An Etymological Dictionary of Astronomy and Astrophysics

The system of → SI units.

See also: → SI units.

A coherent and rationalized system of units, in common use in physics since 1969. The seven basic units are the → meter, the → kilogram, the → second, the → ampere, the → kelvin, the → mole, and the → candela. Same as international system of units.

See also: SI, from Fr. Système Internationale d’unités; → unit.

A person’s brother or sister.

Etymology (EN): M.E., from O.E. sibling “relative, kinsman,” from sibb “kinship, relationship; love, friendship,” cf. O.Saxon sibba, M.Du. sibbe, O.H.G. sippa, Ger. Sippe “kin, related,” Icelandic sifi “related.”

Etymology (PE): Barxâ, from bar- first syllable of barâdar, → brother, + xâ-, short for xâhar (x^wâhar), → sister.
Hamšir, literally “of the same milk,” more specifically “sucking the same breast.” However, a hamšir may not be a person’s brother or sister.

1. An implement for cutting grain, grass, etc., consisting of a curved, hooklike blade mounted in a short handle.
   

   2. → Sickle of Leo.

Etymology (EN): M.E. sikel, O.E. sicol; cf. Du. sikkel, Ger. Sichel, ultimately from L. secula “sickle,” from PIE root *sek- “to cut,” → section.

Etymology (PE): Das “sickle,” Mid.Pers. dâs “sickle;” variant dahre, → scythe; Wakhi δətr, Parachi dêš “sickle;” ultimately from Proto-Iranian *dāθra-; cf. Skt. dātra- “sickle, scythe.”

A sickle-shaped → asterism forming the back of the Lion’s head. It is marked by six stars: ε Leonis, μ Leonis, ζ Leonis, γ Leonis, η Leonis, and α Leonis (→ Regulus).

See also: → sickle; → Leo.

1. A line segment that forms part of the perimeter of a plane geometric figure.
   

2. A surface of a solid geometric figure.

Etymology (EN): M.E., from O.E. side “flanks of a person, the long part of anything;” cf. O.S. sida, O.N. sitha, M.Du. side, O.H.G. sita, Ger. Seite.

Etymology (PE): Bar “side; breadth; breast,” variant var; Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s “wide, broad.”
Pahlu “side, flank;” Mid.Pers. pahlug “side, rib,” Av. pərəsu- “rib,” Ossetic fars “side, flank,” cf. Skt. párśu- “rib,” O.C.S. prbsi “breast,” Lit. piršys (pl.) “horse breast.”
Kenâr “side,” variant karân “edge, limit;” Mid.Pers. karân, karânak, kenâr “edge, limit, boundary;” Av. karana- “side, boundary, end.”
→ octagon, → polygon, → quadrilateral, → triangle.

1. In → radio astronomy, smaller amounts of gain found either side of the main lobe.
   

2. In transmitting antennae, radiation in any direction other than that required.

See also: → side; → lobe.

Either of the two bands of frequencies, one just above (upper side) and one just below (lower side) a carrier frequency, that result from modulation of a carrier wave.

Etymology (EN): → side; → band.

Etymology (PE): Bând, → band; kenâr “side,” variant karân “edge, limit;” Mid.Pers. karân, karânak, kenâr “edge, limit, boundary;” Av. karana- “side, boundary, end.”

Of, relating to, or determined by or from stars.

Etymology (EN): From M.Fr. sidereal, from L. sidereus “starry, astral,” from sidus (genitive sideris) “star, constellation.”

Etymology (PE): Axtari, pertaining to axtar “star,” probably a variant of setâré→ star.

The period of → Earth’s rotation around its axis, the mean value of which is about 23h 56m 04.092s (23.934 470 hours). In other words, the interval between two successive passages of a star across a given → meridian. The sidereal day is 3 minutes 56 seconds shorter than the → mean solar day. The reason is that Earth moves a little less than a degree around the Sun during the time it takes for one full axial rotation. So, for the Sun to appear on the same meridian in the sky again after one full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is also why the stars rise and set about 4 min earlier each day.

See also: → sidereal; → day.

The angle on the celestial sphere measured westward from the hour circle of the vernal equinox to that of the celestial body.

See also: → sidereal; → hour angle.

The time taken by the Moon to complete one → revolution around the Earth with respect to a background, → fixed star. The average time is 27.321 661 days (27d 7h 43m 11.5s). Same as → lunar sidereal orbital period.

See also: → sidereal; → month.

→ sidereal revolution period; → sidereal rotation period.

See also: → sidereal; → period.

The time taken by a planet or satellite to complete one revolution about its primary with respect to stars. For Earth, same as → sidereal year. Sidereal periods of the solar system planets, interms of the sidereal year, are as follows: Mercury 0.240846 (87.9691 days); Venus 0.615 (225 days); Earth 1 (365.25636 solar days); Mars 1.881; Jupiter 11.86; Satrurn 29.46; Uranus 84.01; and Neptune 164.8. That of the Moon is 0.0748 (27.32 days) and for → Sedna 12050.

See also: → sidereal; → revolution; → period.

The rotation period of a celestial body with respect to fixed stars. For Earth, same as → sidereal day.

See also: → sidereal; → rotation; → period.

The time based upon → Earth’s rotation with respect to the stars, with the → sidereal day as the unit of measurement. At the moment when the → vernal equinox crosses the → meridian in → upper culmination, sidereal time is equal to zero hours for that observing position. The → hour angle of the vernal equinox is equal to sidereal time.

