norse hnakki “the nape of the neck.”

Etymology (PE): Gardan “neck;” related to geri, geribân “collar,” gerivé “low hill,” galu “throat;” Mid.Pers. gartan “neck,” galôg, griv “throat;” Av. grīvā- “neck;” cf. Skt. gala- “throat, neck;” Gk. bora “food;” L. gula “throat” (Fr. gueule “(animal) mouth”), gluttire “to gulp down,” vorare “to devour;” PIE base *g^wer- “to swallow, devour.” L. gula; cf. Mod.Pers. galu “throat.”

A slender pointed piece of metal, usually steel. → magnetic needle.

Etymology (EN): M.E. nedle, O.E. naeðlæ, nedlæ
(cf. O.S. nathla, O.N. nal, O.Fris. nedle, O.H.G. nadala, Ger. Nadel); PIE root *(s)ne- “to sew, to spin;” cf. Skt. snayati “wraps up;” Gk. nein “to spin;” L. nere “to spin.”

Etymology (PE): Suzan, Mid.Pers. sôzan, sucan, related to sok “pointed stick for driving cattle;” Av. sūkā- “needle;” cf. Skt. sūcī- “sting;” L. cuneus “wedge;” PIE base kū- “sharp; spike.”

1. To deny the existence or truth of.
   

2. To cause to be ineffective or invalid.

Etymology (EN): From L. negatus p.p. of negare “to say ’no’, deny,” from Old L. nec “not,” from PIE base *ne- “no, not.”

Etymology (PE): Nâyidan infinitive from nâ “no, not,” variants na, ni, ma- (prohitive); from Mid.Pers. nê, ma “no, not;” O.Pers. naiy, nai “not;” Av. nôit, naē “not;” cf. Skt. ná “not;” cf. L. ne-, in-, un-; Gk. ni; Lith. nè; O.C.S. ne “not;” PIE *ne-, as above.

1. The act of denying; → denial.
   

2. The absence or → opposite of something that is actual, positive, or affirmative.
   

3. A → negative statement, idea, doctrine; a contradiction, refutation, or rebuttal.
   

4. Logic: If p is a → proposition, then the statement “not p,” denoted ¬ p, is the negation or opposite of p. If p is “It is sunny,” then ¬ p is “It is not sunny.” If p is → true, then ¬ p is → false, and vice versa.

See also: Verbal noun of → negate.

1. General: Expressing, containing, or consisting of a negation, refusal, or denial.
   

2. Math.: Of or relating to a quantity less than zero. Of or relating to the sign (-).
   

3. Physics: Of or relating to an electric charge of the same sign as that of an electron.
   

4. Photography: Having dark for light and light for dark.
   

5. Opposite of → positive.
   

See also:
→ negative charge, → negative correlation, → negative crystal, → negative feedback, → negative lens, → negative number, → negative polarization, → negative pressure, → negative skewness.

Etymology (EN): From O.Fr. negatif (fem. negative), from L. negativus, “denying, inhibiting (legal actions); denied/refused; negative,” from negare “to refuse, say ’no’” from Old L. nec “not”, from Italic base *nek- “not,” from PIE base *ne- “no, not.”

Etymology (PE): Nâyidâr, from nâyidan, → negate, on the model of xaridâr, foruxtâr, xâstâr, virâstâr, etc.

An electric charge that has the same sign as the electron.

See also: → negative; → charge.

A correlation between two variables such that as one variable’s values tend to increase, the other variable’s values tend to decrease.

See also: → negative; → correlation.

A uniaxial, birefringent crystal in which the velocity of the extraordinary ray surpasses that of the ordinary ray.

See also: → negative; → crystal.

A → feedback process in which the → output reacts on the → input so as to reduce the initial → effect.

See also: → negative; → feedback.

Same as → divergent lens.

See also: → negative; → lens.

A → real number that is less than zero. A negative number is indicated by the → minus sign.

See also: → negative; → number.

A type of polarization in which the direction of polarization becomes reversed.

See also: → negative; → polarization.

A kind of pressure that contrarily to ordinary pressure pushes inward. In contrast with the → Newtonian mechanics, in → general relativity there are situations in which pressure can be negative. Positive pressure gives rise to attractive gravity, whereas negative pressure creates → repulsive gravity.

See also: → negative; → pressure.

Of a distribution function, a skewness in which the left tail (tail at small end of the distribution) is more pronounced that the right tail (tail at the large end of the distribution). → positive skewness.

See also: → negative; → skewness.

An obsolete term denoting a negatively charged electron, as opposed to a positron.

See also: From → negative + → electron.

1. The area or region around or near some place or thing; vicinity. → solar neighborhood.
   

2. Math.: An → open set that contains a given point.

Etymology (EN): From neighbor, M.E., O.E. neahgebur, from neah→ near + -hood suffix denoting “state or condition of being;” M.E. -hode, -hod;
O.E. -had “condition, position,” cognate with Ger. -heit, Du. -heid.

Etymology (PE): Hamsâyegi, noun from hamsâyé “neighbor,” literally “under, sharing the same shade,” from ham-, → syn-, + sâyé,
→ shadow.

A period of → geologic time within the Cenozoic era, between 23 and 2.6 million years ago, which comprises the Miocene and Pliocene epochs.

See also: From L. neo-, → new, + -gene, → gene.

A colorless, odorless, and tasteless gaseous chemical element; symbol Ne.
Atomic number 10, atomic weight 20.179; melting point -248.67°C; boiling point
-246.048°C. It was discovered from its bright red spectral lines by the Scottish chemist William Ramsay and the English chemist Morris William Travers in 1898 from a liquefied air sample. Neon is produced by carbon burning in massive stars and released into the Galaxy when they explode.

See also: From Gk. neon neuter of neos, → new, so called because it was a newly discovered element.

A → nuclear fusion process that takes place in → massive stars and leads to
the → production of → oxygen and → magnesium. It requires high temperatures and densities (around 1.2 × 10⁹ K and 4 × 10⁹ kg/m³).

See also: → neon; → burning.

The branch of meteorology that deals with clouds.

Etymology (EN): From Gk. nephos “cloud,” nephele “cloud;” cognate with Pers. nam “moisture;” Av. napta- “moist,” nabās-cā- “cloud,” nabah- “sky;” L. nebula “mist,” nimbus “rainstorm, rain cloud;” Skt. nábhas- “moisture, cloud, mist;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” PIE base *nebh- “cloud, vapor, fog, moist, sky”

• → -logy.

Etymology (PE): Abršenâsi, from abr “cloud,” from Mid.Pers. awr, abr (Laki owr, Baluchi haur, Kurd. Soriani hewr);
Av. awra- “rain cloud, rain;” cf. Skt. abhra-“thunder cloud;” Gk. afros “scum, foam;” L. imber “rain;” also Sk. ambha- “water;” Gk. ombros “rain,” PIE *mbhros “rain cloud, rain,” from *mbh- + -šenâsi→ -logy.

The eighth planet from the Sun and the fourth largest by size in the → solar system. The equatorial radius of Neptune is 24,764 km (3.883 Earths), its → semi-major axis is 30.11 → astronomical units (4.50 × 10⁹ km), and its → orbital period is 164.8 yr.

Neptune has at least 14 moons, the largest ones are → Triton, → Proteus, and → Nereid, whereas its smaller moons are: Naiad, Thalassa, Despina, Galatea, Larissa, Halimede, Sao, Laomedeia, Psamathe, and Neso. Neptune has an incredibly thick atmosphere comprised of 74% → hydrogen, 25% → helium, and approximately 1% → methane. Particles of icy methane in its upper atmosphere give Neptune its deep blue color. Large storms whirl through Neptune’s upper atmosphere, and high-speed winds track around the planet at up 600 m/s, fastest recorded in the solar system.

One of the largest storms ever seen was recorded in 1989. Called the → Great Dark Spot, it lasted about five years.

Neptune has a very thin collection of → rings. They are likely made up of ice particles mixed with → dust grains and possibly coated with a carbon-based substance.

See also: Named for the Roman god of the sea Neptune, Gk. Poseidon.

A ductile, silvery radioactive metal, a member of the actinide series; symbol Np. Atomic number 93; atomic weight 237.0482; melting point about 640°C; boiling point 3,902°C (estimated). It was discovered in 1940 by Edwin M. McMillan and Philip H. Abelson, who produced neptunium-239 (half-life 2.3 days) by bombarding uranium with neutrons from a cyclotron at the University of California at Berkeley.

See also: The name derives from the planet → Neptune, since it is the next outer-most planet beyond the planet Uranus in the solar system and this element is the next one beyond uranium in the periodic table.

The outermost satellite of Neptune (radius 150-250 km), discovered on May 1, 1949 by Gerard P. Kuiper. Its period is about 360 days and it has the most eccentric orbit (e = 0.76) of any natural satellite.

See also: Named after the Nereids, the 50 sea-nymph daughters of Nereus, a Gk. sea god.

