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

A hypothetical object which can be thought of as infinitely small.

→ point; → mass.

A source of radiation at a great distance from the observer; an ideal source of infinitesimal size.

→ point; → source.

The two-dimensional intensity distribution about the image of a point source.

→ point; → spread; → function.

The two stars that form the front of the Big Dipper's bowl, away from the handle. More specifically, the stars Dubhe (α Ursae Majoris) and Merak (β Ursae Majoris). A line through β to α passes close to the North Star and they are used for finding it.

→ point + -er.

Dorahnemâ, literally "the two guides," from do "two" + rah, râh "way, path" (from Mid.Pers. râh, râs "way, street," also rah, ras "chariot;" from Proto-Iranian *rāθa-; cf. Av. raθa- "chariot;" Skt. rátha- "car, chariot," rathyā- "road;" L. rota "wheel," rotare "to revolve, roll;" Lith. ratas "wheel;" O.H.G. rad; Ger. Rad; Du. rad; O.Ir. roth; PIE *roto- "to run, to turn, to roll") + nemâ agent noun of nemudan "to show" (Mid.Pers. nimūdan, nimây- "to show," from O.Pers./Av. ni- "down; into" (Skt. ni "down," nitaram "downward," Gk. neiothen "from below," cf. E. nether, O.E. niþera, neoþera "down, downward, below, beneath," from P.Gmc. *nitheraz, Du. neder, Ger. nieder; PIE *ni- "down, below") + māy- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure").

The act or process of directing a telescope. → point.
The direction in the sky to which the telescope is pointed. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.

Verbal noun of → point.

A mathematical model that reproduces the diurnal rotation of the Earth and is used to direct a telescope toward a precise position on the sky.

→ pointing; → model.

The unit of viscosity in the c.g.s. system, equal to 1 dyne.s/cm². Symbol: P

Poise, from Jean-Louis-Marie Poiseuille (1797-1869), a French physiologist and physician who studied the flow of liquids through tubes and developed a method for measuring blood pressure.

In fluid dynamics, the law that the rate of flow of a liquid through a horizontal tube of uniform radius is directly proportional to the pressure of the liquid and the fourth power of the radius of the tube and is inversely proportional to the viscosity of the liquid and the length of the tube.

Named after Jean-Louis-Marie Poiseuille (1797-1869), a French physiologist and physician who found the law in 1844; → law.

A → probability function that characterizes → discrete  → random events occurring independently of one another within some definite time or space. It may be regarded as an approximation of the → binomial distribution when the number of events becomes large and the probability of success becomes small. The Poisson distribution is expressed by: f(x) = (λ^xe^-λ)/x!, where λ is the mean number of successes in the interval, e is the base of the → natural logarithm, and x is the number of successes we are interested in.

Named after Siméon Denis Poisson (1781-1840), French mathematician, who developed the application of Fourier series to physical problems and made major contributions to the theory of probability and to the calculus of variations; → distribution.

An equation (∇²φ = 4πGρ) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density).

→ Poisson distribution; → equation.

1) Of or pertaining to the pole of any sphere, a magnet, an electric cell, etc.
2) A subclass of → cataclysmic variables, the prototype being AM Herculis. Polars are short-period systems in which a → late-type → main sequence star transfers mass to a highly magnetized → white dwarf. The strong magnetic field (10-70 million → gauss) prevents the formation of an → accretion disk, locks both stars in synchronous rotation and guides the accreting matter to accretion spots which are the source of intense X-ray radiation (material impacts on to the white dwarf where it is radiated away).

1) Adj. of → pole.
2) From polar(ization) + (st)ar, because of their → circularly polarized light.

The process or the state of making a → telescope's → polar axis → parallel to the → Earth's → rotation axis, that is with the → true North or → South  → celestial pole. When this is accomplished, the sky's motion can be cancelled out simply by turning the axis (either by hand or with a motor → drive) at the same rate as the rotation of the Earth, but in the opposite direction.

→ polar; → alignment.

The axis of an → equatorial mounting that is parallel to the Earth's axis, and consequently points to the celestial pole.

→ polar; → axis.

A chemical bond where the electrons are shared unequally between atoms. The atom that is more electronegative will pull the electrons closer to itself.

→ polar; → bond.

1) Either of the regions around the poles of the Earth that are permanently covered with ice.
2) Either of the two regions around the poles of the planet Mars, consisting of frozen carbon dioxide and water ice.
3) An area of a → pulsar's surface from where open magnetic field lines emanate.

→ polar; → cap.

An imaginary parallel circle on the celestial sphere or on the Earth at a distance of 23°.5 from either poles.

→ polar; → circle.

A coordinate system in which the position of any point (M) in a plane is specified by two coordinates: 1) ρ, which expresses the distance from a fixed point (the pole, denoted O), and 2) the number φ, which is the angle formed by the line segment OM and a fixed reference line passing through the pole.

→ polar; → coordinate.

An area in the Earth's → magnetosphere, where the → magnetosheath plasma has direct access to the → ionosphere.

→ polar; → cusp.

In polar regions, the portion of the year when the Sun is continuously in the sky. Its length changes from twenty hours at the Arctic/Antarctic Circle (latitude 66°33' N or S) to 186 days at the North/South Pole.

→ polar; → day.

The angular distance of an object from a celestial pole. It is equal to 90° minus the object's declination.

→ polar; → distance.