An Etymological Dictionary of Astronomy and Astrophysics

An energy schematic representing the → electronic states
of a → molecule and the → transitions between them. The vertical axis shows energies whereas → energy states are grouped horizontally according to their spin → multiplicity. Radiation-less transitions are symbolized by usual arrows, while → radiative transitions are represented by wavy arrows. The vibrational ground states of each electronic state are indicated with thick lines and the higher → vibrational states with thinner lines.

See also: Named after Aleksander Jablonski (1898-1980), a Polish physicist who was an expert in the field of luminescence and atomic optics; → diagram.

The integral admitted by the equations of a body of infinitesimal mass moving under the → gravitational attractions of two massive bodies, which move in circles about their → center of gravity. The Jacobi integral is the only known conserved quantity for the circular → restricted three-body problem.

In the co-rotating system it is expressed by the equation: (1/2) (x^·2 + y^·2

• z^·2) = U - C_J, where the dotted coordinates represent velocities, U is potential energy, and C_J the constant of integration (→ zero-velocity surface). The Jacobi integral has been used for two different purposes:

1. to construct surfaces of zero velocity which limit the regions of space in which the small body, under given initial conditions, can move, and 2) to derive a criterion (→ Tisserand’s parameter) for re-identification of a → comet whose orbit has suffered severe perturbations by a planet. Also known as Jacobi constant.

See also: Named after Karl Gustav Jacobi (1804-1851), a German mathematician who did important work on elliptic functions, partial differential equations, and mechanics; → integral.

→ Jacobian determinant, → Jacobian matrix.

See also: Named after Karl Gustav Jacobi (1804-1851), a German mathematician who did important work on elliptic functions, partial differential equations, and mechanics. The “Jacobian” first appeared in an 1815 paper of the French mathematician Augustin Louis Cauchy (1789-1857), but Jacobi did write a detailed memoir about it in 1841.

The determinant of a → Jacobian matrix formed by the n² → partial derivative s of n functions of n variables.

See also: → Jacobian; → determinant.

In → vector calculus, the matrix of all → first-order partial derivatives of a vector-valued → function.

See also: → Jacobian; → matrix.

An Iranian solar calendar, based on two successive passages of the Sun through the true → vernal equinox. It results from a reform undertaken by a group of astronomers led by Omar Khayyam (A.D. 1048-1131). The current → Iranian calendar is an improved version of the Jalali calendar.

See also: Jalali, from the name of the ruler Jalâleddin Malek Šâh
of the Saljuqid dynasty, who ordered the reform; → calendar.

A large, infrared space telescope with a mirror 6.55 m in diameter, scheduled for launch in 2018. JWST’s instruments will work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range (0.6 to 28 μm). The scheduled instruments are Near IR Camera (NIRCam, field of 2.2 x 4.4 arcmin, wavelength range 0.6-5 μm), Near IR Spectrograph (NIRSpec, 3.5 x 3.5 arcmin, 0.6-5 μm, resolving powers of ~ 100, ~1000, and ~3000), Mid IR Instrument (MIRI, 1.4 x 1.9 arcmin, 5-27 μm, R ~ 3000), and Fine Guidance Sensor (FGS, 2.3 x 2.3 arcmin, 0.6-5 μm, R ~ 100). The successor to the → Hubble Space Telescope will be placed in an orbit about 1.5 million km from the Earth, at the → Lagrangian point L2. The JWST project is a → NASA-led international collaboration with the → European Space Agency and the Canadian Space Agency. The scientific goals of JWST can be grouped under four broad topics: first light after the Big Bang; galaxy formation; birth of stars and protoplanetary systems; and planetary systems and the origins of life.

See also: Named in honor of James E. Webb (1906-1992), who headed NASA from 1961 to 1968, overseeing all the manned launches in the Mercury through Gemini programs, until before the first manned Apollo flight; → space; → telescope.

The unit of → radio flux density in → radio astronomy, equivalent to 10^-26→ watts per square meter per → hertz.

See also: Named in 1973 by the International Astronomical Union in honor of Karl Guthe Jansky (1905-1950),
an American engineer of Czech descent who first identified radio waves
from beyond the Solar System.

The sixth of Saturn’s known satellites. With a mean diameter of about 178 km it orbits Saturn at a distance of 150,000 km. Discovered by the French astronomer Audouin Dollfus (1924-) in 1966.

See also: Janus was the god of gates and doorways in Roman mythology. He was also thought to represent beginnings, hence he lent his name to January, the first month of the year. He was depicted with two faces looking in opposite directions.

A colored form of natural silica, SiO₂, which is a precious stone.

