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

A → dwarf elliptical galaxy that is a satellite of our → Milky Way. It lies about 285,000 → light-years away in the constellation → Sculptor, and has an → absolute magnitude of -11.28 and a mass of about 3 million → solar masses. The Sculptor Dwarf is a → metal-deficient galaxy containing only 4 percent of the oxygen and carbon elements in our own Galaxy.

→ Sculptor; → dwarf; → elliptical; → galaxy.

The nearest group of galaxies to our → Local Group, lying near the south Galactic pole at about 10 million → light-years distance. The Sculptor Group is dominated by five galaxies, four spiral (NGC 247, 253, 300, and 7793) and one irregular (NGC 55). The brightest of the five is NGC 253. The nearest galaxy in this group is NGC 55 which at a distance of 5 million light-years lies on the border of the Local Group.

→ Sculptor; → group.

The Shield. A small constellation in the southern Milky Way, at 18h 40m right ascension, 10° south declination. Its brightest star has a visual magnitude of 3.85. Scutum contains several open clusters, as well as a globular cluster and a planetary nebula. The two best known deep sky objects in Scutum are M11 (NGC 6705), a dense open cluster, and M26, another open cluster also known as NGC 6694. The globular cluster NGC 6712 and the planetary nebula IC 1295 can be found in the eastern part of the constellation. Abbreviation: Sct; Genitive: Scuti.

Scutum was created by Johannes Hevelius in 1683, who originally named it L. Scutum Sobiescianum "the shield of Sobieski" to commemorate the victory of the Polish forces led by King John III Sobieski in the Battle of Vienna, and thus refers to Sobieski's Janina Coat of Arms. Later, the name was shortened to Scutum "shield."

Separ "shield," from Mid.Pers. spar "shield;" cf. Skt. phalaka- "board, lath, leaf, shield," phálati "(he) splits;" Gk. aspalon "skin, hide," spolas "flayed skin," sphalassein "to cleave, to disrupt;" O.H.G. spaltan "to split;" Goth. spilda "board;" PIE base *(s)p(h)el- "to split, to break off."

A spiral arm of our Galaxy located between the Sagittarius Arm and the Norma Arm, though it is rather less prominent than either of these two better defined spiral arms. It originates relatively close to the Sun's present position in the Galaxy, and follows a sweeping arc of about 80,000 light years to the opposite side of the Galactic disk.

→ Scutum; → Crux; → arm.

An agricultural implement consisting of a long, curving blade fastened at an angle to a handle, for cutting grass, grain, etc., by hand (Dictionary.com).

M.E. sythe, sithe, from O.E. sithe, sigdi "sickle;" cf. West Frisian seine "scythe," Du. zicht "sickle," Ger. Sense "scythe;" from PIE root *sek- "to cut."

Dahre "scythe," variant of dâs, → sickle; dialectal variants (Dari Yazd) dare, (Laki) dara "butcher's cleaver," (Gilân, Lâsgard, Sorxe) dâra, (Tabari) dahra, dâhra, darra.

1) A large lake or landlocked body of water.
2) A large area or great number of something. → Fermi sea.

O.E. sæ "sheet of water, sea, lake;" cf. Du. zee, Ger. See, O.N. sær "sea," Goth saiws "marsh."

Daryâ "sea;" Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- "sea;" cf. Skt. jráyas- "expanse, space, flat surface."

The → apparent horizon formed by the sea.

→ sea; → horizon.

To explore or examine in order to find something.

M.E. serchen, cerchen, from O.Fr. cerchier "to search," from L. circare "to go about, wander, traverse," from circus "circle."

Jost-o-ju interfixed jost and juy past and present stem of jostan/juyidan "to seek, strive for;" Proto-Iranian *iud- "to struggle for something, to fight" (Av. yūδ- "to fight, struggle;" Mod.Pers. justan, juy- "to search, seek, ask for"); cf. Mid.Pers. vijuyihitan "to search, seek."

The scientific attempt to detect → intelligent extraterrestrial → life by surveying the sky to find the existence of → transmissions, especially → radio waves or → light, from a → civilization on a distant → planet. The SETI Institute, that carries out the project, is a private non-profit center founded in 1984. There are many methods that SETI scientific teams use to search for extraterrestrial intelligence. Many of these search billions of radio frequencies that reach Earth from all over the → Universe, looking for an intelligent → radio signal. Other SETI teams search by looking for signals in pulses of light emanating from the stars.

→ search; → extraterrestrial; → intelligence.

The hard shell of a marine mollusk.