When a temperature gradient is maintained through a strip of metal in a magnetic field, the direction of flow being across the lines of force, a potential difference will be produced across the conductor.

Etymology (EN): Walter Nernst (1864-1941), German physical chemist; → effect.

Etymology (PE): Oskar, → effect.

The entropy change for chemical reactions involving crystalline solid is zero at the absolute zero of temperature. Also known as the third law of thermodynamics.

See also: → Nernst effect; → heat; → theorem.

The outermost natural satellite of → Neptune, discovered in 2002. Also known as Neptune XIII, it follows a highly inclined and highly eccentric orbit at about 48 million km from Neptune. According to preliminary estimates, Neso is about 60 km in diameter.

See also: In Gk. mythology, one of the Nereids, the fifty daughters of Nereus and Doris.

Math.: Of an ordered collection of terms, having the property that each term is contained in the preceding one. → nested function, → nested multiplication.

Etymology (EN): From nest, from M.E., O.E. nest “bird’s nest;”
cf. M.L.G., M.Du. nest, Ger. Nest, ultimately from PIE *nizdo- (cf. Skt. nidah “resting place, nest,” L. nidus “nest,” O.C.S. gnezdo, O.Ir. net, Breton nez “nest”), probably from → ni- (PIE) + *sed- “sit” (cf. Pers. nešastan “to sit”), → lander.

Etymology (PE): Tu-dar-tu literally “inside in inside,” from tu “inside, in;” dar, → in-.

In computer programing, a function that is defined inside the definition of another function.

See also: → nested; → function.

A method in the evaluation of polynomials which involves fewer basic operations and allows simpler computation, especially for polynomials of high degree. More specifically, the polynomial P(x) = a₀ + a₁x + a₂x² + a₃x³ + … + a_nxⁿ

can be written in the nested form as:

P(x) = a₀ + x(a₁ + x(a₂ + … + x(a_{n - 1} + a_nx) …)). For example, the polynomial P(x) = x³ - 5x² + 10x - 3 has the following nested form:
P(x) = ((x - 5)x + 10)x - 3.

Same as the → Ruffini-Horner method.

See also: → nested; → multiplication.

Any net-like combination of elements in a system; an interconnection of several communicating entities.

Etymology (EN): O.E. net “mesh,” from P.Gmc. *natjan (cf. Du. net, Swed. nät, O.H.G. nezzi, Ger. Netz, Goth. nati “net”), originally “something knotted,” from PIE *ned- “to twist, knot” (cf. L. nodus “knot;” Skt. nahyati “binds, ties”) + → work.

Etymology (PE): Turbast literally “joined, tied by a net,” from tur “net, fishing net, snare,” related to
târ “thread, warp, string,” tâl “thread” (Borujerdi dialect), tân “thread, warp of a web,” from tanidan, tan-
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch”)

• bast “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).

In → iron meteorites, any of very fine parallel lines that cross each other at various angles. They can be seen after cutting diagonally across the sample.

See also: Named after Johann G. Neumann, who discovered them in 1848 in the iron meteorite Braunau, which fell in 1847; → line.

Grammar: Noting or pertaining to a gender that refers to things classed as neither masculine nor feminine.
Biology: Having no organs of reproduction; without sex; asexual. → neutral.

Etymology (EN): From M.E., from M.Fr., from L. neuter, literally “neither one nor the other,” from ne- “not, no” + uter “either of two;” cf. Av. atāra- “this of the two, which of the two;” Gk. poteros; Lith. katras “which of the two,” Russ. kotoryj “which.”

Etymology (PE): Natâr, from negation prefix na-, → non-,

• Mid.Pers. atâr, from Av. atāra- “this of the two,” cognate with L. uter “either of two;” Av. katāra- “which of two; each of two;” Skt. katará- “who or which of two.”

Physics: Of an atom, molecule, collection of particles, having no net charge; not electrified.
Chemistry: Exhibiting neither acid nor alkaline qualities.

Etymology (EN): From → neuter + → -al.

Etymology (PE): → neuter.

An atom in which the number of → protons equals the number of → electrons and therefore has no net → electric charge.

See also: → neutral; → atom.

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

See also: → neutral; → density; → filter.

Same as → neutral density filter.

See also: → neutral; → filter.

A gas which is not ionized.

See also: → neutral; → gas.

Non-ionized → atomic hydrogen gas which constitutes an important component of the → interstellar medium, accounting for perhaps half its mass, even though its density is very low. Its radio emission
→ 21-centimeter line has made it possible to map the distribution of neutral hydrogen in the → spiral arms of our own Galaxy and other nearby galaxies.

See also: → neutral; → hydrogen.

In hydrodynamic instability theory, a wave solution the amplitude of which does not change with time; it neither grows nor decays. Also called neutral wave.

See also: → neutral; → mode.

1. A point where two fields are equal in magnitude and opposite in direction so that the net force exerted on it is zero.
   

2. One of several points in the sky for which the → degree of polarization of diffuse sky light is zero.

See also: → neutral; → point.

Same as → sodium tail.

See also: → neutral; → tail.

Same as → neutral mode.

See also: → neutral; → wave.

A hypothetical particle predicted by supersymmetry theories, which aim at relating bosons to fermions. Under certain assumptions, the lightest such partner particle would be stable, and if it is neutral (a “neutralino”), would make a good dark matter candidate. Reasonable neutralino masses range from 30 GeV to 10 TeV.

Etymology (EN): From → neutral + -ino diminutive suffix.

In optics, the process of combining two lenses having equal and opposite powers to produce a result having no power.

See also: Verbal noun of → neutralize.

To make neutral; cause to undergo neutralization.
To render electrically or magnetically neutral.

See also: Infinitive from → neutral.

An → elementary particle with zero → charge, → spin 1/2, and very small → rest mass. The three types of neutrino (electron neutrino, muon neutrino, tau neutrino) experience only the → weak nuclear force and gravitational force, and pass easily through matter.

The neutrino undergoes a quantum mechanical phenomenon in which → neutrino flavor changes spontaneously to another flavor (→ neutrino oscillation). The neutrino was first postulated by Wolfgang Pauli in 1931 to account for the problem of energy → conservation in → beta decay. It was discovered in 1956.
See also:

→ antineutrino, → atmospheric neutrino, → cosmic neutrino background (CNB), → cosmogenic neutrino, → high-energy neutrino, → low-energy neutrino, → solar neutrino, → solar neutrino problem, → solar neutrino unit (SNU), → sterile neutrino, → ultra-high-energy neutrino.

See also: Neutrino, coined by Enrico Fermi (1901-1954), from neutr(o)→ neuter + -ino diminutive suffix.

Any of the six different varieties of the neutrinos: electron neutrinos, muon neutrinos, tau neutrinos, and their antiparticles.

See also: → neutrino; → flavor.

The transition between neutrino types (→ neutrino flavor) which is a probabilistic consequence of → quantum mechanics. A neutrino, when produced, is in a quantum state which has three different masses. Therefore, an electron neutrino emitted during a reaction can be detected as a muon or tau neutrino. In other words, the flavor eigenstates are different from the propagation eigenstates. This phenomenon was discovered in → solar neutrinos as well as in → atmospheric neutrinos. Neutrino oscillation violates the conservation of the → lepton number; it is possible only if neutrinos have a mass. First predicted by Bruno Pontecorvo in 1957, neutrino oscillation has since been observed by several experiments. It resolved the long-standing → solar neutrino problem. The smaller the mass difference between the flavors, the longer the oscillation period,
so that oscillations would not occur if all of the flavors were equal in mass or were massless. Moreover, the oscillation period increases with neutrino energy.

See also: → neutrino; → oscillation.

An uncharged → subatomic particle found in the nucleus of every → atom heavier than → hydrogen. It has a → rest mass of 1.67492 x 10^-24 g, 939.566 → MeV, slightly greater than that of the → proton. The neutron is composed of three → quarks (two down and one up). Although the neutron is electrically neutral,
it owns a → spin of 1/2 and a → magnetic moment; it can therefore interact magnetically with matter. A free neutron is unstable and disintegrates by → beta decay to a proton, an → electron, and → antineutrino of the electron type: n→ p + e^- + ν^__e + 0.7823 MeV. Its → mean life is about 15 minutes. The decay of the neutron is associated with a → quark transformation in which a down quark is converted to an up by the → weak interaction.

See also: From neutro-, a combining form representing → neutral, + → -on a suffix used in the names of → subatomic particles.

The → nuclear reaction that occurs when an → atomic nucleus captures a → neutron. Neutron capture is the primary mechanism (principally, the → s-process and → r-process)
by which very massive nuclei are formed in stars and during → supernova explosions.
Instead of → fusion of similar nuclei, heavy, → neutron-capture elements are created by the addition of more and more neutrons to existing nuclei.

See also: → neutron; → capture.