See also: → sidereal; → time.

The interval between two successive passages of the Sun, in its apparent → annual motion around the → celestial sphere, through a particular point relative to stars. It is equal to 365.256356 days for the J2000.0 epoch
and is 20m 24.5s longer than the → tropical year.

See also: → sidereal; → year.

1. A mineral composed of iron carbonate, FeCO₃, that is a valuable ore of → iron.
   
2. A category of → meteorites consisting mainly of → iron and → nickel, commonly referred to as Irons. There are 13 different chemical groups of siderites. These groups are defined based
   on the relative abundance of nickel, gallium, germanium, and iridium. Irons are also categorized by their structural type.

See also: → sidero-; → -ite.

A combining form meaning “iron,” used in the formation of compound words; variant sider- before a vowel.

Etymology (EN): From Gk. sideros “iron.”

Etymology (PE): Âhan-, from âhan, → iron.
Sidero-, loan from Gk., as above.

The category of → meteorites commonly referred to as → stony-irons. The three classes of siderolites are → lodranites, → mesosiderites, and → pallasites.

See also: From Gk. sidero-, from sideros “iron” + -lite a combining form used in the names of minerals.

→ siderophile element.

See also: → sidero-; → -phile.

In the → Goldschmidt classification, any → chemical element that has an → affinity
to combine with → iron rather than some other element. These elements are concentrated in the → Earth’s core. The group includes → iron (Fe), → nickle (Ni), → cobalt (Co), → platinum (Pt), → gold (Au), → tin (Zn), and → tantalum (Ta). The siderophile elements include → highly siderophile element (HSE).

See also: → siderophile; → element.

A mirror arrangement with clock drive that reflects light from a celestial body to a fixed position. → coelostat; → heliostat.

Etymology (EN): From L. sidero-, from sidus (genitive sideris) “star, constellation”

• -stat prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Axtardâštâr, from axtar, → star, + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess;” Mid.Pers. dâštan; O.Pers./Av. dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support.”

The SI unit of electrical conductance, equal to the reciprocal of the ohm and replacing the equivalent MKS unit.

See also: Named for the German electrical engineer Werner von Siemens (1816-1892).

A classical method of finding all → prime numbers up to any given number n.
The method consists of listing all positive integers from 2 up to the given number and then deleting some of them sequentially in steps. For example, if we wish to find the primes less than or equal to 50, we proceed as follows. All integers from 2 to 50 are first written. The integers that are divisible by 2, other than 2, are crossed out from the list. Since 3 is the first integer greater than 2 that is not removed, all the integers divisible by 3, other than 3, are crossed out. We do the same with 5 and then 7. Since all composite integers ≤ 50 are divisible by 2, 3, 5, or 7 (i.e. ≤ √50), all the remaining integers not deleted are prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, and 47.

Etymology (EN): M.E. sive, O.E. sife “sieve;” cf. M.Du. seve, Du. zeef, O.H.G. sib, Ger. Sieb, of unknown origin. Related to sift.

Etymology (PE): Qarbâl (variants qarbil, qarbir, qelber, qalbur, gerbâl), probably related to the PIE base *krei- “to sieve, separate;” cf. Gk. krinein “to separate, decide, judge,” krisis “decision;” L. cribrum “sieve” (Fr. crible), cernere “to sift, separate;” O.E. hriddel “sieve;” O.Ir. criathar; O.Welsh cruitr “sieve.” Pers. qarbâl loaned in Ar. as gharbala “to sift,” itself loaned in It. garbellare; M.Fr. garbeler “to sift;” E. garble “to sift impurities from.”

The SI unit for the dose equivalent of ionizing radiation. One sievert represents a dosage of 1 → joule per kg of tissue, absorbed from the ionizing radiation. The effect of radiation depends on its total amount of energy, the type of radiation, and the energy levels of particular particles. The dose equivalent in sieverts of radiation is the product of the absorbed dose in → gray (Gy)s and a dimensionless numeric factor, called the quality factor or relative biological effectiveness, dependent on the type of radiation. The sievert has replaced the → rem as the unit of dose equivalent. 1 rem = 10^-2 Sv.

See also: The unit honors the Swedish physicist Rolf Sievert (1898-1966), who worked over many years to measure and standardize the radiation doses used in cancer treatment.

The act or fact of seeing; field of vision. → line of sight.

Etymology (EN): M.E., from O.E. gesiht, gesihð “thing seen;” cf. Dan. sigte, Swed. sigt, M.Du. sicht, Du. zicht, O.H.G. siht, Ger. Sicht, Gesicht, related to → sign.

Etymology (PE): Did “sight, vision,” past stem of didan “to see” (Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf.
Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”).

→ line of sight.

See also: → sight; → line.

1. A conventional or arbitrary mark, figure, or symbol used as an abbreviation for the word or words it represents. Related terms: → signal, → signature, → symbol, → index.
   

2. Math.: A plus sign or minus sign used as a symbol for indicating addition or subtraction; also a sign for indicating the positive or negative value of a quantity.
   

3. → sign of the zodiac.

Etymology (EN): M.E., from O.Fr. signe “sign, mark, signature,” from L. signum “mark, token, indication, symbol,” from PIE base *sek^w- “to see.”

