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

A classical set that allows only full membership or no membership at all for its elements, as contrasted with a → fuzzy set. Crisp set can be viewed as a restricted case of the more general fuzzy set concept.

A standard or rule that can serve as basis for a judgment or decision.

From Gk. kriterion "means for judging, standard," from krites "judge," from krinein "to separate, distinguish, judge." L. cribrum "sieve" *krei- "to sieve, discriminate, distinguish."

Sanjidâr verbal noun from sanjid- past tense stem of sanjidan "to compare; to measure" (Mid.Pers. sanjidan "to weigh," from present tense stem sanj-, Av. θanj- "to draw, pull;" Proto-Iranian *θanj-) + suffix -âr.

A person who judges, evaluates, or criticizes.

M.E., from M.Fr. critique, from L. criticus "a judge, literary critic," → critique.

Paržangar, from paržan present stem of paržanidan, "criticize", → critique, + -kâr, -gar suffixes of agent nouns, → -or.

1) Of or pertaining to → critics or → criticism.
2) Of, relating to, or being a state or level at which a significant change takes place (Dictionary.com).

→ critic; → -al.

Angle of incidence of light proceeding from a denser medium toward a thinner, at which grazing refraction occurs (angle of refraction 90°).

→ critical; → angle.

The upper value of mass that a → Bonnor-Ebert sphere must have in order that → hydrodynamic equilibrium be maintained. This → critical mass is given by: M_crit = 1.18 (a⁴/G^3/2)P_ext^-1/2, where a = (kT/m)^1/2 is the isothermal → sound speed inside the sphere, G is the → gravitational constant, and P_ext the pressure of the external medium (see, e.g., F. H. Shu, 1977, ApJ 214, 488).

→ critical; → Bonnor-Ebert mass.

1) Cosmology: The average density of matter in the Universe that would be needed to eventually halt the → cosmic expansion. In a spatially → flat Universe, the critical density is expressed by ρ_c = (3c²/8πG)H_t², where c is the → speed of light, G is the → gravitational constant, and H_t the → Hubble parameter. The critical density is currently 9.3 × 10^-30g cm^-3, about 6 hydrogen atoms per cubic meter (for H₀ = 70 km s^-1 Mpc^-1).
2) In → gravitational lensing, the minimum density that would be needed by an intervening object to bend light rays. It is expressed by: Σ = (c²/4πG)(d_os/d_old_ls), where c is the speed of light, G is the gravitational constant, d_os, d_ol, and d_ls represent angular diameter distances between the observer and the source, the observer and the lens, and the lens and the source respectively. It has units of mass per unit solid angle.
3) Radiative processes: The density at which the collisional → de-excitation rate equals the → radiative transition rate. The critical density for level j is given by: n_c = Σ_{i < j} A_ji = Σ_{i ≠ j} q_ji, where A_ji is the → Einstein coefficient of → spontaneous emission and q_ji is the rate for collisional de-excitation of → energy level j, summed over all possible processes. This expression often simplifies to the ratio of two numbers, since in many cases there is a single important path for emission and a dominant collisional de-excitation process. In the low density limit the → emissivity is proportional to the product N_e (electron density) x N_i (ion density), whereas for densities exceeding the critical density, the emissivity is proportional to N_i. Thus, line emission in a nebula occurs most efficiently near the critical density.

→ critical; → density.

Of a fissile material (²³⁵U or ²³⁹Pu), the minimum mass needed for a sustained nuclear chain reaction, as in an atomic bomb.

→ critical; → mass.

The → metallicity of a → star-forming  → molecular cloud when → cooling → rates by → metals dominate the → gravitational  → heating during → protostellar collapse. The minimum → Jeans mass achieved by gravitational → fragmentation depends on the presence/absence of → coolants in the cloud. Since cooling rate in metal lines is more efficient than in primordial molecular lines (H₂ and HD), metals favor fragmentation in gas and formation of → low-mass stars.