The state of degeneracy created when the density of matter is so high that neutrons cannot be packed any more closely together. This condition occurs in the core of stars above 1.44 solar masses (→ Chandrasekhar limit) where under the gravitational collapse electrons and protons are forced to combine into neutrons. Therefore, in a → neutron star all the lowest neutron energy levels are filled and the neutrons are forced into higher and higher energy levels, since according to Pauli Exclusion Principle no two neutrons (fermions) can occupy identical states. This creates an effective pressure which prevents further gravitational collapse. However, for masses greater than 3 solar masses, even neutron degeneracy cannot prevent further collapse and it continues toward the black hole state.

See also: → neutron; → degeneracy.

A type of radioactive decay of atoms containing excess neutrons, in which a neutron is ejected from the nucleus.

See also: → neutron; → emission.

The excess of → neutrons over → protons in an → atomic nucleus:

η = (N_n - N_p) / (N_n + N_p).

See also: → neutron; → excess.

An extremely compact ball of matter created from the central core of a star that has collapsed under gravity to such an extent that it consists almost entirely of → neutrons. Neutron stars result from two possible evolutionary scenarios: 1) The → collapse of a → massive star during a → supernova explosion; and 2) The accumulation of mass by a → white dwarf in a → binary system. The mass of a neutron star is the same as or larger than the → Chandrasekhar limit (1.4 → solar masses). Neutron stars are only about 10 km across and have a density of 10¹⁴ g cm^-3, representing the densest objects having a visible surface. The structure of neutron stars consists of a thin outer crust of about 1 km thickness composed of
→ degenerate electrons and nuclei, which becomes progressively neutron rich with increasing depth and pressure due to → inverse beta decays. In the main body the matter consists of → superfluid neutrons in equilibrium with their decay products, a few percent protons and electrons. Neutron stars have extremely strong magnetic fields, from 3 x 10¹⁰ to 10¹⁵ gauss. As of 2010 more than 2000 neutron stars have been catalogued, which show a large variety of manifestations, mainly → pulsars.

See also: → neutron; → star.

A → binary system composed of two → neutron stars.

See also: → neutron; → star; → binary; → system.

A → nucleosynthesis process responsible for the generation of the → chemical elements heavier than the → iron peak elements. There are two possibilities for → neutron capture: the slow neutron-capture process (the → s-process) and the rapid neutron-capture process (the → r-process).

The s-process is further divided into two categories: the weak s-component and the main s-component. Massive stars are sites of the weak component of s-process nucleosynthesis, which is mainly responsible for the production of lighter neutron-capture elements (e.g. Sr, Y, and Zr). The s-process contribution to heavier neutron-capture elements (heavier than Ba) is due only to the main s-component. The low- to intermediate-mass stars (about 1.3-8 Msun) in the → asymptotic giant branch (AGB) are usually considered to be sites in which the main s-process occur. There is abundant evidence suggesting that → Type II supernova (SNe II) are sites for the synthesis of the r-process nuclei, although this has not yet been fully confirmed. The observations and analysis on → very metal-poor stars imply that the stars with [Fe/H] ≤ -2.5 might form from gas clouds polluted by a few supernovae (SNe). Therefore, the abundances of → heavy elements in → metal-poor stars have been used to learn about the nature of the nucleosynthetic processes in the early Galaxy (See, e.g., H. Li et al., 2013, arXiv:1301.6097).

See also: → neutron;→ capture; → element.

The reaction that transforms a → proton into a → neutron when a proton and an → electron are forced together to make a neutron: p + e^-→ n + ν^__e. In astronomy, this process occurs during the → core collapse of → massive stars which leads to the formation of → neutron stars.

See also: → neutron; → -ize; → -tion.

1. Not ever; at no time.
   

2. Not at all; absolutely not.

Etymology (EN): M.E., from O.E. næfre “never,” compound of ne “not, no,”
→ un- + æfre “ever.”

Etymology (PE): Hargez, variant hagarz; Mid.Pers. hagriz, hakarc “ever, always, never;” O.Pers. hakarnciy “once”; Av. hakərət “once;” cf. Skt. sakrt “once; repeated; ever; never;” Gk. hapax “once;” L. semel “once,” semper “always;” PIE *smkrt.

However, notwithstanding, in spite of, still.

Etymology (EN): M.E. natheles, notheles, natheless, from O.E. neuerþeles, that is → never +
the “in or by that,” “on that account,” “in or by so much,” + less→ -less, cf. Fr. néanmoins = pas moins; Ger. nichtsdestoweniger.

Etymology (PE): Hagarzkam, from hagarz, → never, + kam “less,” → -less.

Of recent origin, production.
Of a kind now existing or appearing for the first time. → new moon; → New General Catalogue (NGC).

Etymology (EN): O.E. neowe, niowe, niwe; cf. Du. nieuw, Ger. neu, Dan., Swed. ny; cognate with Pers. now, as below, L. novus “new, recent, fresh” (Fr. nouveau, neuf), from PIE *neu- “new, young.”

Etymology (PE): Now, from Mid.Pers. nôg “new, fresh;” Av. nauua- “new, fresh;” cf. Skt. náva- “new, fresh, young;” Gk. neos “new, young;” L. novus, as above, cognate with E. new, as above.

A catalogue of 7,840 non-stellar objects compiled by J. L. E. Dreyer and published in 1888. A further 1,529 objects were listed in a supplement that appeared seven years later, called the → Index Catalogue (IC). The Second Index Catalogue of 1908 extended the supplementary list to 5,386 objects.

See also: → new; → general;
→ catalog

A space mission by → NASA whose main goal is to study the → dwarf planet Pluto and it satellites. New Horizons was launched on January 19, 2006; it swung past → Jupiter for a → gravity assist and scientific studies in February 2007, and conducted a six-month-long reconnaissance → flyby study of → Pluto and its moons in summer 2015, culminating with Pluto closest approach on July 14, 2015. It flew 12,500 km above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. Its science payload includes seven instruments: Ralph (visible and infrared imager/spectrometer), Alice (ultraviolet imaging spectrometer), REX (Radio Science EXperiment), LORRI (Long Range Reconnaissance Imager), SWAP (Solar Wind Around Pluto), PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation), and SDC: (Student Dust Counter). As part of an extended mission, New Horizons has maneuvered for a flyby of → Kuiper belt object 2014 MU69, expected to take place on January 1, 2019, when it is 43.4 → astronomical units (AU) from the Sun.

See also: → new; → horizon.

The Moon’s phase when it is at the same celestial longitude as the Sun and thus totally un-illuminated as seen from Earth.

See also: → new; → moon.

The unit of force in the SI system of units. 1 newton (N) is defined as the force required to give a mass of 1 kilogram an acceleration of 1 m s^-2. 1 N = 10⁵  → dynes.

See also: Named after Sir Isaac Newton (1642-1727), the English highly prominent physicist and mathematician.

Same as the → gravitational constant.

See also: → Newton; → constant.

The formula expressing the value of a → definite integral of a given function over an interval as the difference of the values at the end points of the interval of any → antiderivative of the function: ∫f(x)dx = F(b) - F(a), summed from x = a to x = b.

See also: Named after Isaac → Newton and Gottfried Wilhelm Leibniz (1646-1716), who both knew the rule, although it was published later; → formula.

The incompatibility between → Galilean relativity and Mawxell’s theory of → electromagnetism. Maxwell demonstrated that electrical and magnetic fields propagate as waves in space. The propagation speed of these waves in a vacuum is given by the expression c = (ε₀.μ₀)^-0.5, where ε₀ is the electric → permittivity and μ₀ is the magnetic → permeability, both → physical constants. Maxwell noticed that this value corresponds exactly to the → speed of light in vacuum. This implies, however, that the speed of light must also be a universal constant, just as are the electrical and the magnetic field constants! The problem is that → Maxwell’s equations do not relate this velocity to an absolute background and specify no → reference frame against which it is measured. If we accept that the principle of relativity not only applies to mechanics, then it must also be true that Maxwell’s equations apply in any → inertial frame, with the same values for the universal constants. Therefore, the speed of light should be independent of the movement of its source. This, however, contradicts the vector addition of velocities, which is a verified principle within → Newtonian mechanics. Einstein was bold enough to conclude that the principle of Newtonian relativity and Maxwell’s theory of electromagnetism are incompatible! In other words, the → Galilean transformation and the → Newtonian relativity principle based on this transformation were wrong. There exists, therefore, a new relativity principle, → Einsteinian relativity, for both mechanics and electrodynamics that is based on the → Lorentz transformation.

See also: → Newton; → Maxwell; → incompatibility.

A method for finding roots of a → polynomial that makes explicit use of the → derivative of the function. It uses → iteration to continually improve the accuracy of the estimated root.

If f(x) has a → simple root near x_n then a closer estimate to the root is x_n + 1</SUB where x_n + 1</SUB = x_n - f(x_n)/f’(x_n).
The iteration begins with an initial estimate of the root, x₀, and continues to find x₁, x₂, . . . until a suitably accurate estimate of the position of the root is obtained. Also called → Newton’s method.