Etymology (PE): Nešân “sign, mark,” from Mid.Pers. nišân “sign, mark, omen,” nišitan “to gaze, stare, look into,” niš in marv.niš “taking omen, foretelling, or divining from birds,” nišak “clear, evident,” from Proto-Iranian *niiaš- “to show,” from *ni- “down, below,” → ni- (PIE), + *iaš- “to show, appear;” cf. Armenian loanword niš “sign, mark, spot,” loaned also in Syriac as nyš “sign” (Cheung 2007).

One of the 12 constellations (in fact 13) making up the → zodiac.

Etymology (EN): → sign; → zodiac.

Etymology (PE): Borj originally “tower,” most probably
related to Pers. borz “height, magnitude, greatness,” boland “high,” bâlâ “up, above, high, elevated, height,” Laki dialect berg “hill, mountain;” Mid.Pers. burz “height,” buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. and E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg; PIE base *bhergh- “high.” Borj may have been loaned into Ar. from Mid.Pers. The meaning extension of borj to its astronomical sense of zodiacal sign may have arisen from the conception of the zodiac as a barrier between heaven and Earth through which access was gained by means of twelve gates.

1. Useful part of the received information that can be distinguished from noise.
   

2. Information transmitted by means of a modulated current or an electromagnetic wave and received by telephone, radio, television, etc.

Etymology (EN): M.E., from O.Fr. signal, from M.L. signale “a signal,” from L.L. signalis (adj.) “used as a signal, pertaining to a sign,” from L. signum “signal, sign.”

Etymology (PE): Nešâl, from neš- base of nešân meaning → sign + -âl, → -al.

Concept used to quantify the effects of noise. It is the ratio of a signal to the standard deviation of the signal.

See also: → signal; → noise; → ratio.

1. Physics, Chem.: A distinctive trace or sign that indicates the presence of a substance or the occurrence of a physical process or event.
   

2. Telecommunication: The complete set of electromagnetic and/or acoustic signals received, e.g., from an infrared source, a radio or radar transmitter, an aircraft, or a ship. Signatures may consist of analog or digital signals, or both, and may be analyzed to indicate the nature of their source and assist in its recognition.

Etymology (EN): M.Fr. signature, from M.L. signatura “sign,” in classical L. “the matrix of a seal,” from signatus, p.p. of signare “to mark, sign,” → sign.

Etymology (PE): Nešânzad, literally “striking a mark, sign” (cf. angošt zadan “fingerprint”), from nešân, → sign, + zad past stem of zadan “to strike, beat; to do; to play an instrument” (Mid.Pers. zatan, žatan; O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”); cf. Skt. han- “to strike, beat” (hantar- “smiter, killer”); Gk. theinein “to strike;” L. fendere “to strike, push;” Gmc. *gundjo “war, battle;” PIE *g^when- “to strike, kill”).

1. The state or quality of being significant; → meaning; importance.
   

2. Statistics: An estimated measure of the degree to which a result is not merely a matter of chance. See also → significance level, → significance testing.

Etymology (EN): From L. significantia “meaning, force, energy,” from significans, present participle of significare, → signify.

Etymology (PE): Nešânâri, noun from nešânâr, → significant.

The level of → Type I error which one is willing to risk in a → test of significance. Also called level of significance.

See also: → significance; → level.

Statistics: A procedure that is used to decide whether to accept or reject the → null hypothesis or to determine whether observed samples differ significantly from expected results. Also called → test of significance and → rule of decision.

See also: → significance; → test.

1. Having a special or suggestive → meaning.
   

2. Statistics: Of or pertaining to a result that is not plausible to occur by chance.

Etymology (EN): From L. significant-, stem of significans, from significare, “to → signify.”

Etymology (PE): Nešânâr, from nešân, → sign, + âr short form of âvar present stem of âvardan “to bring, adduce, cause, produce.”

The number of digits used to express a measured or calculated quantity. Rules for deciding the number of significant digits:

The leftmost digit which is not a zero is the most significant digit.

If the number does not have a decimal point, the rightmost digit which is not a zero is the least significant digit.

If the number does have a decimal point, the rightmost significant digit is the least significant digit, even if it’s a zero.

Every digit between the least and most significant digits should be counted as a significant digit.

See also: → significant; → digit.

1. Something that is signified, → meaning, sense.
   

2. The act of signifying. → significance, → significant, → level of significance.

See also: Verbal noun of → signify.

To make known; to be a sign of; to → mean. → significance, → significant, → signification.

Etymology (EN): M.E. signifien, from O.Fr. signifier, from L. significare
“to make a sign, indicate, mention, denote.”

Etymology (PE): Nešânâridan, from nešânâr, → significant,

• -dan suffix of infinitives.

1. Absence of sound or noise.
   

2. The state or fact of being silent.

See also: Noun from → silent.

1. Making no sound; quiet; still.
   

2. Refraining from speech.

Etymology (EN): From L. silentem (nominative silens) “still, calm, quiet,” p.p. of silere “to be quiet or still,” of unknown origin.

Etymology (PE): Bideng, from Kurd. (Kurmanji) bêdeng, bêdêng, Kurd. (Sorani) bedang, from bi- “without,” → a-, + deng “sound, voice,” variants dong, dang, bâng,
→ Big Bang.

A type of heat-resistant glass, especially in the form of flint or → quartz.

Etymology (EN): L. silex “hard stone, flint.” Âtašzané “that used for kindling, ignitor,” from âtaš, → fire, + zadan “to strike,” → beat + -é instrument suffix. Caxmâq loan from Turkish.

A dark disk seen in absorption against the bright nebular background. → proplyd.

