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

Of n numbers a₁, a₂, ..., a_n, the quantity defined as: (a₁ + a₂ + ... + a_n)/n.

→ arithmetic; → mean.

The middle term in a → geometric progression. Of two terms, the geometric mean is the square root of their product. For example, the geometric mean of 4 and 9 is ± 6. For a series of n terms, it is expressed as: (a₁.a₂. ... .a_n)^1/n.

→ geometric; → mean.

The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator.

→ mean; → Greenwich Meridian; → sidereal; → time.

A number whose reciprocal is the → arithmetic mean of the reciprocals of a set of numbers. Denoted by H, it may be written in the discrete case for n quantities x₁, ..., x_n, as: 1/H = (1/n) Σ(1/x_i), summing from i = 1 to n. For example, the harmonic mean between 3 and 4 is 24/7 (reciprocal of 3: 1/3, reciprocal of 4: 1/4, arithmetic mean between them 7/24). The harmonic mean applies more accurately to certain situations involving rates. For example, if a car travels a certain distance at a speed speed 60 km/h and then the same distance again at a speed 40 km/h, then its average speed is the harmonic mean of 48 km/h, and its total travel time is the same as if it had traveled the whole distance at that average speed. However, if the car travels for a certain amount of time at a speed v and then the same amount of time at a speed u, then its average speed is the arithmetic mean of v and u, which in the above example is 50 km/h.

→ harmonic; → mean.

1a) General: A quantity having a value intermediate between the values of other quantities; an average.
1b) → arithmetic mean.
1c) → geometric mean.
1d) → harmonic mean.
1e) → weighted mean.
1f) → root mean square.
2) To have as its sense or signification; signify.

1) From O.Fr. meien, from L. medianus "of or that is in the middle," → median.
2) Verb of → meaning.

1) Miyângin "the middle; middle-sized; the middle pearl in a string," from miyân, → middle, + -gin a suffix forming adjectives of possession.
2) → meaning.

The angle between the periapsis of an orbit and the position of a hypothetical body that orbits in the same period as the real one but at a constant mean angular velocity.

→ mean; → anomaly.

That point on the → celestial sphere at which an object would be seen from the solar system → barycenter affected by the → e-terms → aberration.

→ catalog; → mean; → place.

The average movement of a body along its orbit in one day, usually expressed in degrees.

→ mean; → diurnal; → motion.

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.