See also: → Newton found the method in 1671, but it was not actually published until 1736;
Joseph Raphson (1648-1715), English mathematician, independently published the method in 1690.

The arrangement of the seven colors of the rainbow on a disk. When the disk rotates very fast, the eye cannot distinguish between individual colors and the disk is perceived as white. This apparatus demonstrates the discovery made by Newton (Opticks, 1704) that light is composed of seven colors.

See also: → Newton; → color; → wheel.

Same as the → gravitational constant.

See also: → Newton; → constant.

A device consisting of a series of equal → pendulums
in a row used to demonstrate the laws of → conservation of momentum and → conservation of energy.

See also: → Newton; → cradle.

→ Newton’s color wheel.

See also: → Newton; → disk.

In → geometric optics, an expression relating the → focal lengths of an → optical system (f and f’) and the object x and image x’ distances measured from the respective focal points. Thus, ff’ = xx’. Same as Newton’s formula.

See also: → Newton; → equation.

A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

See also: → Newton; → first; → law; → motion.

An approximate empirical relation between the rate of → heat transfer to or from an object and the temperature difference between the object and its surrounding environment. When the temperature difference is not too large: dT/dt = -k(T - T_s), where T is the temperature of the object, T_s is that of its surroundings, t is time, and k is a constant, different for different bodies.

See also: → Newton; → law; → cooling.

The universal law which states that the force of attraction between any two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G (m₁.m₂)/r², where G is the → gravitational constant.

See also: → Newton; → law; → gravitation.

The three fundamental laws which are the basis of → Newtonian mechanics. They were stated in Newton’s Principia (1687). → Newton’s first law, → Newton’s second law , → Newton’s third law.

See also: → Newton; → law; → motion.

Same as the → Newton-Raphson method.

See also: → Newton; → method.

Colored circular → fringes formed when light beams reflected from two polished, adjacent surfaces, placed together with a thin film of air between them, interfere.
→ interference.

See also: → Newton; → ring.

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

See also: → Newton; → second; → law; → motion.

In classical mechanics, an analytical method applied to a material sphere to determine the gravitational field at a point outside or inside the sphere. Newton’s shell theorem states that:

1. The gravitational field outside a uniform spherical shell (i.e. a hollow ball) is the same as if the entire mass of the shell is concentrated at the center of the sphere.

2. The gravitational field inside the spherical shell is zero, regardless of the location within the shell.

3. Inside a solid sphere of constant density, the gravitational force varies linearly with distance from the center, being zero at the center of mass.
   For the relativistic generalization of this theorem,
   see → Birkhoff’s theorem.

See also: → Newton; → shell; → theorem.

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

See also: → Newton; → third; → law; → motion.

Of or pertaining to Sir Isaac Newton or to his theories or discoveries.

See also: Newtonian, from → Newton + -ian a suffix forming adjectives.

A particular solution of the → general relativity when the → gravitational mass is small. The → space-time is then approximated to the → Minkowski’s and this leads to
→ Newtonian mechanics.

See also: → Newtonian; → approximation.

Same as the → gravitational constant.

See also: → Newtonian; → constant; → gravitation.

The use of → Newtonian mechanics to derive homogeneous and isotropic solutions of → Einstein’s field equations, which represent models of expanding Universe. The Newtonian cosmology deviates from the prediction of → general relativity in the general case of anisotropic and inhomogeneous models.

See also: → Newtonian; → cosmology.

Any → fluid with a constant → viscosity at a given temperature regardless of the rate of → shear.

See also: → Newtonian; → fluid.

The focus obtained by diverting the converging light beam of a reflecting telescope to the side of the tube.

See also: → Newtonian; → focus.

The limit attained by → general relativity when velocities are very smaller than the → speed of light or gravitational fields are weak. This limit corresponds to the transition between general relativity and the → Newtonian mechanics. See also → Newtonian approximation.

See also: → Newtonian; → limit.

A system of mechanics based on → Newton’s law of gravitation and its derivatives. Same as → classical mechanics.

See also: → Newtonian; → mechanics.

A potential in a field of force obeying the inverse-square law such as → gravitational potential.

See also: → Newtonian; → potential.

The Newton’s equations of motion, if they hold in any → reference frame,
they are valid also in any other reference frame moving with uniform velocity relative to the first.

See also: → Newtonian; → principle; → relativity.

The laws of physics are unchanged under → Galilean transformation. This implies that no mechanical experiment can detect any intrinsic diff between two → inertial frames. Same as → Galilean relativity.

See also: → Newton; → relativity.

A telescope with a concave paraboloidal objective mirror and a small plane mirror that reflects rays from the primary mirror laterally outside the tube where the image is viewed with an eyepiece.

See also: → Newtonian; → telescope.

Immediately following in time, order, place, and so on.

Etymology (EN): M.E., from O.E. next, nehst, niehsta, nyhsta “nearest, closest,” superlative of neah “nigh” + superlative suffix. Cognate with Du. naast “next,” O.H.G. nahisto “neighbor,” Ger. nächst “next.”

Etymology (PE): Podâ, literally “placed after,” from *upada-, from *upa- “near, next, after,” + *dâ- “to place, put,” → thesis; cf. Baluci pôši “the day after tomorrow,” from pô- contraction of *upa- + *aušah- “dawn,”

• → aurora, + suffix -i; cf. also Baluci godâ (from *uidâ-?) “then, next;” Bašâgardi, Baluci randâ “next, then.”

A → Seyfert galaxy, which is the central, dominant member of the large and relatively nearby → Perseus cluster of galaxies.
A powerful source of X-rays and radio emission, NGC 1275 accretes matter (→ accretion) as
intercluster material falls into it, ultimately feeding a → supermassive black hole (SMBH) at the galaxy’s core. NGC 1275, hosts a narrow-line radio source, Perseus A (3C 84), which interacts with the intracluster gas through its jets and bipolar outflows.

See also: NGC, → New General Catalogue.

A prominent → star cluster, and the ionizing core of giant → H II region → N66 in the → Small Magellanic Cloud galaxy. NGC 346 hosts the largest sample of young, → massive stars in the whole SMC, containing 33 → O-type stars among which 11 are of type O6.5 or earlier. This is young massive star cluster with an estimated age of about 3 million years.

See also: 346, a serial number in the → New General Catalogue.

The most massive and luminous visible → starburst region in the Galaxy. This is our local → giant H II region lying at a distance of about 6-7 kpc in the → Carina arm (→ right ascension = 11h, → declination = -61°). Its central starburst cluster hosts the largest known concentration of extremely young, mostly unevolved → high-mass stars in the Galaxy. With an age of only 1-2 Myr for its most massive stars, NGC 3603 is one of the youngest starburst clusters known.
It has about 40 known → O stars and → W-R stars, producing a → Lyman continuum flux of 10⁵¹ s^-1, about 100 times the ionizing
power of the Orion → Trapezium cluster.
The OB stars contribute to more than 2000 → solar masses to the cluster mass. With a bolometric luminosity over 10⁷→ solar luminosities, NGC 3603 has about 10% of the luminosity of → 30 Doradus and looks in many respects very similar to its core, → R136. A total mass of 7,000 solar masses is measured in the inner 1 pc from the cluster center, whereas the → low-mass stars extend out to at least 5 pc. The mass segregated core of the
cluster, with 10⁵ solar masses per pc³, displays the highest local stellar density outside the Galactic Center region. The spectral
analysis of the W-R like massive component in the cluster core (→ HD 97950) suggests a → metallicity close or equal to solar (See, e.g., Melena et al. 2008, AJ 135, 878, and references therein).

See also: 3603, a serial number in the → New General Catalogue.

A → binary star lying in the core of the Galactic → giant H II region  → NGC 3603. NGC 3603-A1 is double-eclipsing → Wolf-Rayet binary of type → WN6ha with an orbital period of 3.77 days. Their masses have been derived to be

M1 = (116 ± 31) Msun for the primary and M2 = (89 ± 16) Msun for the secondary component of A1. The primary in A1 is the most massive star weighed so far (Schnurr et al., 2008, MNRAS 389, L38).

See also: → NGC 3603.

A galaxy discovered in 1784 by William Herschel that happens to lie nearly along the line of sight to the star → Beta Andromedae. More specifically, it lies at an angular separation of seven arc-minutes. For this reason it is known also as → Mirach’s Ghost. NGC 404 is in fact a → dwarf galaxy lying at a distance of about 10 million → light-years (3.07 ± 0.37 Mpc). NGC 404 harbors a low-luminosity → active galactic nucleus powered by the lowest-mass (< 150,000 Msun) central → massive black hole (Nyland et al., 2017, ApJ 845, 50).

See also: NGC, → New General Catalogue.