Etymology (EN): M.E. jaspe, jaspre, from M.Fr., O.Fr. jaspe,
from L. iaspidem (nominative iaspis), from Gk. iaspis “jasper,” via an Oriental language, probably an Eastern Iranian language, see below.

Etymology (PE): Yašm, variants, yasp, yasb, yašf “jasper;” Sogd. iešp “jasper,” iešpênê “of jasper, crystalline.”

Sir James Hopwood Jeans (1877-1946), English mathematical physicist, astrophysicist, and popularizer of science. He made important contributions to theoretical astrophysics, especially to the theory of stellar formation. → Jeans escape, → Jeans instability, → Jeans length, → Jeans mass, → Jeans scale, → Rayleigh-Jeans law, → Rayleigh-Jeans spectrum, → thermal Jeans mass, → turbulent Jeans mass, → Jeans escape.

A → thermal escape process by which the atmosphere of a planet loses gases to outer space. This form of thermal escape occurs because some molecules, especially low mass ones, are within the higher-velocity end of the → Maxwell-Boltzmann distribution. The possibility for the gases to escape occurs when the thermal energy of air molecules becomes greater than the → gravitational potential energy of the planet: (3/2)kT = (1/2)mv²  >  GmM/R where v is upward velocity of a molecule of mass m, M is the mass of the planet, and R is the radius of the planet at which thermal escape occurs.

The minimum velocity for which this can work is called the → escape velocity is:

v_e = (2MG/R)^1/2.

Hydrogen molecules (H₂) and helium, or their ions tend to have velocities high enough so that they are not bound by Earth’s gravitational field and are lost to space from the top of the atmosphere.

This process is important for the loss of hydrogen, a low-mass species that more easily attains escape speed at a given temperature, because v ~ (2kT/m)^1/2. As such, Jeans’ escape was likely influential in the atmospheric evolution of all the early terrestrial planets. Jeans' escape currently accounts for a non-negligible fraction of hydrogen escaping from Earth, Mars, and Titan, but it is negligible for Venus because of a cold upper atmosphere combined with relatively high gravity

(see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → Jeans; → escape.

An instability that occurs in a → self-gravitating  → interstellar cloud which is in → hydrostatic equilibrium. Density fluctuations caused by a perturbation may condense the material
leading to the domination of gravitational force
and the cloud collapse. The advent of instability involves a threshold called the → Jeans length or the → Jeans mass.

See also: → Jeans; → instability.

The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud’s internal pressure is sufficient to resist collapse. The Jeans length is defined by:

λ_J = (π c_s²/Gρ)^1/2 = 0.2 pc (T/10 K)^1/2(n_H2/10⁴ cm^-3)^-1/2, where c_s is the → sound speed, G is the → gravitational constant, ρ is the gas density,
T is the gas temperature, and n_H2 is the
molecular hydrogen density.

See also: → Jeans; → length.

The → minimum mass for an → interstellar cloud below which the → thermal pressure of the gas prevents its → collapse under the force of its own → gravity. It is given by the formula M_J = (π^5/2 / 6) G ^-3/2ρ₀^-1/2c_s³, where G is the → gravitational constant, ρ₀ the initial → density, and c_s the isothermal → sound speed.

It can be approximated to M_J
~ 45 (T_K) ^3/2 (n_cm^-3) ^-1/2 in units of solar masses, where T_K is the temperature in → Kelvin, and n_cm^-3 the gas density per cm³. High density favors collapse, while high temperature favors larger Jeans mass. See also: → thermal Jeans mass, → turbulent Jeans mass.

See also: → Jeans; → mass.

Same as → Jeans length.

See also: → Jeans; → scale.

1. A soft somewhat elastic food product made usually with gelatin or pectin; especially, a fruit product made by boiling sugar and the juice of fruit.
   

   2. A substance resembling jelly in consistency (Merriam-Webster.com).

Etymology (EN): M.E. gely, from O.Fr. gelee “a jelly,” from L. gelare “to freeze, congeal, stiffen,” from PIE *gel- “cold; to freeze.”

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

Any of various marine coelenterates of a soft, gelatinous structure, especially one with an umbrella like body and long, trailing tentacles; medusa (dictionary.com).

Etymology (EN): → jelly; → fish.

Etymology (PE): Medusâ, from Gk. Medousa, literally “guardian,” from medein “to protect, rule over.”

A type of galaxy exhibiting “tentacles” (tails) of material that appear to be stripped from the main body of the galaxy, making it resemble a jellyfish. Such type of galaxies occur in → galaxy clusters and are produced by a process called → ram pressure stripping. The mutual → gravitational attraction between galaxies causes them to fall at high speed into the clusters, where they encounter a hot → intracluster medium (ICM) with dense gas. The falling galaxy feels a powerful wind, forcing tails of gas out of the galaxy’s disk and triggering → starbursts within it.

