An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

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.

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.

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.

→ Jacobian; → determinant.

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

→ 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.

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.

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.

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.

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.

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.

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).

→ 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.

→ 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.

→ 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.

→ Jeans; → mass.

Same as → Jeans length.

→ 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).

M.E. gely, from O.Fr. gelee "a jelly," from L. gelare "to freeze, congeal, stiffen," from PIE *gel- "cold; to freeze."

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).

→ jelly; → fish.

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).

→ 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.

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").

Ešân, from ešândan, → eject; šân contraction of ešân.