→ sea; → shell.

Sadaf, loan from Ar. Kelâcak from Tabari, variant kelâcin, cf. Gilaki guš kuli. The component kel-, kul might be related to PIE *qarq- "to be hard," → crab.

One of the four periods of the year astronomically defined by the position of the Sun with respect to the equator. As a result of the obliquity of the ecliptic, the angular distance between the Sun and the equator varies in the course of the year. This circumstance gives rise to seasons. The current lengths of the astronomical seasons, around the year 2000, are about: spring 92.76 days, summer 93.65 days, autumn 89.84 days, and winter 88.99 days. The seasons are unequal because the Earth's orbit is slightly elliptical and the Sun is not exactly at the center of the orbit. Moreover, the Earth moves faster when it is close to the Sun than when it is farther away, so the seasons that occur when the Earth is close to the Sun pass more quickly.

M.E. sesoun, seson, from O.Fr. seison "a sowing, planting," from L. sationem (nominative satio) "a sowing," from p.p. stem of serere "to scatter seed over land."

Fasl, from Ar. faSl "cutting, dividing; section."

1) Geometry: A straight line that intersects a curve in two or more points.
2) Trigonometry: For an → acute angle of a → right triangle, the function defined as the ratio of the → hypotenuse to the adjacent side. For any angle, the function defined as the ratio of the → radius vector to the → abscissa. Abbreviation sec.

From L. secant-, stem of secans, pr.p. of secare "to cut," → section.

1) Sekanjân, agent noun from sekanjidan "to shave, cut, scape," cognate with šekastan "to break," → section.
2) Sekânt, loan from Fr.

A pioneering work in → spectral classification conducted in the 1860s. Secchi divided stars into four main groups based on the visual observation of spectra. Class I: The white and bluish stars with a continuous spectrum crossed by hydrogen bands, the metallic bands being absent or weak. Examples, → Sirius, → Vega. Class II: Yellow stars, with spectra in which the hydrogen bands were less prominent and the metallic lines more strong. Examples, Sun, → Capella. Class III: Red or orange stars, showing bands or flutings. Examples, → Antares, → Betelgeuse. Class IV: Red stars, showing bands similar to Class III, but with the sharp edge of the flutings toward the other end of the spectrum. Secchi's scheme was superseded by the photographic → Harvard classification system.

Pietro Angelo Secchi (1818-1878), Italian astronomer and Jesuit priest; → classification.

1) Next after the first in place, time, or value.
2) The unit of time in the → International System of Units; symbol s. It is defined by taking the fixed numerical value of the cesium frequency Δν_Cs, the unperturbed → ground state  → hyperfine transition  → frequency of the → cesium-133 atom, to be 9 192 631 770 when expressed in the unit → hertz (Hz), which is equal to s^-1.

M.E., from O.Fr. second, from L. secundus "following, next in order," from root of sequi "to follow;" PIE base *sek^w- "to follow;" cf. Pers. az from; Mid.Pers. hac "from;" Av. hac-, hax- "to follow," hacaiti "follows" (O.Pers. hacā "from;" Av. hacā "from, out of;" Skt. sácā "with"); Skt. sácate "accompanies, follows;" Gk. hepesthai "to follow;" Lith. seku "to follow."

1) Dovom, dovomin "ordinal number of do, two" (Mid.Pers. do; Av. dva-; cf. Skt. dvi-; Gk. duo; L. duo; (Fr. deux; E. two; Ger. zwei).
2) Sâniyé, from Ar. sâniyat (feminine) "second."

Math: In calculus, limiting an equation to its → second derivative, for example: e^x≅ 1 + x + x²/2. Also called linear approximation. → first approximation.

→ second; → approximation.

An early evolutionary period in the process of star formation which succeeds the → first collapse. When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H₂ molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 10³⁰ g (1.5 x 10^-3M_sun) and 9 x 10¹⁰ cm (1.3 R_sun) respectively, and the central density and temperature are about 2 x 10^-2 g cm^-3 and 2 x 10⁴ K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271).

→ second; → collapse.

The beginning of the total phase of a solar eclipse when the leading edge of the Moon touches the eastern edge of the Sun completely obscuring the Sun.

→ second; → contact.

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

→ second; → core.

In → calculus, the → derivative of a → first derivative. It is usually written as f''(x), d²y/d²x, or y''.

→ second; → derivative.

A method, used in → calculus, for determining whether a given → stationary point of a → function is a → local minimum or → local maximum.

→ second; → derivative; → test.