A → lenticular galaxy (S0) in → Hydra constellation located about 130 million → light-years (40 ± 8 Mpc, → redshift z = 0.009680) from Earth, discovered by William Herschel in 1789. On 17 August 2017 the → Laser Interferometer Gravitational-Wave Observatory (LIGO) and the → Virgo Interferometer both detected → gravitational waves (→ GW170817) from the collision of two → neutron stars within this galaxy. The → electromagnetic counterpart GRB 170817A was detected 1.7 seconds later. The event was associated with a → kilonova offset 10.6 arcsec (corresponding to 2.0 kpc) to north-east from the nucleus of NGC 4993.

See also: NGC, → New General Catalog

A → lenticular galaxy in the constellation of the → Draco. It lies about 50 million → light-years (15.3 Mpc) distant and has a size of about 60,000 light-years. Due to its thin → rdge-on appearance, it is also called the → Spindle Galaxy. Known also as Messier 102.

See also: 5866, a serial number in the → New General Catalogue.

A → reflection nebula located in the constellation → Cepheus at a distance of 1,300 → light-years. It was discovered by Sir William Herschel on October 18, 1794. The nebula, also known as the → Iris Nebula and → Caldwell 4, is about 6 light-years across.

At the center of NGC 7023 lies an extremely young star named V380 Cep or SAO 19158 (mag. +7.1), which is associated with plenty of local dust.

NGC 7023 is located 6 degrees northwest of the brightest star in Cepheus, Alderamin (α Cep - mag. +2.5) with third magnitude Alfirk (Beta Cephei) positioned about 3.5 degrees to the northeast. Only a degree east-northeast of NGC 7023 is Mira type variable star T Cep, which fluctuates between magnitudes +5.2 and +11.3 over a period of 388.14 days.

See also: 7023, a serial number in the → New General Catalogue.

PIE prefix *ni- “down, below.”

Etymology (EN): E. nether is from this PIE root; M.E. nethere, O.E. neothera, nithera “down, downward, below, beneath” (cf. O.S. nithar, O.N. niðr, O.Fris. nither, Du. neder, Ger. nieder); akin to Pers. ne-, ni-, as below.

Etymology (PE): Mod.Pers. ne-, ni- “down, below” (as in negâh “look, watch,” nešastan “to sit down,” nehoftan “to conceal,” nehâdan “to place, put,” nemudan “to display,” nefrin “curse,” etc.); Mid.Pers. ni-, O.Pers. preposition and verbal prefix ni- “down;” Av. nī- “down, in, into;” cf. Skt. ni- “down,” nitaram “downward;” Gk. neiothen “from below” (from ne-[io]- “below” + -then “from, since;” other usage examples of -then: po-then “from where,” paidio-then “since childhood,” panta-ho-then “from everywhere.”); E. nether, as above.

Metallic chemical element belonging to the iron group; symbol Ni. Atomic number 28; atomic weight 58.69; melting point about 1,453°C; boiling point about 2,732°C. It was discovered by the Swedish metallurgist Axel-Fredrik Cronstedt (1722-1765) in 1751.

See also: Nickel, from shortening of Swedish kopparnickel “copper-colored ore,” from which it was first obtained, a half-translation of Ger. Kupfernickel, literally “copper demon,” from Kupfer “copper” + Nickel “demon, rascal” (from Nikolaus; cf. E. Old Nick “the devil;” the ore so called by miners because it looked like copper but yielded none.

Optical device constructed from a crystal of calcite, used for obtaining plane polarized light.

See also: Named after John Pringle Nicol (1804-1859), British physicist; → prism.

The period between → sunrise and → sunset, especially the hours of darkness.

Etymology (EN): M.E., from O.E. niht (O.H.G. naht, Du., Ger. Nacht, O.N. natt, Goth. nahts), from PIE *nok(w)t- “night;” cf. Gk. nuks; L. nox (Fr. nuit; Sp. noche);
Skt. nákt-; Av. *naxtar- “night,” upa.naxtar- “adjoining the night” (Kurd. Soriani nûtak (?) “sheer darkness”); Lith. naktis; Russ. noch’.

Etymology (PE): Šab, from Mid.Pers. šab, šap “night;” O.Pers. xšap- “night;” Av. xšapan-, xšafn-, xšap- “night;” cf. Skt. ksáp- “night;” PIE base *k(w)sep- “night.”

A specialized technician in an observatory who is in charge of functioning a telescope and helping visiting astronomers during their observation run.

Etymology (EN): From → night + assistant; M.E. assistent, from L. assistent-, stem of assistens, pr.p. of assistere “assist, stand by,” from → ad- “to” + sistere “take a stand, cause to stand,” cognate with Pers. istâdan “to stand,” → histogram.

Etymology (PE): Dastyâr “assistant,” from dast “hand” (Mid.Pers. dast; O.Pers. dasta-; Av. zasta-; cf. Skt. hásta-; Gk. kheir; L. praesto “at hand;” Arm. jern “hand;” Lith. pa-žastis “arm-pit;” PIE *ghes-to-) + yâr “helper; companion” (Mid.Pers. hayyâr “helper,” hayyârêh “help, aid, assistance,” Proto-Iranian *adyāva-bara-, cf. Av. aidū- “helpful, useful”).

An eye disease which is the difficulty in seeing at night or in dim light. Opposite of → hemeralopia. Also called → nyctalopia.

See also: → night; → blindness.

Same as → airglow.

See also: → night; → glow.

A yellow star of visual magnitude 2.84, the second brightest in the constellation → Lepus. It is a giant of → spectral type G5, lying some 159 light-years away. Nihal is double, with a companion, 2.5 seconds of arc apart.

See also: From Ar. an-nihal (النهال) “the thirsty camels, drinking camels,” plural form of an-nâhil (الناهل).

A → cardinal number between → eight and → ten.

Etymology (EN): M.E., from O.E. nigen, nigan, nigon, akin to M.Du. neghen, Du. negen, O.H.G. niun, Ger. neun, Goth. niun “nine,” Pers. noh, as below; from PIE *newn “nine.”

Etymology (PE): Noh, from Mid.Pers. nô; Av. nava; cognate with Skt. nava-; Gk. ennea; L. novem.

The small conical projection in the centre of the areola of each breast, which in women contains the outlet of the milk ducts. Also called: mamilla, papilla or teat (TheFreeDictionary).

Etymology (EN): M.E. nyppell, neple, “teat,” from neble, diminutive of neb “bill, beak,” + -le, → -ule.

Etymology (PE): Mamak, diminutive of mame “breast” in child language, maybe related to mâm, → mother.

Gaseous chemical element; symbol N. Nitrogen is the most abundant constituent of dry air. It comprises 78.09% (by volume). → Atomic number 7; → atomic weight 14.0067; → melting point -209.86°C; → boiling point -195.8°C. Nitrogen is a colorless, odorless, tasteless diatomic gas. It was discovered by the Scottish physician and chemist Daniel Rutherford in 1772.

Etymology (EN): From Fr. nitrogène, coined 1790 by Fr. chemist Jean Antoine Chaptal (1756-1832), from combining form of Gk. nitron “sodium carbonate” + Fr. gène “producing.”

Etymology (PE): Azot, loan from Fr. azote, from Gk. azotos “lifeless,” from negation prefix → a- + zotos “vital.”

Science of → snow.

Etymology (EN): From Fr. nivologie, from niveus “snow; snowy,” → snow, + -logie, → -logy.

Etymology (PE): Barfšenâsi, frp barf, → snow,

• -šenâsi, → -logy.

The second satellite of → Pluto discovered in 2005 by astronomers using the → Hubble Space Telescope images. It has an estimated diameter of between 46 and 137 km and an orbital period of 24.9 days. Also called Pluto II (P2).

See also: Nix, from Nyx in Gk. mythology the goddess of darkness and night. The conversion of “i” into “y” was to avoid conflict with the asteroid 3908 Nyx.

The proposal whereby the → Universe would not have begun with a → singularity. Instead, the → Big Bang would be an ordinary point of → space-time. The proposal, advanced by James Hartle and Stephen Hawking (1983) results from an attempt to combine aspects of → general relativity and → quantum mechanics. Based on an imaginary time assumption, it predicts a closed Universe that would start at a single point, that can be compared to the North Pole of the Earth on a two-dimensional space.
Before the → Planck era there was space, but
the real time began with the Big Bang event. → Hartle-Hawking initial state.

See also: → boundary; → hypothesis.

There are only three parameters that can be applied by an outside observer relating to a → black hole: → mass, → electric charge, and → angular momentum. The collapse of a star into a black hole wipes out all other details of its structure, and the observer can never discover any other properties of the star which formed the black hole. In other words, none of its characteristics leave any trace outside the black hole, and that is what is meant by “hair.”

Etymology (EN): No, M.E., from O.E. na “never, no,” cognate with Pers. na, nâ, → non-; → hair;
→ theorem.

Etymology (PE): Farbin, → theorem; bimuyi, noun from bimu “without hair,” from bi- “without” (→ in-) + mu, → hair.
Kacali “baldness,” from kacal “bald,” also “crooked, bandy-legged,” from kajal, from kaj “crooked, curved, bent” + → -al; probably unrelated to kal “bald,” → colure.

