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

The → antiparticle of the → electron, which has the same → mass, → spin, and → electric charge as the electron, but the charge is → positive. Positrons may be generated by → radioactive decay or by → pair production from energetic → gamma ray photons.

From posi(tive), → positive + (elec)tron→ electron.

A short-lived bound state of a positron and an electron.

From → positron + -ium (as in barium, titanium), from N.L., from L. neuter suffix.

1) The act or fact of possessing; the state of being possessed.
2) A thing possessed (Dictionary.com).

→ possess; → -tion.

Dârešt, verbal noun of dâštan "to have, possess" (on the model of konešt, from kardan; xoršt, from xordan; bâlešt, from bâlidan; râmešt, from râmidan; (Lori) zenešt, from zadan; (Nowdân, Fârs) perešt, from paridan); Mid.Pers. dâr-, dâštan "to have, hold, preserve;" O.Pers./Av. dar- "to hold, keep back, maintain, keep in mind;" Skt. dhr- "to hold, keep, preserve," dharma- "what is established or firm; law;" Gk. thronos "elevated seat, throne," L. firmus "firm, stable," Lith. daryti "to make," PIE *d^her- "to hold, support."

→ post-Newtonian formalism.

→ post-; → Newtonian; → expansion.

An approximate version of → general relativity that applies when the → gravitational field is → weak, and the matter → velocity is → small. Post-Newtonian formalism successfully describes the gravitational field of the solar system. It can also be applied to situations involving compact bodies with strong internal gravity, provided that the mutual gravity between bodies is weak. It also provides a foundation to calculate the → gravitational waves emitted by → compact binary star systems, as well as their orbital evolution under radiative losses. The formalism proceeds from the Newtonian description and then, step by step, adds correction terms that take into account the effects of general relativity. The correction terms are ordered in a systematic way (from the largest effects to the smallest ones), and the progression of ever smaller corrections is called the → post-Newtonian expansion.

→ post-; → Newtonian; → formalism.

To put off to a later time; defer.

From L. postponere "put after; neglect; postpone," from → post- "after" + ponere "to put, place," → position.

Pas afkandan, literally "to throw after," "to postpone" (Dehxodâ), from pas- "after," → post-, + afkandan "to throw," → stopword.

A function of the form f(x) = xⁿ, where n is a → real number.

→ power; → function.

For a → random variable X, any → distribution which has the form: P(X ≥ x) = (k/x)^α, where x is a value in the range defined for X, k > 0 is a parameter termed location parameter, and α > 0 is the → slope parameter.

→ power; → law; → distribution.

A loss of → orbital angular momentum by tiny ring particles associated with their absorption and re-emission of → solar radiation. Also known as the → Poynting-Robertson effect (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ Poynting-Robertson; → drag.

The effect of → solar radiation on a small (centimeter-sized) particle in → orbit around the Sun that causes it to lose velocity and fall gradually into the Sun. The particle → absorbs solar radiation and → radiates the energy → isotropically in its own frame. The particle thereby preferentially radiates (and loses → angular momentum) in the forward direction in the → inertial frame of the Sun (aberration effect). This leads to a decrease in the particle's angular momentum and causes it to spiral sunward. In contrast, the → Yarkovsky effect is anisotropic; the object may be accelerated or decelerated.

→ Poynting's theorem; Howard Percy Robertson (1903-1961), American physicist and mathematician; → effect.

A professional man, especially in medicine and the law.

Alteration of practician, → practice (+ -ian) + -er, → -or.

Varzmand, from varz, → practice, + -mand, → -ist.

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.

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.

The component of general precession caused by the gravitational attraction of the planets on the Earth's center of mass. It causes the equinox to move eastward by about 0''.12 per year in the opposite direction to the → precession of the equator. This terminology replaces → planetary precession, according to an IAU resolution adopted in August 2006.

→ precession; → ecliptic.

That component of general precession caused by the combined effect of the Moon, the Sun and the planets on the equatorial protuberance of the Earth, producing a westward motion of the equinoxes along the ecliptic about 50'' per year. According to an IAU resolution adopted in August 2006, the present terminology replaces lunisolar precession.

→ precession; → equator.

The slow motion of the equinoxes along the ecliptic, resulting from the combined motion of the equator (→ precession of the equator) and the ecliptic (→ precession of the ecliptic), or in other words the precession of the Earth's axis of rotation. Also know as → general precession. The First Point of Aries moves westward along the ecliptic at 50.38 arcseconds per year (1 degree every 71.6 years), causing the equinoxes to occur about twenty minutes earlier each sidereal year. See also → nutation.

→ precession; → equinox.

The gradual change in he orbital planes of a binary system.

→ precession; → node.

The interval with which a rotating body precesses. The precession period of the Earth is 25,770 years. For a → spinning top it is given by: T_p = (4π²I)/(mgrT_s), where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top.

→ precession; → period.

A time interval over which an orbit precesses by 2π radians in its plane.

→ precession; → time.