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

In a → directed graph, a path in which all → edges are oriented in the same direction.

→ directed; → edge.

An → ESA spacecraft that was launched on December 3, 2015 to test technologies needed for the Evolved Laser Interferometer Space Antenna (eLISA), an ESA → gravitational wave observatory planned to be launched in 2034. LISA Pathfinder paves the way for future missions by testing in flight the very concept of gravitational wave detection.

LISA, short for → Laser Interferometer Space Antenna; → pathfinder.

The mean distance which a particle moves between two successive collisions with other particles of the medium. Mean free path is inversely proportional to the number of particles per cm³ (n), and the collision → cross section (σ). In the case of a gas with molecules having a diameter of d, the cross section is equal to the area of a circle of radius d, i.e. σ = πd², and the mean free path is given by: l = 1/(nσ). Taking into account the relative velocity distribution of the colliding molecules, l = 1/(√2 . nσ). For a gas at one atmosphere pressure and room temperature, the average distance between molecules is roughly 3.5 × 10^-7 cm, that is some 35 times the diameter of a molecule. Taking the gas density n = 2.4 × 10¹⁴ molecules cm^-3, and a typical diameter d = 2 × 10^-8 cm for a molecule, the mean free path is 3.3 × 10^-5 cm. This means that the average distance between collisions is about 95 times the average distance between molecules.

→ mean; → free; → path.

Puyeš, verbal noun of puyidan "to run, trot; wander," from Mid.Pers. pôy-, pwd- "to run;" cf. Gk. speudein "to hasten;" Lith. spudinti; âzâd, → free; miyângin, → mean.

In → geometric optics, the distance a light ray would travel in a vacuum in the same time it travels from one point to another, a specified distance, through one or more optical media.

→ optical; → path.

1) A route, course, or track along which something moves.
2) A course of action, conduct, or procedure.
3) Math.: A continuous curve that connects two or more points.
4) In → graph theory, a → graph whose → vertices can be arranged in a sequence.

O.E. paþ, pæþ; cf. O.Fris. path; M.Du. pat; Du. pad; O.H.G. pfad; Ger. Pfad "path."

Pah "path, way," from O.Pers. paθi- "path, way;" Av. paθ-, variants paθi-, paθā-, pantay- (with conversion of -θ- to -h-, as in ciθra-/cehr, xšaθra-/šahr/šâh, vərəθraγna-/bahrâm, → Mars); Mid/Mod.Pers. pand "path, advice, counsel;" Khotanese pande "road, path;" Ossetic fœndœg "path, road;" cf. Skt. pánthā- "road, path, course;" Gk. patos "path, way," pontos "sea;" L. pons "bridge, path;" P.Gmc. *finthanan "to find;" E. find; PIE base *pent- "to go, to tread."
Râh, → way.

The path followed by an individual particle of fluid over an interval of time. It contrasts with the → streamline that represents an instantaneous picture of the motion of particles.

→ path; → line.

The path (up to 320 km wide) that the Moon's shadow traces on the Earth during a total solar eclipse.

→ path; → totality.

1) A person who makes or finds a way, especially through unexplored areas or fields of knowledge.
2) An airplane, or a person dropped from a plane, sent into a target area to illuminate the area for succeeding aircraft (Dictionary.com).
3) → LISA pathfinder.

→ path; → finder.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

→ thermodynamic; → path.

Of a → solar eclipse, the path of the → umbra across the → Earth. The totality path is usually about 100 km across, but under the most favorable conditions, when the → Moon is at its nearest → distance to Earth and the Earth is at its farthest distance from the Sun, the umbra can have a diameter of about 270 km.

→ totality; → path.