Noachian era = dowrân-e Nuhiyâné     دوران ِ نوحیانه

Fr.: ère noachienne

The oldest geological era on Mars, which started from the planet’s birth and lasted until about 3.8 billion years ago. Features such as dried-up river valleys and delta features suggest that the climate may have been warmer and wetter. It is believed also that lakes and oceans could have existed. → Amazonian era; → Hesperian era.

See also: The oldest geological era on Mars, which started from the planet’s birth and lasted until about 3.8 billion years ago. Features such as dried-up river valleys and delta features suggest that the climate may have been warmer and wetter. It is believed also that lakes and oceans could have existed. → Amazonian era; → Hesperian era.

Noachian, named for → Noachis Terra “Land of Noah;” → era.

An extensive southern terrain of the planet Mars. One of the oldest terrains on Mars, it lies roughly between the latitudes -20° and -80° and longitudes 30° west and 30° east.

Etymology (EN): L. Noachis Terra “Land of Noah.”

Etymology (PE): Zamin “land,” → earth; Nuhiyâné adj. of Nuhi “related to Nuh,” from Nuh (نوح) “Noah.”

A gaseous chemical element that does not readily enter into chemical combination with other elements. Examples are helium, argon, krypton, xenon, and radon. Same as → inert gas.

Etymology (EN): Noble, from O.Fr. noble, from L. nobilis “well-known, famous, of superior birth,” earlier gnobilis, literally “knowable,” from gnoscere “to come to know” (Fr. connaître; Sp. conocer); cognate with Pers. šenâxtan “to know, discern, distinguish, be acquainted with;”
Mid.Pers. šnâxtan, šnâs- “to know, recognize,” dânistan “to know;” O.Pers./Av. xšnā- “to know, learn, come to know, recognize;”
cf. Skt. jñā- “to recognize, know,” jānāti “he knows;” Gk. gignoskein “to know, think, judge;” P.Gmc. *knoeanan; O.E. cnawan; E. know; Rus. znat “to know;” PIE base *gno- “to know;” → gas.

Etymology (PE): Gâz, → gas; huped “noble,” from Mid.Pers. hupid “noble,” from hu- “good, well” (Mid.Pers. hu-; O.Pers. ^hu- “good, well” (ukāra- “having good people”); Av. hu-, hū- “well, good, beautiful” (hu-kərp- “well-shapen,” hūxta- “well spoken,” hu-manah- “good-minded”); cf.
Skt. su- “good” (svasti “well-being, good luck,” sumánas- “good-minded,” sūktá- “well spoken”); L. from Gk. eu “well,” combining form of eus “good” (hu-gies “healthy”); Gallic su-; O.S. su-; Welsh hy-; PIE base *su-) + pid “father,” variant of pidar; Mod.Pers.
pedar “father” (O.Pers. pitā- “father;” Av. patar-; Skt. pitár-; Gk. pater; L. pater, O.H.G. fater).

The end part of an → arrow having a notch for holding the bowstring.

Etymology (EN): M.E. nok(ke); of uncertain origin; cf. Swedish nock “notch;”
also M.L.G. nokk, Dutch nok “tip of a sail.”

Etymology (PE): Sufâr “the notch in the arrow which receives the string,” also “the eye of needle,” related to softan, sombidan “to pierce;” Mid.Pers. sôftan “to pierce, bore;” cf. Pashtu suray, Wakhi sərv “hole, cavity,” Munji surv “hole,” Yidgha surv, Kurd. sontin; Proto-Ir. sup, sub, sump, sumb “to pierce.”

The highest clouds in the Earth’s atmosphere, at altitudes between 75 and 90 km. They resemble thin cirrus clouds with a bluish or silverish color, and are visible only during twilight.

Etymology (EN): From nocti- combining form of L. nox, → night, + L. lucent p.p. of lucere “to shine,” from lux→ light; → cloud.

Etymology (PE): Abr, → cloud, + šabtâb “noctilucent,” from šab, → night, + tâb present stem of tâbidan “to shine,” → luminous.

Relating to or of the nature of a node.

See also: → node; → -al.

The line connecting the ascending and descending nodes of an orbit.

See also: → nodal; → line.

Any of the two points on the → axis of a → lens system, such that if the → incident ray passes through one, travelling in a given direction, the → emergent ray passes the other in a parallel direction.

See also: → nodal; → point.

For a ring inclined to the planet’s equator, the points at which the ring crosses the equator (nodes) slowly move around the planet (regress) in a direction opposite to that of the ring’s orbital motion. (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → nodal; → regression.

1. A point of zero → amplitude in a system of → stationary waves. See also → antinode.
   

2. A → junction point in a network.
   

3. Same as → orbital node; see also → lunar node.
   

4. In → graph theory, same as → vertex.

Etymology (EN): From L. nodus “knot;” cognate with necto “I bind,” Skt. nahyati “binds, ties,” Av. naska-, “bundle,” Old Irish nascim “to bind,” Old Norse knutr, Norwegian knut, Icelandic hnutur, O.E. cnotta, Modern E. knot, O.H.G. knotto, Ger. Knoten; PIE root *ned- “to bind, tie.”

Etymology (PE): Gereh “knot,” from Mid.Pers. grih “knot;” Sogdian γr’nš “knot, bond, joining;” Khotanese grantha- “knot;” cf. Skt. granthi- “knot.”

The time interval between two successive passages of the Moon through its ascending node (27.212220 days, i.e., 27 days 5 hours 5 minutes 35.8 seconds). Nodical month is important for predicting eclipses. Also called → draconic month.

Etymology (EN): Nodical adj. of → node; → month.

Etymology (PE): Mâh, → month; gerehi, adj. of gereh,
→ node; gowzahri→ draconic month.

1. A small node, knot, or knob.
   

   2. A small, rounded mass or lump.

Etymology (EN): From L. nodulus “small knot;” → node, → -ule.

Etymology (PE): Gerhule, from gereh, → node, + -ul, → -ule.

A → symmetry in a physical system leads to a → conserved quantity. For example, symmetry under → translation corresponds to conservation of → momentum, symmetry under → rotation to conservation of → angular momentum, and symmetry in → time to conservation of → energy. The Noether symmetry theorem is a fundamental tool of modern theoretical physics and the calculus of variations,
allowing to derive conserved quantities from the existence of variational symmetries.

See also: Named in honor of the German-American woman mathematician Amalie Emmy Noether (1182-1935), who published the theorem in 1918 (“Invariante Variationsprobleme,” Nachr. D. König. Gesellsch. D. Wiss. Zu Göttingen, Math-phys. Klasse 1918: 235-257).

1. The → random → fluctuations
   that are always associated with a measurement that is repeated many times over. Any unwanted disturbance, random or → systematic, which contaminates the → signal from an object under study. More specifically:
   

2. Electronics: An undesired signal within the useful frequency band.
   

3. Acoustics: Any extraneous sound tending to interfere with the perception of wanted sound.
   

See also:
→ atmospheric noise, → background noise, → cosmic radio noise, → dark current noise, → Galactic radio noise, → Johnson-Nyquist noise, → noise source, → noise temperature, → noise voltage, → noise-equivalent power, → photon noise, → quantum noise, → radio noise, → random noise, → readout noise, → receiver noise, → Schottky noise, → shot noise, → signal-to-noise ratio, → speckle noise, → stationary noise, → system noise, → thermal noise, → white noise.

Etymology (EN): Noise, of obscure origin; it has been related to O.Fr. noise “uproar, brawl,” apparently from L. nausea “disgust, annoyance,” literally “seasickness.” Alternatively the O.Fr. word is traced to L. noxia “hurting, injury, damage.”

Etymology (PE): Nufé “noise,” related to Mod.Pers. noyidan “to cry loud, lament,”
navidan, nâvidan “to lament,” noyé, nôyah “plaint, mown,” navâ “sound, song,” (with prefix *uz-) zenudan, zenav-, zonudan “to wail,” Ossetic niwyn/newun “to howl,” O.Khotanese nuva- “to make a noise,”
Yaqnâvi nuyok “crying, howling,” novva “sound,” Shahmirzâdi nâv- “to cry; cf. Skt. nav- “to sound loudly, roar,” náva- “show of joy or triumph;” L. nuntius “messenger,” adnuntiare “to annoince;” Tocharian AB nu- “to roar, PIE *neu- “to shout”.

An electronic device designed to generate known amounts of radio noise in order to test and calibrate the receivers of radio telescopes.

See also: → noise; → source.

A means for specifying the noise generated as unwanted → electromagnetic radiation
in a receiver system or one of its components. It is usually measured in terms of the equivalent temperature in a → Rayleigh-Jeans spectrum. Noise temperature is used mainly in radio astronomy.

See also: → noise; → temperature.