Jellyfish galaxies have mainly been observed in nearby clusters (e.g., Virgo, Coma, A1367, A3627, Shapley). A few examples have been identified in clusters at → redshifts z ~ 0.2-0.4, and there is accumulating evidence for a correlation between the efficiency of the stripping phenomenon and the presence of shocks and strong gradients in the X-ray → intergalactic medium

(Poggianti et al., 2016, AJ 151, 78).

See also: → jellyfish; → galaxy.

1. A stream of a liquid, gas, or small solid particles forcefully shooting forth from a nozzle, orifice, etc.
   
2. Fountain-like formations of gas and/or dust gushing out from compact regions of some astronomical objects. → astrophysical jets.
   
3. Meteo.: A common abbreviation for → jet stream.
   
4. Shortened of → jet plane.

Etymology (EN): Jet, from M.Fr. jeter “to throw,” from V.L. *jectare, alter. of L. jactare, from jac- “throw” + -t- frequentative suffix + -are infinitive suffix; PIE base *ye- “to do” (cf. Gk. hienai “to send, throw;” Hittite ijami “I make”).

Etymology (PE): Ešân, from ešândan, → eject; šân contraction of ešân.

An → engine that works by taking in air at the front and expelling exhaust gases at the rear so that the reaction to this exhaust propels the vehicle forward.

See also: → jet; → engine.

The mechanism whereby → astrophysical jets are thrown out of → accretion disks . Observed correlations between emission from the accretion disk and from the jet provide evidence that the jets are launched from the disks directly.

As the energy emitted from the jets is a → synchrotron radiation, the presence of a → magnetic field is deduced for the ejection.

The most promising model for such “accretion-ejection” structures is based on a scenario where a large-scale magnetic field threads an accretion disk. This model, using a → magnetohydrodynamic (MHD) approach, shows that the magnetic field can azimuthally brake the matter inside the disk (carrying off → angular momentum allowing accretion) and accelerate matter above the disk surface. The → collimation of the flow is achieved via → magnetic tension due to the presence of a → toroidal component of the magnetic field. The magnetic field provides an effective alternative to the radially outward transport of disk angular momentum by → viscosity. The interaction of the magnetic structure with the disk plasma can create a MHD → Poynting flux leaving the disk along the magnetic surface. This energy flux can then be converted into → kinetic energy of the matter within the jet. Because the → mass density in the jet is smaller than in the disk, it is thereby possible to reach high → terminal velocities for a given amount of angular momentum removed from the disk

(Casse & Keppens, 2002, ApJ 581, 988, and references therein).

See also: → jet; → launch; → -ing.

An airplane moved by → jet propulsion.

Etymology (EN): → jet; plane, short for airplane, from Fr. aeroplane, from aero-, → air, + plane feminine of plan “flat, level,” from L. planus, perhaps by association with forme plane; apparently coined and first used by Fr. sculptor and inventor Joseph Pline in 1855.

Etymology (PE): → jet; havâpeymâ “airplane,” from havâ, → air, + peymâ “travelling; traveller,” from peymudan, peymâyidan “to travel, traverse, pass over,”
from Mid.Pers. patmudan, paymudan “to measure (against),” from *pati-māya-. The first element *pati- “against, back” (cf. Mod.Pers. pâd- “against, contrary to;” Mid.Pers. pât-; O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti; Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti). The second element from *mā- “to measure;” O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.” Apart from peymâ, several other terms in Mod.Pers. are related to this second element, which occurs also as mun, mân, man, mâ, mu, and mây:
pirâmun “perimeter,” âzmun, âzmây- “test, trial,”
peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,” man “a measure weighing forty seers),”
nemudan, nemâ- “to show, display,”
âmâdan, âmây- “to prepare.”

Powerful, forward thrust that results from the rearward expulsion of a jet of fluid, especially propulsion by jet engines.

See also: → jet; → propulsion.

Meteo.: An area of relatively strong winds that are concentrated in a narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres.

See also: → jet; → stream.

Same as → Hebrew calendar

Etymology (EN): Jewish, adj. of jew, from M.E. jewe, giu, gyu, ju, from O.Fr. juiu, juieu, gyu,
from L.L. judeus, from L. juaeus, from Gk. ioudaios, from Aramaic yehudhai, from Heb. yəhudhi “Jew,”
from Yəhudah “Judah,” literally “celebrated,” name of Jacob’s fourth son and of the tribe descended from him; → calendar.

Etymology (PE): Gâhšomâr→ calendar; yahud, from Ar., from Heb., as above.