→ critical; → metallicity.

The strong → scattering that occurs in a fluid near the → critical point. Large density fluctuations bring about inhomogeneities of all sizes. This results in the scattering of light at all wavelengths giving a milky appearance to the fluid.

→ critical; → opalescence.

1) Velocity of → fluid through a pipe at which the motion changes from → laminar to → turbulent flow.
2) Same as → break-up velocity.

→ critical; → velocity.

1) The act of passing judgment as to the merits of anything.
2) The act or art of analyzing and evaluating or judging the quality of a literary or artistic work, musical performance, art exhibit, dramatic production, etc.
3) A critical comment, article, or essay; → critique (Dictionary.com).

→ critic; → -ism.

To judge or discuss the merits and faults of.

→ critic; → -ize.

1) An article or essay criticizing a literary or other work; detailed evaluation; review.
2) A criticism or critical comment on some problem, subject, etc.
3) The art or practice of criticism (Dictionary.com).

M.E. critick, changed to Fr. spelling, from M.Fr. critique, from L. criticus "a judge, literary critic," from Gk. kritikos "able to make judgments," from krinein "to sift, separate, decide;" cognate with Pers. qarbâl "sieve," → sieve of Eratosthenes, from PIE base *krei- "to sift, distinguish."

Paržan, from Tâti Karingâni puržen, Zazaki pirožin, Tabari parjan "sieve," related to parvizan "sieve" (Laki vežonen "to sift"); ultimately from Proto-Ir. *pari-uaicana-, from the base *uaic- "to separate, sift, select, sort out;" Av. vaēc- "to select, sort out, sift;" Mod.Pers. bixtan, biz- "to sift."

1a) Cultivated plants or agricultural produce, such as grain, vegetables, or fruit, considered as a group (e.g. a wheat crop, farmers harvest crop).
1b) The total yield of such produce in a particular season or place.
2) To harvest (TheFreeDictionary.com).

M.E. crop, croppe, from O.E. cropp "head or top of a sprout or herb, any part of a medicinal plant except the root," also "bird's craw," cognate with O.H.G. kropf, Du. krop "crop."

Kâšâr, literally "yield of cultivating," from kâš, contratction of kâšt-, kâštan "to cultivate, plant, sow," + âr contraction of âvar present stem of âvardan "to bring; to cause, produce," → format.

1a) The Southern Cross, → Crux. 1b) A figure or pattern formed by the intersection of two lines.
2a) (adj.) Involving interchange; reciprocal.
2b) Lying or passing crosswise; intersecting.
3) To move or go across (something); traverse or intersect.

M.E. cros, from O.E., probably from Old Norse kross, from O.Ir. cros, from L. crux "stake, cross."

Calipâ, loanword from Aramaic.
Xâj loanword from Arm.
Gozaštan, Mid.Pers. vitartan "to pass," Sogdian wytr "to go," O.Pers./Av. vitar- "to put across," from prefix vi- "away, apart," + tar- "to cross over" (Skt. tr- "to cross over," tarati "crosses," cf. Av. taro, Skt. tirah "through, beyond," L. trans "beyond," Ger. durch, E. through; PIE base *tr- "through").

In radio astronomy, the process performed by a → cross correlator or the result of the process.

→ cross; → correlation.

In radio interferometry, a device that multiplies together the signals received by each pair of antennas in an array and performs several operations on the signal (filtering out the noise added to the signal by the receiver electronics, correcting for the Doppler shift and geometric delay due to the position and motion of the antennas). The correlated signal, corresponding to the spatial frequency given by the distance between the two antennas, is then combined with the other spatial frequency to reconstruct the map of the source.

→ cross; → correlator.

A device producing cross dispersion.

→ cross; → disperser.

Dispersion of a light beam by using two dispersing elements (grating, grism), one for separating spectral orders, the other for resolving spectral features within an order.

→ cross; → dispersion.