Fluctuations of electric potential in a physical system due to spontaneous disturbances in the system.

See also: → noise; → voltage.

A measure of the sensitivity of an electronic detector, defined as
the power input to the detector that will create a signal to noise ratio of one for an integration time of half a second.

See also: → noise; → equivalent; → power.

A set or system of names or terms assigned to objects or items in a particular science or art.

Etymology (EN): From Fr. nomenclature, from L. nomenclatura “calling of names,” from nomenclator, variant of nomenculator “one who announces names, namer,” from nomen, → name,

• calator “caller,” from calare “to call.”

Etymology (PE): Nâmgozâri, from nâm, → name, + gozâri, verbal noun of gozâštan “to place, put; to allow, let,” related to gozaštan “to pass, proceed, go over;” Mid.Pers. vitar- “to let pass, lead; to pass;” O.Pers. vitar- “to go across,” viytarrayam “I put across;” Av.
tar- “to cross over,” vī-tərəta- “taken away, isolated;” ultimately Proto-Ir. *ui-tar-.

1. Of, pertaining to, or constituting a name or names, a noun or nouns.
   

2. Describing a value quoted for commodity, expectation, or its current estimate, as opposed to the actual value. → nominal value.
   

3. Within acceptable limits; as expected.
   

4. a) Grammar: Relating to a noun or to any word or word group that functions as a noun.
   b) A word which differs from a noun but functions as one.

Etymology (EN): From L. nominalis “pertaining to a name or names,” from nomen “name,” cognate with Pers. nâm, as below.

Etymology (PE): Nâmenâl, from nâmen, from O.Pers./Av.nâman-, → name, + suffix -al.

An approximate value of the → focal length, used for the classification of lenses, mirrors, or cameras, as opposed to the measured one.

See also: → nominal; → focal; → length.

The assigned, specified, or intended value of a quantity.

See also: → nominal; → value.

1. To propose (someone) for appointment or election to an office; to appoint to a duty or office.
   

   2. To name; designate.

Etymology (EN): From L. nominatus, p.p. of nominare “to name, call by name, give a name to,” also “name for office,” from nomen, → name.

Etymology (PE): Nâmenidan, from nâmen, from O.Pers./Av.nâman-, → name, + infinitive suffix -idan.

Having received a → nomination.

See also: Past participle of → nominate.

An act or instance of nominating, especially to office. The state of being nominated (Dictionary.com).

See also: Verbal noun of → nominate.

1. → nominative case.
   

   2. Similar to such a case in function or meaning.
      

   3. Made out in a person’s name (Dictionary.com).

Etymology (EN): From L. nominativus “belonging to naming,” from nomen, → name.

Etymology (PE): Karini, relating to or denoting karin, → subject.

Grammar: In certain inflected languages (as Sanskrit, Avestan, Greek, Latin, German, and Russian), relating to or denoting a case of nouns, pronouns, and adjectives having as its function the indication of the subject of a verb. Same as subjective case.

See also: → nominative; → case.

One who nominates.

See also: → nominate; → -or.

A person who is nominated as a candidate for a job, position, or award.

Etymology (EN): → nominate + -ee a suffix that adds to transitive verbs to form nouns denoting a person who is the object or beneficiary of the act specified by the verb (addressee; employee; grantee).

Etymology (PE): From nâmen-, present stem of nâmenidan, → nominate, + agent noun suffix -bar (from bordan “carry, bear”), as in payâmbar, râhbar, farmânbar.

A prefix meaning “not.”

Etymology (EN): From O.Fr. non-, from L. non “not,” from Old L. noenum “not one;” in some cases perhaps from M.E. non “not” (adj.), from O.E. nan.

Etymology (PE): Nâ- “no, not,” variants na, ni, ma- (prohibitive) “not;” from
Mid.Pers. nê, ma “no, not;” O.Pers. naiy, nai “not;” Av. nôit, naē “not;” cf. Skt. ná “not;” cf. L. ne-, in-, un-; Gk. ni; Lith. nè; O.C.S. ne “not;” PIE *ne-.

A → transcendental function. Examples are: exponential, logarithmic, and trigonometric functions.

See also: → non-; → algebraic + → function.

→ Dark matter composed of → non-baryonic particles.

See also: → non-; → baryonic; → dark; → matter.

Matter that, unlike the ordinary matter, is not made of baryons (including the neutrons and protons). It is proposed as a possible constituent of dark matter.

See also: → non-; → baryonic matter.

Radiation having waves that are out of phase in space and/or time; radiation which is not → coherent.

See also: → non-; → coherent; → radiation.

Describing a → proposition that is either → true in every possible circumstance or → false in every possible circumstance. A proposition that is not → contingent.

See also: → non-; → contingent.

A medium in which the → phase velocity is independent of frequency.

See also: → non-; → dispersive; → medium.

Any of several geometries which do not follow the postulates and results of Euclidean geometry. For example, in a non-Euclidean geometry through a point several lines can be drawn parallel to another line. Or, the sum of the interior angles of a triangle differs from 180 degrees. According to Einstein’s general relativity theory, gravity distorts space into a non-Euclidean geometry.

See also: → non-; → Euclidean geometry.

A → multiple star system that lacks the characteristics of a → hierarchical multiple system.

See also: → non-; → hierarchical; → multiple; → system.

A → magnetohydrodynamics approach dealing with → plasmas which is an improvement with respect to → ideal magnetohydrodynamics. Non-ideal magnetohydrodynamics allows for a drift between particles, redistributing the → magnetic flux and acting on both the → angular momentum and magnetic flux conservation issues.

See also: → non-→ ideal; → magnetohydrodynamics.

Any frame of reference in which the law of inertia does not apply, such as in accelerating and rotating frames. For example,
the Earth is a non-inertial frame because it is rotating about its axis. But the rotation is so slow that the associated acceleration is negligible compared to other accelerations commonly encountered in everyday life. However, the non-inertial nature of the Earth appears in, e.g., the → Coriolis effect. → inertial reference frame.

See also: → non-; → inertial; → frame.

Not ionized, → neutral.

See also: → non-; → ionized.

A physical condition in which the assumption of the → local thermodynamic equilibrium does not hold.

See also: → non-; → local thermodynamic equilibrium.

Not ordinary matter. Same as → dark matter.

See also: → non-; → luminous; → matter.

A → tumbling motion of an → asteroid. See also → tumbling asteroid.

See also: → non-; → principal axis; → rotational; → motion.

A type of stellar pulsation in which waves run in different directions on and beneath the surface of a star.

See also: → non-; → radial;
→ pulsation.

An process in which an excited state returns to the ground state
without emitting radiation. → radiative process.

See also: → non-; → radiative; → process.

Not concerned with or based on the → theory of relativity. See also:
→ non-relativistic electron, → non-relativistic mechanics, → relativistic.

See also: → non-; → relativistic.

An electron that does not experience relativistic effects since its velocity is very small compared to that of light.

See also: → non-relativistic; → electron.

Mechanics in which the masses under consideration move at speeds much slower than the speed of light.

See also: → non-relativistic; → mechanics.

The nature of a → non-thermal radiation.

See also: → non-; → thermal.

→ non-thermal radiation.

See also: → non-thermal; → emission.

Any of many long and slender structures visible in → radio continuum images of the inner hundred parsecs of the → Galactic Center. NTFs are typically tens of parsecs long and only a fraction of parsec wide. They may occur in isolation or in bundles, such as those comprising the linear portion of the prominent → radio Arc. Their → non-thermal spectrum and strong → linear polarization indicate → synchrotron radiation. The magnetic fields in the NTFs have been estimated from various means. Early estimates centered on the radio Arc, and focused on a comparison between the → magnetic pressure and the estimated → ram pressure from nearby → molecular cloud interactions, indicated magnetic field strengths as high as 1 mG (Morris and Yusef-Zadeh 1985). More recent observations, however, have pointed to significantly weaker magnetic fields among the population of NTFs. Synchrotron models of the radio spectrum imply equipartition magnetic fields between 50-200 μG. Theoretically, it has been challenging to understand the nature of these filaments that resemble extragalactic → radio jets but are not accompanied with any obvious source of acceleration of charged particles to high energy → relativistic energies. Although a number of detailed models have been considered, there is no consensus as to the origin of the
NTFs. These models suggest that molecular and ionized gas clouds, mass-losing stars, → Galactic winds, magnetic activity of the → supermassive black hole at the Galactic center, and → lepton production due to → dark matter annihilation play a role in the processes that lead to the production of the NTFs (Linden et al. 2011, ApJ 741,95, and references therein). In most models, the magnetic field is strong and its global geometry in the central region of the Galaxy is considered to be → poloidal and static. However, some recent models have argued that the magnetic field is local and dynamic.

See also: → non-thermal; → filament.

The electromagnetic radiation whose characteristics do not depend on the temperature of the emitting source. In contrast to → thermal radiation, it has a different spectrum from that of → blackbody radiation. The three common types of non-thermal radiation in astronomy are: → synchrotron radiation, → bremsstrahlung radiation, and → maser  → stimulated emission.