1. Three → atmospheric windows in the → near infrared portion of the → electromagnetic spectrum at 1.25 (J), 1.65 (H), and 2.20 (K) μm.
   

2. An extension of Johnson’s → photometric system into near infrared using filters corresponding to the atmospheric windows J (1.25 μm), H (1.65 μm), and K (2.20 μm), with → bandwidths 0.3 μm, 0.4 μm, and 0.6 μm respectively.

See also: Letters of alphabet, used conventionally; → system.

The biggest meteorite ever witnessed falling and the largest stone meteorite known.
It happened near Jilin, an industrial port city located northeastern China, on March 8, 1976. Of the four tons of fragments of the type H5 chondrite recovered, one piece weighs 1.774 tons and measures about 100 x 80 x 50 cm. The meteorite exploded in the sky and produced a shower covering an area of more than 500 square kilometers.

See also: Jilin, from the name of the Chinese city, known also as Chi-lin City or Kirin City. → meteorite.

Short term random variations either in amplitude or phase of a radio signal.

Etymology (EN): Jitter, may be variant of chitter “tremble, shiver,” from M.E. chiteren “to twitter, chatter.”

Etymology (PE): Jastojah, from jast and jah past and present stems of jahidan “to jump, leap, spring forward; to shake or tremble,” from Mid.Pers. jastan, jahidan “to jump,” figuratively “to happen, occur;” Av. yaēš-, yas- “to boil;” cf. Skt. yas-, yásyati “to boil, to heat; to make effort” + -o- euphonic infix, on the model of kandokâv, oftoxiz, and so on.

A mode of observation in which a series of short exposures are used to enhance the final image quality through appropriate data processing. The first exposure is assumed to be centered on the point of interest; the following ones are slightly offset from the first position with separations not larger than a reasonably small fraction of the detector size. The offsets should be optimally distributed on the sky in order to maximize the chances of being able to filter out the sky background.

See also: → jitter; → mode.

A coupling scheme of electronic → spin angular momenta
and → orbital angular momenta
for heavy atoms (generally Z > 30), where the spin and orbital angular momenta of individual electrons couple strongly, and therefore the → LS coupling scheme does not apply. The coupling between spin and orbital angular momentum of each electron is much stronger than the coupling between different electrons. Therefore, the total angular momentum, j_i, for the i-th electron is obtained by combining l_i and s_i and then coupling these j’s together to give the total angular momentum J = Σ_i j_i. In the jj coupling scheme the total orbital angular momentum quantum number, L, and the total spin angular momentum number, S, are not specified.

See also: j referring to the symbol of the total angular momentum for individual electrons; → coupling.

The random fluctuation of voltage across a resistor caused by the thermal excitation of electrons within it, and the dissipation of power associated with these fluctuations. More generally, an intrinsic noise generated by thermal agitation of electrons by all bodies whose temperature is above 0 K. Also called → thermal noise, Johnson noise, or Nyquist noise.

See also: Named after John Bertrand Johnson (1887-1970) and Harry Nyquist (1889-1976) Swedish-born American engineers and physicists, who did important work on thermal noise
and information theory. → noise.

A square matrix with a constant value λ (nonzero) along the diagonal, 1’s on the superdiagonal, and all other elements 0.

See also: Named after Marie Ennemond Camille Jordan (1838-1922), French mathematician who pioneered group theory, wrote on the theory of linear differential equations, and on the theory of functions, which he applied to the curve which bears his name. → matrix.

A relativistic theory of gravitation which involves a → scalar field in addition to the → metric (→ tensor field) used in rarr; general relativity. It obeys the → equivalence principle, but tries at the same time to comply with → Mach’s principle owing to possible spatial and temporal variations of the → gravitational constant, which is inversely proportional to the scalar field. The theory uses a new dimensionless parameter to determine the discrepancy between its predictions and those of general relativity. So far there is no firm indication of its validity. Same as → scalar-tensor theory.

See also: Named after the creators, Carl Brans (1935-) and Robert Dicke (1916-1997), who presented the theory in 1961, based on the initial work of Pascual Jordan (1902-1980); → theory.

A quantum mechanical → tunnel effect allowing the flow of a continuous current across two weakly coupled → superconductors which are separated by a very thin insulating barrier.

See also: Named after the British physicist Brian David Josephson, who predicted the existence of the effect in 1962; → effect.

A type of electronic circuit involving → Josephson effect, capable of switching at very high speeds when operated at temperatures approaching → absolute zero.

See also: → Josephson effect; → junction.

A unit of → energy in the → International System of Units equal to the → work performed by one → newton over a distance of 1 → meter.
1 J is equivalent to 10⁷ ergs = 1 Watt second =
2.78 &times 10^-7 kWh = 0.2389 calories =
6.24 × 10¹⁸ eV.

