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

An element of an adopted reference orbit that approximates the actual, perturbed orbit. Mean elements may serve as the basis for calculating perturbations.

→ mean; → element.

The orientation the Earth's equator would have if the nutation was subtracted.

→ mean; → equator.

A fictitious equinox whose position is that of the vernal equinox at a particular epoch with the effect of nutation removed.

→ mean; → equinox.

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.

The average amount of time an unstable radioisotope exists before it decays, It is equal to 1.44 times the half-life.

→ mean; → life.

The total atomic or molecular weight divided by the total number of particles. For instance, the mean molecular weight of a plasma of pure ionized ⁴He would be 4 (the atomic mass number) divided by 3, the total number of particles (1 nucleus plus 2 electrons), i.e. 4/3.

→ mean; → molecular; → weight.

A fictitious Moon that has the same average motion as the true Moon but that is not subject to any gravitational perturbations by other bodies.

→ mean; → moon.

The average angular velocity of a satellite in an elliptical orbit.

→ mean; → motion.

The parallax, derived by means of statistical studies of brightness and motions, for a large group of stars whose individual parallaxes cannot be measured.

→ mean; → parallax.

An object's celestial position as determined for a given mean equator and equinox. → mean position.

→ mean; → place.

The direction of the Earth's axis at a particular epoch if the nutation is ignored.

→ mean; → pole.

Same as → mean place.

→ mean; → position.

The shape of a pulsar's pulse as determined by averaging several pulses.

→ mean; → profile.

The average time interval between two successive → upper transits of the → mean equinox.

→ mean; → sidereal; → day.

The hour angle of the mean equinox for a given observer.

→ mean; → sidereal; → time.

The average length of the apparent solar day. In other words, the interval between successive transits of the mean Sun for a given observer.

→ mean; → solar; → day.

The time since the mean Sun crossed the meridian with 12 hours added to make the day begin at midnight.

→ mean; → solar; → time.

A plot of the mass-to-charge ratio of elementary particles, sorted by their isotopic mass.

→ mean; → spectrum.

A hypothetical Sun that moves along the ecliptic at a uniform rate equal to the average motion of the real Sun.

→ mean; → sun.

In → syllogism, the term which is common to both → premises and is excluded from the → conclusion.

→ mean; → term.