Etymology (EN): From Fr. silhouette, in allusion to Étienne de Silhouette (1709-67), Fr. controller general of finances in 1759; perhaps from his ephemeral tenure; → disk.

Etymology (PE): Gerdé, → disk; siyah-rox, from siyah, → black, + rox, variant of
ru(y) “face, surface; aspect; appearance,” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”).

A widely found mineral of silicon dioxide SiO₂, which is used as an ore of → silicon. It is
a hard, white, or colorless crystalline compound with a high melting point, occurring abundantly as quartz, sand, flint, rock crystal, etc.

See also: From L. silic- stem of → silex “hard stone, flint, boulder.”

A common rock-forming → mineral belonging to a group formed from → silicon and → oxygen combined with various elements and classified by their crystalline structures. SiO₄ in the shape of a tetrahedron is the main component of the group. It has been estimated that about 95% of the Earth’s crust is made up of silicate minerals. A salt of → silicic acid.

See also: From → silica + → -ate.

1. Containing → silicon.
   

2. Of or pertaining to → silica or acids derived from it.

See also: From silic-, from → silic + → -ic.

A general name for a family of chemical compounds containing the element silicon attached to oxide and hydroxyl groups.

See also: → silicic; → acid.

A non-metallic chemical element; symbol Si. Atomic number 14; atomic weight 28.0855; melting point 1,410°C; boiling point 2,355°C; specific gravity 2.33 at 25°C. It occurs in a combined state in minerals and rocks and constituting more than one fourth of the earth’s crust.

Etymology (EN): From silic(a), from L. silex (genitive silicis) “flint, pebble” + -on, as in carbon and boron.

Etymology (PE): Silisiom, from Fr. silicium, from silic(a), as above,

• -ium as in magnesium, barium.

The → nucleosynthetic process taking place in the interior of → massive stars whereby → silicon is transmuted into iron, nickel, and neighboring nuclei collectively called the → iron peak elements.

See also: → silicon; → burning.

The soft, lustrous fiber obtained as a filament from the cocoon of the silkworm (Dictionary.com).

Etymology (EN): M.E., from O.E. seolc, sioluc, from L. sericum “silk garment, silk,” from neuter of sericus “silken,” from Gk. serikos, from Seres, an eastern Asian people, probably the Chinese (cf. Chinese si “silk”, Manchurian sirghe, Mongolian sirkek).

Etymology (PE): Abrišam “silk,” from Mid.Pers. abrešom “silk,” ultimately from Proto-Ir. *au-uris-, from *uris- “to turn, spin;” cf.
rešté “thread, line, file,” reštan, ristan,
“to spin;” Mid.Pers. ‘rws- “to turn to;” Av. uruuaēs- “to twist, turn.”

The smoothing of primordial → density fluctuations at high frequencies caused by photon → diffusion. Before the → decoupling era, photons and → baryons were tightly coupled to each other by
→ Compton scattering. However, the transition to a transparent → Universe was not instantaneous. As the → opacity of the Universe dropped, the photons started diffusing away from the positions they had while opaque, hence undergoing a → random walk. Since the → acoustic waves in the decoupling era were driven by photon pressure, the photon diffusion also led to damping of the → baryon acoustic oscillations. Silk damping suppresses all perturbations with masses smaller than about 10¹³ Msun. The implication for a theory of → structure formation is that individual galaxies must have formed in a → top-down structure formation mechanism, i.e. by the fragmentation of larger objects. However, in theories of structure formation which include → non-baryonic dark matter galaxies can form from smaller objects in a → bottom-up scenario.

See also: Joseph Silk (1942-); → damping.

A metallic → chemical element;
symbol Ag (L. argentum). → Atomic number 47; → atomic weight 107.8682; → melting point 961.93°C; → boiling point 2,212°C; → specific gravity 10.5 at 20°C. Pure silver is nearly white, lustrous, soft, very ductile, malleable, and an excellent conductor of heat and electricity.

Silver is generated in stars through a special rapid process of → neutron captures. This process defers from the main → r-process, the main and weak → s-process, and charged particle freeze-outs. In other words, during a → supernova explosion, silver is formed in a different → fusion process from the one that forms → gold (Hansen et al., 2012, A&A 545, A31).

Etymology (EN): M.E. silver(e), selver(e), selfer; O.E. seolfor “silver;” cf. O.S. silvbar, O.N. silfr, M.Du. silver, Du. zilver, O.H.G. sillabar, Ger. silber, Goth. silubr, akin to Serbo-Croatian srebro, Rus. serebo, Lith. sidabras “silver.”

L. argentum “silver, white money,” from PIE *arg-ent-, from PIE *arg- “to be bright, shine;” cf. Av. ərəzata- “silver,” auruša- “white” (Mid.Pers. arus “white, bright”); O.Pers. ardata- “silver;” Mod.Pers. arziz “silvery metal tin;” Skt. arjuna- “white, shining,” rajata- “silver;” Gk. argos “white,” arguron “silver;” Arm. arcat; Hittite harki- “white.”

Etymology (PE): Noqré from Sogdian nâkartak “uncoined (silver),” literally “undone,” from nâ- negation prefix “not, no” (Mid.Pers. ne, O.Pers. naiy, Av. na-, (particle of negation noit), Skt. na-, (ned), Gk. né- “not,” L. ne-, in-, un-, PIE *ne-) + kartak “done,” p.p. of kardan “to do, to make;”
kâr “work,” variant kar
(Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”), Sogdian nâktênê “of silver.” Note that another word in Pers. for silver is sim, which has a semantic form comparable to that of noqré, since it derives from Mid.Pers. asêm, from Gk. asemon “without mark, uncoined, shapeless, formless,” from argurion asemon “uncoined money.”