Etymology (EN): In honor of the English physicist James Prescott Joule (1818-1889), who established that the various forms of energy (mechanical, electrical, and heat) are basically the same and can be changed, one into another.

Etymology (PE): Joule is in Pers. pronounced as žul, loaned from the Fr. rendering of the E. name.

A → conductor becomes heated by the passage of an electric current through it due to the → resistance of the conductor. Same as → Ohmic dissipation.

See also: → joule; → effect.

The change in the temperature of a gas in the → throttling process.

See also: → Joule; → Thomson; → effect.

The proportional relationship of mechanical energy to thermal energy, equal to 4.184 joules per calorie. Also called mechanical equivalent of heat.

See also: → joule; → constant.

Of or pertaining to the → planet  → Jupiter.

Etymology (EN): From L. Jovius “Jupiter,” Roman god of the sky, cognate with deus “god;” Gk. Zeus “supreme god;” Pers. div “devil, demon” (Mid.Pers. dêw;
O.Pers. daiva- “evil god, demon;” Av. daēva- “evil spirit, false god;” Skt. deva-; PIE base *deiwos “god,” from *dei- “to gleam, to shine”).

Etymology (PE): Hormozi, related to Hormoz, → Jupiter.

A planet that does not have a well-defined → solid  → crust, such as any of the four Solar System outer, gaseous planets: → Jupiter, → Saturn, → Uranus, and → Neptune.

See also: → Jovian; → planet.

Sunspot pairs or groups are tilted with the → leader spots closer to the equator than the → follower spots. The tilt of bipolar sunspot pairs increases with latitude.

See also: Alfred Harrison Joy (1882-1973), an American astronomer; → law.

1a) A public officer authorized to hear and decide cases in a court of law; a magistrate charged with the administration of justice.

1b) A person qualified to pass a critical judgment.

2a) To pass legal judgment on; pass sentence on (a person).

2b) to form a judgment or opinion of; decide upon critically (Dictionary.com).

Etymology (EN): M.E. jugen, from Anglo-Fr. juger, O.Fr. jugier “to form an opinion about; make a decision,” also “to try and pronounce sentence upon (someone) in a court,” from Anglo-Fr juger, O.Fr. jugier “to judge, pronounce judgment; pass an opinion on,” from L. iudicare “to judge, to examine officially; form an opinion upon; pronounce judgment,” from iudicem “a judge,” a compound of ius “right, law,” → just,

• root of dicere “to say.”

Etymology (PE): Dâdras “justice administrator,” from dâd, → justice, + ras present stem and agent noun of rasidan “to attain, to arrive, to mature,” → access.

11 An act or instance of judging.

2. The ability to judge, make a decision, or form an opinion objectively, authoritatively, and wisely, especially in matters affecting action; good sense (Dictionary.com).

See also: → judge; → -ment.

1. Pertaining to judgment in courts of justice or to the administration of justice: judicial proceedings; the judicial system.
   

2. Pertaining to courts of law or to judges; judiciary: judicial functions.
   

3. Of or relating to a judge; proper to the character of a judge; judgelike: judicial gravity.
   

4. Inclined to make or give judgments; critical; discriminating: a judicial mind.
   

5. Decreed, sanctioned, or enforced by a court (Dictionary.com).

Etymology (EN): From L. iudicalis “of or belonging to a court of justice,” from iudicium “judgment, decision,” from iudicem, → judge.

Etymology (PE): Dâvarâné, dâvarik, of or relating to dâvari, → judgment.

A → solar calendar established by Julius Caesar in 46 B.C. to replace the → Roman calendar. It was inspired by the Egyptian calendar year of 365 days.
The astronomer Sosigenes set up the months (January to December)
and added an extra day in February every fourth year (→ leap year).
This gave an average year of 365.25 days. The Julian calendar remained unchanged for 1,600 years, and was replaced by the → Gregorian calendar to correct its errors.
The Roman calendar before the reform was running 80 days out of alignment with the seasons of a true year. Sosigenes fixed the calendar by having 445 days in the year 46 B.C., which brought the seasons in line with the calendar. The year 45 B.C. is known as the first leap year of the Julian calendar. However, Sosigenes’ work was misinterpreted and they were placing leap years every 3 years instead of every 4 years.

See also: Julian, adj. of L. Julius.

A timekeeping system which does not have months and years. It is
used primarily by astronomers to avoid confusion due to the use of different calendars at different times and places. Julian date is the interval of time in days and fractions of a day since noon 1 January 4713 B.C. (12h Universal Time). For example, January 1, 1970 is JD 2440588. Decimal fractions
correspond to fractions of a day so that, for example, an observation
made at 15h on June 24, 1962 is given as JD 2437840.13.
→ modified Julian date (MJD). Note that the “Julius” involved is not Julius Caesar, and this system is unrelated to the Julian calendar, as explained below.

