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

The forces of jets from a comet's nucleus that can cause a rocket-like effect and alter a comet's direction of motion slightly.

→ non-; → gravitational; → force.

A suffix used to form abstract nouns from verbs to express action, state, or associated meanings.

From L. -tionem, accusative of noun suffix -tio (genitive -tionis).

-eš, from Mid.Pers. -išn.

A blue star of → apparent visual magnitude 4.96 lying in the → Orion constellation. It has other designations, including: ψ¹ Orionis, BD+01 1005, HR 1789, and HIP 25302. 25 Ori is a → main sequence star of → spectral type B1 V. It lies at a distance of 1,040 → light-years and has a → luminosity of 9,300 MSun, a radius of 5.5 Rsun, and a mass of about 10 Msun. 25 Ori is in fact a → Be star.

A group of nearly 200 low-mass → pre-main-sequence stars, concentrated within ~ 1 of the early → B star  → 25 Orionis, in the component a of the → Orion OB1 Association. The group also harbors the → Herbig Ae/Be star V346 Ori and a dozen other early-type stars. The velocity distribution for the low-mass stars shows a narrow peak at 19.7 km s^-1, offset ~ 10 km s^-1 from the velocity characterizing the younger stars of the Ori OB1b subassociation, and 4 km s^-1 from the velocity of widely spread young stars of the Ori OB1a population. This indicates that the 25 Ori group is a distinct kinematic entity. The low-mass members follow a well-defined band in the → color-magnitude diagram, consistent with an age of ~ 7-10 Myr (Briceno et al., 2007, ApJ 661, 1119).

→ 25 Orionis; → group.

A blue star with → apparent visual magnitude 4.59 in → Orion's Sword. Also known as → c Orionis, HD 37018, HR 1892, and HIP 26237. More specifically, it is a → main sequence star of → spectral type B1 V, lying in the → H II region→ NGC 1977. 42 Orionis is approximately 900 → light-years away based on parallax. It is the major source for ionizing photons in NGC 1977. 42 Ori is a → mutiple star system. The primary star, Aa, of magnitude 6.3 has a → companion companion Ab at a separation of 0.16'', and a more distant companion B of 7.5 magnitude at 1.6'' separation. An irradiated → circumstellar disk near 42 Ori has been detected by Bally et al. (2012) in the → HST image using Hα filter (F658N). They identified a bent → protostellar jet HH1064 from Parenago 2042 (the Spindle) in NGC 1977 with numerous → bow shock features. They argue that the arc feature in the Hα Spindle is centered on the star and its brightened side of the arc is facing toward 42 Ori, suggesting that it may be a → proplyd (Kim et al., 2016, arXiv::1606.08271).

Number 42 in → Flamsteed designation; Orionis, genitive of → Orion.

In physics and chemistry, a calculation that relies on basic and established laws without additional assumptions or special models. Experimental input in ab initio calculations is limited to the determination of values of fundamental physical constants.

→ ab initio; → calculation.

In → geometric optics, a condition for eliminating → spherical aberration and → coma in an → optical system. It is expressed by the relationship: sin u'/U' = sin u/U, where u and U are the angles, relative to the → optical axis, of any two rays as they leave the object, and u' and U' are the angles of the same rays where they reach the image plane. A system which satisfies the sine condition is called → aplanatic.

Named after Ernst Karl Abbe (1840-1905), a German physicist; → sine; → condition.

1) An imperfection in the imaging properties of a → lens or → mirror. The main aberrations are → chromatic aberration, → spherical aberration, → coma, → astigmatism, → distortion, and → field curvature.
2) → aberration of starlight.
See also: → aberration angle→ aberration constan→ aberration of starlight→ aberration orbit→ annual aberration→ chromatic aberration→ constant of aberration→ diurnal aberration→ optical aberration→ planetary aberration→ relativistic aberration→ secular aberration→ spherical aberration→ stellar aberration.

Aberration, from L. aberrationem, from aberrare "go astray," → aberrate.

Birâheš, from birâidan, → aberrate.

The angle tilt required by the → stellar aberration phenomenon in order that a moving telescope points directly to a star.

→ aberration; → angle.

Same as → constant of aberration.

→ aberration; → constant.

→ aberration of starlight

→ aberration; → light.

An apparent displacement in the observed position of a star. It is a result of the finite speed of light combined with the relative motion of the Earth through space. Suppose that you walk through a vertically falling rain with an umbrella over your head. The faster you walk, the further you must lower the umbrella in front of yourself to prevent the rain from striking your face. For starlight to enter a telescope, a similar phenomenon must occur, because the Earth is in motion. The telescope must be tilted in the direction of motion by an angle: tan θ =(v/c), where v the Earth velocity and c the speed of light. The aberration of starlight was discovered by the English astronomer James Bradley (1693-1762) in 1729 by observing → Gamma Draconis. The tilt angle is θ = 20''.50, from which the Earth's orbital speed, 29.80 km s^-1, can be deduced, using the above equation. See also → annual aberration; → diurnal aberration; → secular aberration. → Special relativity modifies the classical formula for aberration, predicting results which differ substantially from those of classical physics for objects moving at a substantial fraction of the speed of light; → relativistic aberration.

→ aberration; → star; → light.

The apparent path described by a star on the → celestial sphere due → annual aberration. A star at the → ecliptic pole is seen to move around a circle of angular radius about 20".50, once a year. A star on the → ecliptic oscillates to and fro along a line of angular half-length 20".50. At an intermediate → celestial latitude, β, the aberration orbit is an ellipse, with semi-major axis 20".50 and semi-minor axis (20".50) sin β.

→ aberration; → orbit.

Of or pertaining to → aberration.

→ aberration; → -al.

A → Besselian day number denoted by C or D.

→ aberration; → -al; → day; → number.

The → locus of points on the → celestial sphere occupied by a star during the annual → revolution of the → Earth about the → Sun due to → annual aberration. annual aberration.

→ aberrational; → ellipse.

The → erosion of a surface through a process such as → vaporization or → friction.

L. ablatio, ablation, from ablatus, from ab- "away" + latus "carried."

Farsâb from far-, prefix denoting "abundance, excess" + sâb present stem of sâbidan "to rub, wear out," variants sâyidan, pasâvidan "to touch," Khotanese sauy- "to rub," Sogdian ps'w- "to touch," Proto-Iranian *sau- "to rub."

The stopping of a process; a result of such termination.

Verbal noun of → abort.

For a body that moves with respect to a rotating → reference frame, the vector sum of the observed acceleration, the → Coriolis acceleration, and the → centrifugal acceleration. See also the → Coriolis theorem.

→ absolute; → acceleration.

1) General: The process or fact of absorbing.
2) Physics: The action of energy or matter penetrating or being assimilated into a body of matter with no reflection or emission. → adsorption, → desorption, → sorption.
See also: → absorption band, → absorption coefficient, → absorption curve, → absorption feature, → absorption line, → absorption nebula, → absorption spectrum, → atmospheric absorption, → discrete absorption component, → foreground absorption, → intergalactic absorption, → internal absorption, → interstellar absorption, → mass absorption coefficient, → photoabsorption, → selective absorption, → self-absorption.

Verbal noun of → absorb; → -tion, from L. absorptionem.