A large on-line astronomical database, developed at the Centre de Données astronomiques de Strasbourg, → CDS in
France. It provides a large collection of astronomical data, including cross-identifications, bibliography, and measurements for astronomical objects outside the solar system.

See also: SIMBAD, short for Set of Identifications, Measurements, and Bibliography for Astronomical Data.

1. Geometry: Having the same shape; representing the same figure drawn to different scales (same corresponding angles and proportional sides).
   

2. Math.: Related by means of a → similarity transformation.

Etymology (EN): From Fr. similaire, from L. similis “like,” → simulate.

Etymology (PE): Hamânand, contraction of hammânand, from ham-, → com-, + mânand “resembling, like,” → simulate.

Two → square matrices A and B that are related by B = X^-1AX, where X is a square → nonsingular matrix.

See also: → similar; → matrix.

Polygons that are exactly the same shape, but can be different sizes.

See also: → similar; → polygon.

The state of being similar; likeness; resemblance.

See also: → similar; → -ity.

1. A transformation that preserves angles and changes all distances in the same ratio.
   

2. A transformation of the form B = X^-1AX relating two → square matrices A and B.

See also: → similarity; → transformation.

1. Chem.: Composed of only one → substance or → element.
   

2. Math.: Consisting of, involving, or describable by → terms of the → first degree.
   

3. Music: Uncompounded or without overtones.

Etymology (EN): M.E., from O.Fr. simple, from L. simplus “simple, single,” variant of simplex, from PIE root *sem- “one, together;” cf. Pers. ham “together,” → com-, Skt. sam “together;” + *plac- “-fold,” from PIE *plek- “to plait,” → multiply.

Etymology (PE): Sâdé “simple, unmixed, smooth, erased, plain;” cf. Khotanese sāta- “smooth;”
Baluchi sāt/sāy-, sāh- “to shave;” Av. si-, sā- “to sharpen, cut;” Skt. śā- “to sharpen, whet” (Cheung 2007); see also → precise.

Statistics: An event consisting of a single point of the → sample space.

See also: → simple; → event.

A fraction in which the → numerator and → denominator are positive → integer numbers. Compare → compound fraction.

See also: → simple; → fraction.

The motion of a body subjected to a restraining force which is directly proportional to the displacement from a fixed point in the line of motion. The equation of simple harmonic motion is given by x = A sin(ωt + θ₀), where x is the body’s displacement from equilibrium position, A is the → amplitude, or the magnitude of harmonic oscillations, ω is the → angular frequency, t is the time elapsed, and θ₀ is the → initial phase angle.

See also: → simple; → harmonic; → motion.

An oscillator whose force is proportional to its extension, according to → Hooke’s law. The way the oscillator moves is called → simple harmonic motion.

See also: → simple; → harmonic; → oscillator.

A set of stars resulting from a spatially (≤ few pc) and temporally (≤ Myr) correlated star formation event.

See also: → simple; → population.

A → rootx₀ of function f(x), if f(x₀) = 0 and df/dx | x₀ = 0. See also → double root.

See also: → simple; → root.

A generalization of the simplest closed configuration that can be made from straight line segments. For example, a → triangle is a 2-simplex because it is in two → dimensions, and → tetrahedron is a 3-simplex because it is in three dimensions (Steven Schwartzman, An Etymological Dictionary of Mathematical Terms Used in English, 1994).

Etymology (EN): Simplex, literally “uncomplicated, → simple,” from sim-, from PIE root *sem- “one, once, together” + plek- “to fold.” “folded [only] once.”

Etymology (PE): Taktâft, literally “folded once,” from tak “→ single, alone,” + tâft, contraction of tâfté “plated, twisted, fold,” as in hamtâft, → complex.

An → algorithm for solving the classical → linear programming problem; developed by George B. Dantzig in 1947. The simplex method is an → iterative method, solving a system of → linear equations in each of its steps, and stopping when either the → optimum is reached, or the solution proves infeasible. The basic method remained pretty much the same over the years, though there were many refinements targeted at improving performance (e.g. using sparse matrix techniques), numerical accuracy and stability, as well as solving special classes of problems, such as mixed-integer programming (Free On-Line Dictionary of Computing, FOLDOC).

See also: → simplex; → method.

The act of simplifying, or the fact of being simplified.

See also: Verbal noun of → simplify.

1. To make less complex or complicated.
   

2. Math.: Reduce an → expression by → algebraic manipulations.

See also: → simple + epenthesis -i- + → -fy.

Minute images or replicas of objects supposed by ancient atomist philosophers to be shed from any object and used in the explanation of vision. According to Democritus (c. 460-c. 370 BC) and Epicurus (341-270 BC), these replicas or effigies, called eidola, were perpetually peeled off
the surfaces of things and caused vision by entering in the eye.

See also: L. translation of eidola by Lucretius (1st Century BC), from L. simulacrum “likeness, image,” from simulare “to → simulate.”

To create a likeness or model of something (a situation, system, or the like).

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

Etymology (PE): Mânadidan verb from mânand “resembling, like,” variant mânestan “to resemble;” Mid.Pers. mânag “like, resembling;” Av. man- “to resemble;” hamânad sâxtan, from hamânand, from ham-, as above, + mânad + sâxtan “to make, build.”