See also: The system was proposed by the French scholar Joseph Justus Scaliger (1540-1609)
in 1583 and named after his father, Julius Caesar Scaliger.
His choice of starting year was based on the convergence in 4713 B.C. of
three calendrical cycles (indication cycle, Metonic cycle, and solar cycle). → date.

Same as → Julian date.

See also: → Julian date; → day.

A way of specifying the date as a year with a decimal based on the Julian year of 365.25 days and the Barycentric Dynamical Time (TDB). The standard epoch currently in use is J2000.0, which corresponds to January 1, 2000 12:00 Terrestrial Time.

See also: → Julian calendar; → epoch.

A period of 365.25 days adopted in the Julian calendar for the length of the year.

See also: → Julian calendar; &rarr ;year.

A point of discontinuity in a function or a derivative of a function.

Etymology (EN): Etymology unclear, probably akin to L.G. gumpen “to jump.”

Etymology (PE): Jaheš, verbal noun of jahidan, jastan “to jump, to leap,” from Mid.Pers. jastan, jahidan “to jump,” figuratively “to happen, occur;” Av. yaēš-, yas- “to boil;” cf. Skt. yas-, yásyati “to boil, to heat; to make effort.”

Very different values of pressure and density (or temperature or energy) across a shock wave.

See also: → jump; → condition.

In a → semiconductor device, a region of transition between semiconducting regions of different electrical properties.

Etymology (EN): Junction “act of joining,” from L. junctionem, noun of action from jungere “to join,” cognate with Pers. yuq, juhé, as below; PIE base *yeug- “to join,”

Etymology (PE): Juheš, from juh, variant of yuq “yoke,”
Mid.Pers. jug, ayoxtan “to join, yoke;” Av. yaog- “to yoke, put to; to join, unite;” cf.
Skt. yugam “yoke;” Hittite yugan “yoke;” Gk. zygon “yoke,” zeugnyanai “to join, unite;” L. jungere “to join,” as above; O.C.S. igo, O.Welsh iou, Lith. jungas O.E. geoc.

1. Of more recent appointment or admission, as to an office or status; of lower rank or standing.
   

2. (in American universities, colleges, and schools) noting or pertaining to the class or year next below that of the senior
   

3. Younger, designating the younger of two men bearing the same full name, as a son named after his father; often written as Jr. or jr. following the name (Dictionary.com).

Etymology (EN): From L. iunior, comparative of iuvenis “young, young man,” cognate with Pers. javân, → young.

Etymology (PE): Kehtar, comparative of keh “small, little,” → decrease.

A → NASA → space mission devoted to the study of the planet → Jupiter. Juno was launched on August 5, 2011 and traveled over a total distance of roughly 2.8 billion km (1.8 → astronomical units) to reach Jupiter on July 4, 2016, after a journey of about five years. Two years after its launch Juno used a → gravity assist through an Earth → flyby in October 2013. The spacecraft will make 37 turns around Jupiter in a → polar orbit over the course of 20 months, until February 2018. Juno has nine different instruments to achieve its scientific goals. Its main goal is to understand the origin and evolution of Jupiter. Among Juno’s scientific objectives, it will:

1. Determine how much water is in Jupiter’s atmosphere, which helps find out which planet formation theory is correct (or if new theories are needed);
   

2. Look deep into Jupiter’s atmosphere to measure composition, temperature, cloud motions and other properties;
   

3. Map Jupiter’s magnetic and gravity fields, revealing the planet’s deep structure;
   

4. Explore and study Jupiter’s → magnetosphere near the planet’s poles, especially the auroras, providing new insights about how the planet’s enormous → magnetic field

affects its atmosphere.

See also: The spacecraft’s name comes from Greco-Roman mythology. The god Jupiter drew a veil of clouds around himself to hide his mischief, but his wife, the goddess Juno, was able to peer through the clouds and see Jupiter’s true nature.

The largest → planet in the → Solar System and the fifth from the Sun, lying at a mean distance of about 5.2 → astronomical units from the Sun. Jupiter is a → gas giant,
mostly → hydrogen and → helium, with a mass of 1.898 × 10²⁷ kg, or about 0.001 → solar masses, or 318 times → Earth masses. It is more than twice as massive as all the other solar system planets combined. Jupiter’s diameter measures 11 times that of Earth. Its → rotation period, 9.93 hours (Jupiter/Earth ratio = 0.41), is the shortest of all the solar system planets. Its → orbital period is 11.857 Earth years. Jupiter has an extensive family of → satellites (79 known) and a faint → ring system; → Jupiter’s ring. Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth masses. Above the core lies the main bulk of the planet in the form of liquid → metallic hydrogen. This exotic form of the most common of elements is possible only at pressures about 3 million bars, as is the case in the interior of Jupiter (and Saturn). Under the extreme pressure found deep inside Jupiter, the electrons are released from the hydrogen molecules and are free to move about the interior. This causes hydrogen to behave as a metal; it becomes conducting for both heat and electricity. See also → Jupiter’s atmosphere.

