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

A branch of → semiotics dealing with the relation between language and the users, especially the constraints they encounter in using language in social interaction, and the corresponding effects on other users in the communication.

→ pragmatic; → -ics.

Philo.: The doctrine that the truth or value of a concept or assertion is determined by its practical effects upon human interests.

→ pragmatic; → -ism.

A dimensionless number representing the ratio of the fluid viscosity to the thermal conductivity of a substance; a low number indicates high convection.

Named after the German physicist Ludwig Prandtl (1875-1953); → number.

1) The practice and practical side of knowledge or skills, as opposed to the → theory.
2) Accepted practice or custom.

M.L. praxis "practice, action," from Gk. praxis "practice, action, doing," from stem of prassein "to do, to act."

Varzidâr, from varzid past stem of varzidan, → practice, + -âr prefix forming nouns of action, such as kerdâr, raftâr, didâr, goftâr, jostâr, etc.

A prefix meaning "before, prior to, in advance of, early, beforehand, in front of."

M.E., from O.Fr. pré-, from L. præ (adverb) "before."

Piš- "before; in front," from Mid.Pers. pêš "before, earlier," O.Pers. paišiya "before; in the presence of."

A precursor of a small, loosely bound → star cluster (→ bound cluster) as well as an → OB association, with masses ranging from about 10 to 1000 → solar masses or more.

→ pre-; → cluster; → core.

Same as → PG 1159 star.

→ post-; → degenerate; → star.

A technique in spectroscopy which uses a combination of several dispersive elements (prisms in series or a grism) before focusing the light on the primary disperser, usually a grating, in order to achieve high spectral resolutions.

→ pre-; → dispersion.

A → binary system whose components are → pre-main sequence stars.

→ pre-; → main sequence; → binary.

A → Herbig AeBe star displaying → forbidden emission lines in its spectrum.

→ pre-; → main; → sequence; → B[e] star.

A star that evolves in the → Hayashi phase and has not yet reached the → zero-age main sequence.

→ pre-; → main sequence, → star.

The stage of a star before its eruption to become a nova.

→ pre-; → nova; → stage.

An adjective relating to a stage before the formation of a → protostar. → pre-stellar core.

→ pre-; → stellar.

A small, gravitationally unstable molecular → clump of typical size of less than 0.1 pc resulting from → gravitational collapse and → fragmentation of a larger → molecular cloud. It is a centrally concentrated structure which evolves into a → class 0 object, where eventually a single star or a stellar system is formed. Core masses range between 0.5 and 5 solar masses, with a mean number density of at least 10⁴-10⁵ cm^-3, and a temperature as low as about 10 K. A pre-stellar core evolves into a → Class 0 object. Also called dense core.

→ pre-stellar; → core.

A → post-planetary nebula star that is approaching the top of the → white dwarf sequence. These stars have exhausted the capacity of → nuclear burning in their cores.

→ pre-; → white; → dwarf.

Existing before or making possible the appearance of living organisms. Prebiotic molecules are biologically essential components such as amino acids, sugars, and nucleic-acid bases which are precursors of the origin of life.

→ pre- + → bio- + -tic equivalent of → -ic.

1) To correct celestial coordinates for → precession.
2) To undergo → precession.

Back formation from → precession.

Back formation from pišâyân, → precession.

The apparent position of a celestial object corrected for the epoch → precession.

Precessed, p.p. of → precess; → coordinates.

The periodic motion of the → rotation axis of a body such as a → spinning top in which the axis of rotation gradually sweeps out a conical shape. In the case of the spinning Earth, it is due to the combined → gravitational attractions of the → Sun, the → Moon, and → planets on Earth's → equatorial bulge. Since the Earth's axis is tilted to its → orbital plane or → ecliptic, the gravitational force of the Sun and the Moon on the Earth's equatorial bulge tend to pull it back toward the plane of ecliptic. As a result, the axis → precesses. Earth's axis of rotation precesses with a period of about 25,770 years, describing one complete circle on the → celestial sphere (→ precession constant). This circle has a radius of approximately 23°.5, equal to the → inclination of the Earth's orbit. Since the → vernal equinox is the reference direction for the → equatorial coordinate system, the coordinates of "fixed" objects change with time and must therefore be referred to an → epoch at which they are correct. → sign of zodiac.
See also → general precession; → precession of the ecliptic; → precession of the equator; → precession of the equinoxes; → precessional circle; → precession time; → geodetic precession; → general precession in longitude; → general precession in right ascension; → orbital precession; → perihelion precession; → planetary precession; → precession period.

L.L. præcissionem "a coming before," from L. præcessus, p.p. of præcedere "to happen before," from the fact that the equinoxes occur earlier each year with respect to the preceding year, from præ- "before," → pre-, + cedere "to walk, to go, to happen."

Pišâyân, literally "coming before," from piš- "before" → pre- + ây- (present stem of âmadan "to come, arrive, become"), from Av. ay- "to go, to come," aēiti "goes;" O.Pers. aitiy "goes;" Skt. e- "to come near," eti "arrival;" L. ire "to go;" Goth. iddja "went," Lith. eiti "to go;" Rus. idti "to go;" + -ân suffix of space and time.

The amount by which the equinoctial points drift westward annually due to precession. Its value for epoch J2000.0 is 50''.26, resulting from the westward → precession of the equator (50".38), and the eastward → precession of the ecliptic (0".12).

→ precession; → constant.