The construction of a mathematical model to reproduce the characteristics of a phenomenon, system, or process, often using a computer, in order to infer information or solve problems.

See also: Verbal noun of → simulate.

The property of events occurring → simultaneously.

See also: Noun from → simultaneous; → -ity.

1. General: Happening, existing, or operating at the same time.
   

2. In a → space-time diagram, the points that have the same time values. Two events that are simultaneous in one → inertial frame are not, in general, simultaneous in another inertial reference frame moving relative to the first. → relativity of simultaneity. Compare → synchronous.

Etymology (EN): From L.L. simultaneus, from L. simul “at the same time” + -taneous, abstracted from → spontaneous.

Etymology (PE): Hamzamân, from ham-, → syn- + zamân, → time.

In trigonometry, the function of an acute angle of a right triangle represented by the ratio of the opposite side to the hypotenuse.

Etymology (EN): Greek mathematicians were not aware of the advantages of sine and instead used chord. The invention of this function is a great Indian contribution. It seems that Aryabhata (c. AD 500) was the first who coined a term in Skt. for this concept: árdha-jiyā- “half chord,” which was later shortened to jiyā- “chord.” This Skt. word was subsequently loaned in Ar. and corrupted to jayb (جیب). Later on Gerard of Cremona (1114-1187),
who translated Ar. scientific texts in L., took the corrupted Skt. form jayb for Ar. jayb “pocket, bundle, bosom, fold,” and translated it to L. sinus “pocket, fold, curve.” The Skt. jiyā- “chord, bow-string;” is cognate with Av. jiiā- “bow-string;” Pers. zeh “chord, string;” → zij “astronomical table;” PIE base *g^whi- “thread, tendon” (from which derive also Gk. bios “bow;” L. filum “thread;” Russ. žca “thread”).

Etymology (PE): Sinus loanword from Fr., as above.

A periodic oscillation that is defined by the function y = sin x.

See also: → sine; → wave.

Only one in number; one only; unique; sole.

Etymology (EN): M.E., from O.Fr. sengle “being one, separate,” from L. singulus “one, individual, separate,” from sim- (stem of simplus) + diminutive suffix, → -ule.

Etymology (PE): Tak“single, alone,” related to tâq “odd, single,”
tâ, tâh “piece, part; fold, plait, ply;” Mid.Pers. tak “single,” tâg, tâk, tâi “unit, piece.”

A type of scattering where photons are scattered only once. Single scattering dominates in → optically thin media, since photons have a high probability of exiting the medium (e.g., a thin cloud) before being scattered again.

See also: → single; → scattering.

A radio astronomical observation which uses only one antenna, in contrast to interferometric observations.

See also: → single; → dish; → observation.

A → spectroscopic binary in which only one set of → spectral lines is detectable. The binary nature of the system is deduced from the fact that the spectral lines exhibit periodic → Doppler shifts due to orbital motions in the system. Same as → SB1 binary. See also: → double-lined binary.

See also: → single; → line; → binary.

A single unit; an unpaired or separate item. → doublet; → octet; → quadruplet.

Etymology (EN): From → single + -et diminutive suffix, M.E. from O.F. -et (masc.), -ette (fem.).

Etymology (PE): Taktâyé, literally “single-folded,” from tak, → single, + -tâyé, from tâ- “fold, plait, ply; piece, part;” Mid.Pers. tâg “piece, part” + -yé nuance suffix.

In atomic physics, the electronic state of an atom or molecule for which the total → spin angular momentum is zero.

See also: → singlet; → state.

An atom that has lost one electron and has become a positive ion.

See also: → single; → ionized; → atom.

A carbon atom → singly ionized by a photon of energy 11.3 eV. The ion C⁺ emits a → fine-structure line (²P_3/2→ ²P_1/2) at 157.7 μm when excitation conditions are satisfied (critical density ~ 3 x 10³ cm^-3). In → photodissociation regions, [C II] 157.7 μm is a major cooling line for regions exposed to significant → far ultraviolet (FUV) photon fluxes. In Galactic → H II regions, as well as in the central regions of external galaxies, the luminosity of the [C II] line is typically ~ 0.05-0.5% of the FUV luminosity and correlates well with → carbon monoxide (CO) line intensities.

See also: → single; → ionized; → carbon.

Math.: 1) Of or pertaining to a linear transformation from a vector space to itself that is not one-to-one.

2. Of or pertaining to a matrix having a determinant equal to zero.

Etymology (EN): M.E., from O.Fr. singuler “single, separate,” from L. singularis “single, solitary,” from singulus “one, individual, separate,” from sim- (stem of simplus) + diminutive suffix.

Etymology (PE): Takin, from tak “single, alone,” related to tâq “odd, single,”
tâ, tâh “piece, part; fold, plait, ply;” Mid.Pers. tak “single,” tâg, tâk, tâi “unit, piece, after numerals”

• -in adj. suffix.

In models of star formation, an isothermal sphere in which the density distribution in the static or nearly static outer envelope obeys an r^-2 power law. In the limit of infinite central concentration, the unstable equilibrium approaches the singular isothermal sphere which has the density and mass distributions ρ(r) = (a²/2πG)r^-2 and M(r) = (2a²/G)r, where a is the isothermal → sound speed inside the cloud, G is the → gravitational constant, and r the distance from the center (F. H. Shu, 1977, ApJ 214, 488).

See also: → singular; → isothermal; → sphere.

A → square matrix that does not have a → matrix inverse.