Etymology (EN): Jupiter “the king of ancient Roman gods, the ruler of Olympus,” from L. Iupeter, from PIE *dyeu-peter- “god-father,” from *deiw-os “god” (cf. Pers. div “devil, demon;” Mid.Pers. dêw;
O.Pers. daiva- “evil god, demon;” Av. daēva- “evil spirit, false god;” Skt. deva-; Gk. Zeus “supreme god;” from *dei- “to gleam, to shine”)

• *peter “father” (cf. Pers. pedar “father;” O.Pers. pitā- “father;” Av. patar-, ptā-; Skt. pitár-; Gk. pater; L. pater, O.H.G. fater).

Etymology (PE): Hormoz, from Mid.Pers. Ohrmazd “name of the highest god in Zoroastrianism,” from O.Pers. aura-mazdā-, Av. ahura-mazdā- “Wise Lord,” from ahura- “lord, god;” cf. Skt. ásura- “god, lord;”
Hittite hassu- “king;” M.H.G. Asen “name of a group of gods;” O.N. āss “god;” PIE *ansu- “spirit, demon” + mazdā- “wisdom,” mazdāθa- “what must be borne in mind,” mazdāh- “memory;” cf.
Skt. medhā- “mental power, wisdom, intelligence;” Gk. mathein “to learn, to know” (root of → mathematics).

An interplanetary mission currently in development by the → European Space Agency planned for launch in 2020. It is aimed mainly at in-depth studies of three potentially ocean-bearing satellites, → Ganymede, → Europa, and → Callisto.

JUICE will complete a unique tour of the Jupiter system including several flybys of each planet-sized world, culminating with orbit insertion around Ganymede, the largest moon in the Solar System, followed by nine months of operations in its orbit.

JUICE will carry the most powerful scientific payload ever flown to the outer Solar System. It consists of 10 state-of-the-art instruments plus one experiment that uses the spacecraft telecommunication system with ground-based instruments.

See also: → Jupiter; → icy; → explorer; → moon.

A quantity of mass equal to 1.898 × 10²⁷ kg, about 0.000954
→ solar masses, or 317.83 → Earth masses. Jupiter mass, M_J, is used as a → unit to describe masses of the → gas giant, such as the outer planets and → extrasolar planets. Similarly, → brown dwarf masses are expressed in terms of Jupiter mass.

See also: → Jupiter; → mass.

The gaseous envelope surrounding Jupiter. It is about 90% → hydrogen and 10% → helium (by numbers of atoms, 75/25% by mass) with traces of → methane, → water, and → ammonia. This is very close to the composition of the primordial → solar nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. The outermost layer is composed primarily of ordinary → molecular hydrogen and helium.

Visually, Jupiter is dominated by two atmospheric features; a series of ever-changing atmospheric cloud bands arranged parallel to the equator and an oval atmospheric blob called the → Great Red Spot.

See also: → Jupiter; → atmosphere.

Any of several faint, dark, narrow rings around Jupiter. Jupiter’s rings are so faint and tenuous that are only visible when viewed from behind Jupiter and are lit by the Sun, or directly viewed in the infrared where they faintly glow. Unlike → Saturn’s rings full of large icy and rock chunks, they are composed of tiny rock fragments and dust. Jupiter’s rings are continuously losing material and being resupplied with new dust from → meteorite impacts with Jupiter’s four inner moons (→ Metis, → Adrastea, → Amalthea, and → Thebe). Jupiter’s rings were discovered by NASA’s Voyager 1 in 1979.

They are composed of three parts: the → Main ring, a → Halo ring that orbits closer to Jupiter, and a very wide → Gossamer ring that extends far from Jupiter.

See also: → Jupiter; → ring.

→ Jurassic era.

See also: Named for the Jura Mountains on the border between France and Switzerland, where rocks of this age were first studied, + -assic, suffix extracted from Triassic.

A period of the Mesozoic era, spanning the time between the Triassic and the → Cretaceous periods, about 200 to 145 million years ago. The start of the period is marked by the major Triassic-Jurassic → mass extinction event.

See also: → Jurassic; → era.