See also: → non-thermal; → radiation.

A radio emission with a negative → spectral index. In this type of emission the intensity of the emitted radiation increases with wavelength.

See also: → non-thermal; → spectrum.

Not equal to zero.

See also: → non-; → zero.

A → polynomial that at least has one non-zero → coefficient. See also → zero polynomial.

See also: → non-zero; → polynomial.

The absorption of a photon and its re-emission at a different frequency (in the observer’s frame of reference) by scattering atoms.

See also: → non-; → coherent scattering.

A plasma in which the → mean free path of the charged particles between two collisions is much larger than the size of the system containing the plasma.

See also: → non-; → collisional; → plasma.

An equation that is independent of the units of measurement as it only
involves nondimensional numbers, parameters, and variables.

See also: → non-; → dimensional; → equation.

Describing something that is partially or totally occupied.

See also: → non; → empty.

A set containing at least → one → element.

See also: → nonempty; → set.

Math.: The fact that no → solution exists for a given → problem. In mathematics, nonexistence usually is a matter of impossibility.

See also: → non-; → existence.

Not → fuzzy. → nonfuzzy set.

See also: → non-; → fuzzy.

A set that obeys the rules of → classical logic, a → crisp set, as contrasted with a → fuzzy set.

See also: → nonfuzzy; → set.

A mechanical system with constraints on their velocity that are not derivable from position constraints. Nonholonomic systems arise, for instance, in mechanical systems that have rolling contact (for example, the rolling of wheels without slipping) or certain kinds of sliding contact (such as the sliding of skates). They are a remarkable generalization of classical Lagrangian and Hamiltonian systems in which one allows position constraints only.

See also: → non-; → holonomic; → system.

Not homogeneous. → nonhomogeneous linear differential equation.

See also: → non-; → homogeneous.

A → linear differential equation if Q(x)≠ 0 on interval I.

See also: → nonhomogeneous; → linear; → differential; → equation.

The quality of an atom that has neither lost nor gained any electrons.

See also: → non-; → ionized.

Not a linear function of the relevant variables.
Containing a variable with an exponent other than one; opposite of → linear.

See also: → non-; → linear.

An electronic device whose output is not directly proportional to its input. For example, in a → diode the current is a nonlinear function of the voltage; its voltage-current characteristics do not obey → Ohm’s law.

See also: → nonlinear; → device.

Same as → chaos.

See also: → nonlinear; → dynamics.

The instability of a physical or mathematical system that arises from the nonlinear nature of relevant variables and their interactions within the system.

See also: → nonlinear; → instability.

A system in which small changes can result in large effects, and large changes in small effects.

See also: → nonlinear; → system.

The property, condition, or state of being → nonlinear.

See also: → non-; → linearity.

A star that is never seen above the horizon from a given position. These stars are located between the celestial pole and a diurnal circle with an angular distance larger than the altitude of the pole.

Etymology (EN): Nonrising, from → non- + rising adj. of → rise; → star.

Etymology (PE): Setâré, → star; hamiše penhân, literally “always hidden,” coined by Biruni (A.D. 973-1050) in his at-Tafhim, from hamišé “always” (Mid.Pers. hamêšag “always”) + penhân “hidden.”

Word or language having no meaning. → absurd.

Etymology (EN): → non- + → sense.

Etymology (PE): Jafang “nonsense, futile.”

A star that is always seen above the horizon from a given position. These stars are located between the celestial pole and a diurnal circle with an angular distance smaller than the altitude of the pole. Same as → circumpolar star.

Etymology (EN): Nonsetting, from → non- + setting adj. of → set; → star.

Etymology (PE): Setâré, → star; hamiše peydâ literally “always visible,” coined by Biruni (A.D. 973-1050) in his at-Tafhim, from hamišé “always,” → perpetual,

• peydâ, → visible.

A → square matrix that is not a → singular matrix.

See also: → non-; → singular; → matrix.

See also → non-thermal.

See also: → non-; → thermal.

Same as → suprathermal escape.

See also: → non-; → thermal; → escape.

Fluid mechanics: Having no → viscosity. Same as → inviscid.

See also: From → non- + → viscous.

The time of day when the Sun crosses the observer’s meridian and is at its highest point above the horizon. At this point, the Sun lies due south of an observer in the northern hemisphere and due north of an observer in the southern hemisphere.

Etymology (EN): M.E. none; O.E. non, from L. nona hora “ninth hour” of daylight by Roman reckoning, about 3 p.m.

Etymology (PE): Nimruz, from nim “mid-, half” (Mid.Pers. nêm, nêmag “half;” Av. naēma- “half;” cf. Sk. néma- “half”) + ruz→ day.

General: A standard, model, or pattern regarded as typical.
Math.:

1. The magnitude of a vector expressed as the square root of the sum of the squares of the absolute values of the components of the vector.
   
2. The value that has the highest frequency within a statistical range.

Etymology (EN): From Fr. norme, from O.Fr., from L. norma “carpenter’s square, rule, pattern,” of unknown origin.

Etymology (PE): Hanjâr “a straight road; way, rule, law; habit, custom; conduct; a mason’s rule, a plumb-line, a level;” Mid.Pers. hanjâr “right, correct;” from Proto-Iranian *ham-cara-, *han-cara- prefixed *cara- “to move, walk” (cf. Av. car- “to move, go, walk,” carāni “I would go,” carāt “he would go;” Mod.Pers. caridan “to graze,” gozârdan “to explain,” gozâreš “explanation”); cf. Skt. samcara- “passage, way, road, path; going about, moving,” from prefix sam- + cara- “moving, going, walking;” Gk. pelomai “to move;” L. colere “to till, cultivate, inhabit.”

The Carpenter’s Square. A small and inconspicuous southern constellation which lies between → Scorpius and → Centaurus. Its brightest star is only of magnitude 4.0. Abbreviation: Nor; genitive: Normae

Etymology (EN): Initially Norma et Regula, L. translation of l’Équerre et la Règle “the Set Square and the Ruler,” as named by Abbé Nicolas Louis de Lacaille (1713-1762).

Etymology (PE): Guniyâ “carpenter’s square,” probably related to konj “angle, corner, confined place” (variants xong “corner, angle,” Tabari kânj, Kurd. kunj, Hamadâni kom) and zânu “knee” (Av. žnu-),
Skt. kona- “angle, corner,” Gk. gonia “angle,”, gony “knee,” L. genu “knee,” cuneus “a wedge,” Albanian (Gheg dialect) kân “angle, corner,” Albanian (Toks) kënd “angle, corner;” PIE base PIE base *g(e)neu-.

1. Conforming to the usual standard, type, custom; not abnormal; regular; natural.
   2a) Math.: A perpendicular, particularly a line perpendicular to the tangent of a curve or curved surface at the point of contact of the tangent with the curve or the curved surface. See also → perpendicular; → vertical.
   2b) Optics: A line drawn perpendicular to the surface of a reflecting or refracting medium.

Etymology (EN): From L.L. normalis “standing at right angle, in conformity with rule,” from L. normalis “made according to a carpenter’s square,” from norma “rule, pattern,” literally “carpenter’s square.”

Etymology (PE): Hanjârvar, hanjârmand, adjectives of hanjâr, → norm.

The dispersion in which a shorter wavelength is associated with a higher → refractive index. Contrasted with the → anomalous dispersion.

See also: → normal; → dispersion.

A theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve with a mean at the center of the curve and tail widths proportional to the standard deviation of the data about the mean. Same as → Gaussian distribution.

See also: → normal; → distribution.

Of an atom, the same as → ground state.

See also: → normal; → state.

General: The fact or condition of being normal.
Chemistry: Measure of the number of gram-equivalent weights of a compound per liter of solution.

See also: From → normal + → -ity.

1. A mathematical technique for adjusting a series of values (typically representing a set of measurements) according to some transformation function in order to make them comparable with some specific point of reference.
   
2. Process of introducing a numerical factor into an equation so that the area under the corresponding graph (if finite) shall be made equal to unity.
   
3. To change in scale so that the sum of squares or the integral of squares of the transformed quantity is zero.

See also: Verbal noun of → normalize.

To change in scale so that the sum of squares, or the integral of the square, of the transformed quantity is unity.

See also: → normal + → -ize.

The quality of something that has undergone → normalization.

See also: P.p. of → normalize.

A dimensionless parameter expressed by h(z) = H(z)/H₀, where H(z) is the → Hubble parameter at → redshift z and H₀ is the → Hubble constant.

See also: → normalized; → Hubble; → parameter.

The → cardinal point at which the → meridian cuts the → horizon below the → north celestial pole.

Etymology (EN): M.E., O.E. norð, from P.Gmc. *nurtha- (cf. O.N. norðr, M.Du. nort, Du. noord, Ger. nord), ultimately from PIE *ner- “left, below.”