See also: → singular; → matrix.

The point M₀(x₀,y₀) of the curve F(x,y) = 0, where at least one of the → partial derivatives ∂F/∂x and ∂F/∂y vanishes. See also → ordinary point.

See also: → singular; → point.

Of partial differential equations, the solution which cannot be obtained from the general solution by particular choice of arbitrary functions. → general solution; → particular solution.

See also: → singular; → solution.

1. Math.: A point at which a given mathematical object is not defined.
   

2. Physics: A point in → space-time in which gravitational force causes space-time to have infinite curvature; matter is crushed to infinite density and zero volume. This happens at the center of a → black hole where space and time cease to exist as we know them, and the known laws of physics break down. See also
   → Schwarzschild singularity; → initial singularity.

See also: → singular + → -ity.

1. A region within a system where mass or energy is given up, in contrast to a → source, where mass or energy is released.
   

2. In hydrodynamics simulation codes of gaseous collapse and → accretion, such as → smoothed particle hydrodynamics (SPH), a region of the flow that accretes incoming material but that does not allow it to leave. In in theses simulations an enormous dynamic range is usually encountered,
   which makes the full problem computationally infeasible. Indeed dynamic range
   requires enough resolution elements in both space and time to resolve the largest and smallest scales present in the problem. Sinks provide a way of concentrating resolution in regions of interest and evolving different regions with different time-steps. These entities can have stellar scale masses. See also → sink particle.

Etymology (EN): M.E. sinken, O.E. sincan, from verb; cf. O.S. sinkan, O.N. sökkva, M.Du. sinken, Du. zinken, O.H.G. sinkan, Ger. sinken, Goth. sigqan “to sink.”

Etymology (PE): Câhak, from câh “a well” (Mid.Pers. câh “a well;” Av. cāt- “a well,” from kan- “to dig,” uskən- “to dig out;”
O.Pers. kan- “to dig;” Mod.Pers.
kandan “to dig;” cf. Skt. khan- “to dig,” khanati “he digs,” kha- “cavity, hollow, cave, aperture”) + -ak diminutive suffix.

In hydrodynamics codes, a way of treating a collapsing or accreting region, such as a star, as a simple → point mass. Indeed, in many situations, the scale of interest is much larger than the scale of the → accreting object itself and it would be impossible to perform the calculation otherwise. → Sinks are generally modeled as → Lagrangian particles (see, e.g., Bates et al. 1995, MNRAS 277, 362; Krumholz et al. 2004, ApJ 611, 399; Federrath et al. 2010, ApJ 713, 269).

See also: → sink; → particle.

The outermost of Jupiter’s known confirmed satellites, also known as Jupiter IX, discovered by Seth B. Nicholson (1891-1963) in 1914. With a visual magnitude of 18.3, it has a diameter of 28 km and orbits Jupiter at a mean distance of 23,848,000 km every 753 days.

See also: In Gk. mythology a Naias Nymphe who was abducted by Zeus to a Black Sea coast where the city of Sinope was named for her. According to most sources, she tricked Zeus
into swearing an oath promising her her virginity.

Having the characteristics of a sine function; same as → sine wave.

See also: From sinus, → sine, + → -al.

1. To drink (a liquid) a little at a time; take small tastes of.
   

2. An instance of sipping; a small taste of a liquid; a small quantity taken by sipping
   (Dictionary.com).

Etymology (EN): M.E. sippen (v.), akin to Low German sippen “to sip.”

Etymology (PE): Cašel, from Pashto cašəl “to drink,” caceq “to drip;” related to cašidan “to taste,” → taste.

A ∩-shaped tube with unequal arms that is used to move a liquid from one level to a lower level via a third level higher than either. Once the short arm is filled, for example, by suction, the liquid flows down in the long arm under the action of gravity due to mass excess in it.

See also: From Fr. siphon, from L. sipho (genitive siphonis), from Gk. siphon “pipe, tube,” of unknown origin.

The white star in the constellation → Canis Major that is the brightest star of the sky (V = -1.46). Its other designations include HD 48915, HR 2491, and BD-16°1591.

Its particular brightness is mostly due to its proximity to the Earth, being a mere 8.6 → light-years away, the fifth closest star system. Sirius is a → dwarf star of → spectral type A0 or A1 V with an → effective temperature of 9,880 K, a mass of 2.063 ± 0.023 Msun (Bond et al., 2017, ApJ 840, 70), and a → luminosity of 26 Lsun. Sirius has a radius of 1.75 solar and a minimum equatorial rotation speed of 16 km s^-1. Its → rotation period is less than 5.5 days. This star is a → visual binary (separation 4.6 arcsec, period 50 years), the companion → Sirius B being the first → white dwarf to be discovered. Sirius is a → metal-rich star, its iron content triple that of the Sun, most likely from some sort of → element diffusion.

Etymology (EN): From L. Sirius, from Gk. Seirios, literally “scorching,” because of its brightness.

Etymology (PE): Tištar, from Mid.Pers. Tištar, from Av. Tištrya- “(name of the deified star) Sirius,” literally “the one who belongs to the three stars,” in reference to the three stars of → Orion’s Belt; ultimately from PIE *tri-str-o-m- “group of three stars,” then *tri-str-iia- and by dissimulation Indo-Iranian *ti-str-iia-, Av. *Tištriia- and Vedic Skt. Tisyà (A. Panaino, in Iranica, under Tištrya).

Same as → companion of Sirius.