1. Of or relating to the administration of justice.
   

2. Of or relating to law or jurisprudence; legal (Dictionary.com).

Etymology (EN): From L. iuridicalis “relating to right; pertaining to justice,” from iuridicus, from ius “right, law,” → jurist,

• dicere “to say, to speak.”

Etymology (PE): Dâdšenâxti, of or relating to dâdšenâxt, → jurisprudence

1. The right, power, or authority to administer justice by hearing and determining controversies.
   

2. The extent or range of judicial, law enforcement, or other authority.
   

3. The territory over which authority is exercised (Dictionary.com).

Etymology (EN): M.E., from O.Fr. juridiccion and directly from L. iurisdictionem “administration of justice, jurisdiction,” from ius “right, law,” → just, + dictio “a saying; extent or range of administrative power.”

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

• baxš “division; donor, distributor, divider,” from baxšidan “to divide, distribute, grant,” → division, + -ân suffix of attribution and nuance

1. The science or philosophy of law.
   

2. A body or system of laws.
   

3. Civil Law: Decisions of courts, especially of reviewing tribunals (Dictionary.com).

Etymology (EN): M.E., from Fr. jurisprudence and directly from L.L. iurisprudentia “the science of law,” from iuris “of right, of law” + prudentia “knowledge, a foreseeing, foresight, sagacity.”

Etymology (PE): Dâdšenâxt, literally “science of justice,” from dâd, → justice, + šenâxt “knowledge, science,” → -logy.

A person versed in the law, as a judge, lawyer, or scholar.

Etymology (EN): M.E., from M.Fr. juriste, from M.L. iurista “jurist,” from L. ius “law,” → just.

Etymology (PE): Dâdšenâs, literally “knower of justice,” → jurisprudence.

1. Guided by truth, reason, justice, and fairness.
   

2. Done or made according to principle; equitable; proper.
   

3. Based on right; rightful; lawful (Dictionary.com).

Etymology (EN): M.E. juste, from O.Fr. juste “just, righteous,” from L. iustus “upright, equitable,” from ius “right,” especially “legal right, law,” from O.L. ious, perhaps literally “sacred formula.”

Etymology (PE): Dâdmand, from dâd “law, → justice.”
Râst, → right.
Dorost “whole, complete, right,” → integral.

1. The quality of being just; righteousness, equitableness, or moral rightness.
   

2. The moral principle determining just conduct.
   

3. The administering of deserved punishment or reward (Dictionary.com).

Etymology (EN): M.E., from O.Fr. justice “justice, legal rights, jurisdiction,” from L. iustitia “righteousness, equity,” from iustus “upright, → just.”

Etymology (PE): Dâd “justice, law” from Mid.Pers. dâd “law, justice, scriptures with legal content;” related to Mid.- and Mod.Pers. daheš “creation,” dâdan “to give;” Av. dā- “to place upon, give;” → datum.
Dâdmandi noun from dâmand, → just.
Dâdgari “administration of justice,” from dâdgar, literally “just doer,” “an administrator of justice,” from dâd + -gar, → -or.
Dâdgostari, from Dâdgostar, literally “justice spreader,” with gostar, from gostardan “to spread, → expand.”

1. A reason, fact, circumstance, or explanation that justifies or defends. What is offered as grounds for believing an assertion.
   
2. An act of justifying.

Etymology (EN): Verbal noun of → justify.

Etymology (PE): Râstâvard, from râst “right, true; just, upright, straight” (Mid.Pers. râst “true, straight, direct;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz-
“to direct, put in line, set,” razan- “order;” cf. Skt. raj-
“to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Ger. recht; E. right; PIE base *reg- “move in a straight line,” hence, “to direct, rule”)

• âvard past stem of âvardan “to bring; to adduce, bring forward in argument or as evidence” (Mid.Pers. âwurtan, âvaritan; Av. ābar- “to bring; to possess,” from prefix ā- + Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother;” Mod.Pers. bordan “to carry;” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry”).

1. To show (a claim, statement, act, etc.) to be right, reasonable, or proper.
   
2. To defend or uphold as warranted or well-grounded.
   
3. To adjust (a line of type) to fill a line of constant width.

Etymology (EN): Justify, from O.Fr. justifier “to show (something) to be just or right; to administer justice,” from L. justificare “act justly toward, make just,” from justificus “dealing justly, righteous,” from justus “just, upright, equitable,” from jus (gen. juris) “right,” from O.Latin ious, from PIE base *yewes- (cf. Av. yaož-da- “to purify ritually, to revitalize;” Skt. yos- “(long) life” + root of facere “to do” (from PIE base *dhe- “to put, to do;” cf. Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Râstâvard kardan, râstâvardan, infinitives of râstâvard,
→ justification.