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

In a → ferroelectric substance, the transition layer between two → domains magnetized in different directions. It is of finite thickness ans has nonuniform → magnetization.

→ domain; → wall.

A method for deriving the distance to a binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler's laws and the mass-luminosity relation, the distance of the binary star can be calculated.

→ dynamical; → parallax.

Light exhibiting → elliptical polarization.

→ elliptic; → polarized; → light.

In an exponential manner.

→ exponential; → -ly.

A collected meteorite whose arrival on Earth is witnessed, as opposed to a → find.

M.E. fallen, from O.E. feallan, from P.Gmc. *fallanan (cf. O.N. falla, O.H.G. fallan), from PIE base *phol- "to fall" (cf. Arm. p'ul "downfall;" Lith. puola "to fall").

Oft, stem of oftâdan "to fall;" Mid.Pers. opastan "to fall," patet "falls;" Av. pat- " to fly, fall, rush," patarəta- "winged;" cf. Skt. patati "he flies, falls," pátra- "wing, feather, leaf;" Gk. piptein "to fall," pterux "wing;" L. penna "feather, wing;" O.E. feðer "feather;" PIE base *pet- "to fly, rush."

Logic: An → error in → reasoning that renders an → argument logically → invalid such as affirming the → consequent and → denying the → antecedent.

From L. fallacia "deception," from fallere "to deceive, trick, cheat; fail, be defective."

Titâl (Dehxodâ) "deceit; deceiving speech, fallacious words;" cf. Tabari titâl hâ kərdan "to deceive (somebody) wheedlingly," Pashtu titâl "duplicity, guile, deceit, fraud."

A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide.

From → fire + ball, from O.E., from O.N. bollr "ball," from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- "to swell."

Tašguy, from taš "fire," variant of âtaš→ fire + guy "ball, sphere," variants golulé, gullé, goruk, gulu, gudé (cf. Skt. guda- "ball, mouthful, lump, tumour," Pali gula- "ball," Gk. gloutos "rump," L. glomus "ball," globus "globe," Ger. Kugel, E. clot; PIE *gel- "to make into a ball").

The motion of a body under the influence of → gravity alone. See also → free-fall time.

→ free; → fall.

The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: t_ff = (3π/32 ρ₀ G)^1/2, where ρ₀ denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.

→ free fall; → time.

In the absence of air resistance, any two bodies that are dropped from rest at the same moment will reach the ground at the same time regardless of their mass.

Galileo (1564-1642) was the first to determine, at the start of the seventeenth century, the law of constant acceleration of free-falling bodies. → law; → fall; → body.

The metallicity derived from observations of the gas component of a galaxy. It is mainly measured from optical → emission lines using primarily oxygen abundances. The gas → metallicity is one of the most important tools to investigate the evolutionary history of galaxies. The reason is that the gas metallicity of galaxies is basically determined by their star-formation history. Recent observational studies has allowed the investigation of the gas metallicity even in → high redshift beyond z = 1, such as → Lyman break galaxies, submillimeter-selected high-z galaxies, and so on. Such observational insights on the metallicity evolution of galaxies provide constraints on the theoretical understandings of the formation and the evolution of galaxies.

→ gas; → metallicity.

The difference between the direction of an object as seen from a point on the surface of the Earth and the direction in which it would be seen from the Earth's center. Also known as → diurnal parallax.

→ geocentric; → parallax.

Any of the small circles on the → reference ellipsoid parallel to the → geodetic equator.

→ geodetic; → parallel.

Objects held in orbit about each other by their → gravitational attraction. Such objects are part of a → bound system.

→ gravitational; → bound.

The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known → periodic comet, hence its → designation 1P/Halley.

Named after the English astronomer Edmond Halley (1656-1742), who first computed its orbit and predicted its return in 1758; → comet.

The parallax of a celestial body when viewed from two points in the Earth's orbit around the Sun. More specifically, the angular difference in a celestial object's position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax.

→ heliocentric; → parallax.

The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator.

→ horizontal; → parallax.

The fall of matter toward the center of a region of gravitational influence.

Infall, from → in- + → fall.

Darun-oft, from darun- "in, into; within" (Mid.Pers. andarôn "inside," from andar, → inter-, + rôn "side, direction;" Av. ravan- "(course of a) river") + oft, → fall.

1) To place in position or connect for service or use.
2) To establish in an office, position, or place (Dictionary.com).

From M.L. installare, from L. → in- + M.L. stallum "stall," from a Germanic source (compare O.H.G. stal "standing place, stand, place, stable, stall," Ger. Stall "stable," Stelle "place"), from PIE root *stel- "to put, stand;" cf. Gk. stele "standing block, slab," stellein "to set in order, arrange, array, equip, make ready."

Darhidan, literally "to place in," from dar "in, into," → in-, + -hidan present stem of (ne)hâdan "to put, place, establish" → position.

1) Something installed, as machinery or apparatus placed in position or connected for use.
2) The act of installing.
3) The fact of being installed (Dictionary.com).

→ install; → -tion.