See also: → Sirius; B, letter of alphabet by convention.

Same as → Alpheratz.

Etymology (EN): Sirah, contraction of Ar. As-Surrat al-Faras (السره‌الفرس) “The Horse’s Navel,” from surrat (سره) “navel” + faras (فرس) “horse.” The name refers to the location of the star in the figure of → Pegasus. However, the star is now considered to belong to the constellation → Andromeda.

Etymology (PE): Ra’s-ol-Mosalsalé, from Ar. Ar-Ra’s al-Mar’ah al-Musalsalah “The head of the chained woman,” from Ra’s “head” + Mar’ah “woman” + Musalsalah “chained”.

In a → superheterodyne receiver, a → mixer which consists of a sandwich structure of two superconducting leads separated by a thin isolator. SIS mixers give a good noise performance especially for → millimeter wavelengths.

See also: SIS, acronym for Superconductor-Insulator-Superconductor; → mixer.

A female person having the same parents as another person.

Etymology (EN): M.E. suster, sister, from O.E. sweostor, swuster or Old Norse systir, in either case ultimately from PIE *swesor; cf. Skt. svásar-, Av. x^vaηnhar-, Pers. xâhar, xwâhar, as below, Gk. eor, L. soror (Fr. sœur), O.C.S., Rus. sestra, O.Ir. siur, Sw. syster, Dan. soster, O.Sax. swestar, M.Du. suster, Du. zuster, O.H.G. swester, Ger. Schwester.

Etymology (PE): Xâhar, xwâhar, from Mid.Pers. xwah(ar) “sister;” Av. x^vaηnhar- “sister;” cf. Khotanese hvar- “sister;” cognate with E. sister, as above.

1. To rest with the body supported by the buttocks or thighs; be seated.
   

2. To be located or situated.

Etymology (PE): Nešastan “to sit down; to settle down; to sink;” Mid.Pers. nišastan “to sit;” O.Pers. nišādayam [1 sg.impf.caus.act.] “to sit down, to establish,” hadiš- “abode;” Av. nišasiiā [1 sg.subj.acr.] “I shall sit down,” from nihad- “to sit down,” from → ni- “down; into” + had- “to sit;” PIE base *sed- “to sit;” cf. Skt. sad- “to sit,” sidati “sits;”
Gk. hezomai “to sit,” hedra “seat, chair;” L. sedere “to sit;” O.Ir. suide “seat, sitting;” Welsh sedd “seat;” Lith. sedmi “to sit;” Rus. sad “garden;” Goth. sitan, Ger. sitzen; E. sit.

See also: → reside, → settle.

The position or location of a building, observatory, etc. especially as to its environment. → astronomical site.

Etymology (EN): M.E., from L situs “position, arrangement, site,” from sinere “to let, leave alone, permit,” cognate with Av. šiti- “place, abode, residence,” as below.

Etymology (PE): Sit, from Av. šiti- “place, abode, residence,” šitāy- “habitation, dwelling,” from ši- “to live;” cognate with Skt. ksay- “to live, to stay,” kséti “he dwells;” Gk. ktizein “to inhabit, build;” L. situs “position, site; situated.”

The process of choosing a site for an astronomical observatory based on meteorology, seeing conditions, and access to the site.

See also: → site; → selection.

To place in a site or context; to locate.

Etymology (EN): From M.L. situatus, p.p. of situare “to place, locate,” from L. situs “place, position.”

Etymology (PE): Sitidan, from sit, → site, + -idan infinitive suffix.

Having a site, situation or location.

See also: P.p. of → situate.

1. The manner of being placed with respect to surroundings.
   

2. Momentary state; a set of circumstances.

See also: Verbal noun of → situate.

A cardinal number, five plus one.

Etymology (EN): M.E. six, sex;
O.E. siex, syx, seox, sex, from P.Gmc. *sekhs (cf. O.S. seks, O.N., O.Fris. sex, M.Du. sesse, Du. zes, O.H.G. sehs, Ger. sechs, Goth. saihs), from PIE *seks-, cognate with Pers. šeš, as below.

Etymology (PE): Šeš, from Mid.Pers. šaš; Av. xšuuaš- “six;” cf. Skt. sás- “six;” Gk. hex; L. sex (Fr. six; Sp. seis); O.C.S. sesti; Lith. sesi; O.Ir. se; Welsh chwech; E. six, as above.

A cardinal number, ten times six. → sexagesimal.

Etymology (EN): M.E.; O.E. sixtig, from → six + -tig a suffix of numerals denoting multiples of ten.

Etymology (PE): Šast “sixty;” Mid.Pers. šast “sixty;” Av. xšuuašti- “sixty;” cf. Skt. sasti- “six;” L. sexaginta “sixty.”

The spatial dimensions, extent, proportions, amount, or degree of something.

Etymology (EN): M.E. syse originally “control, regulation, limit,” from O.Fr. sise shortened form of assise “session, regulation, manner.”

Etymology (PE): 1) Andâzé “measure, size” from Mid.Pers. andâzag, handâcak “measure,” handâxtan, handâz- “to measure,”
Manichean Mid.Pers. hnds- “to measure,” Proto-Iranian *hamdas-, from
ham-, → com-, + *das- “to heap, amass;” cf. Ossetic dasun/dast “to heap up;” Arm. loanword dasel “to arrange (a crowd, people),” das “order, arrangement,”

The number of → edges.

See also: → size; → graph.

A quantity that defines the type of → scattering.

See also: → size; → parameter.