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

The highest frequency that can be determined in a Fourier analysis of a discrete sampling of data.

→ Nyquist formula; → frequency.

The minimum number of resolution elements required to properly sample a signal, such as a star image, without causing erroneous effects known as aliasing. For electronic imaging, this number is generally taken as 2 pixels across the seeing disk diameter at the half intensity points. Also called → Shannon's sampling theorem and → sampling theorem.

Named after Harry Nyquist (1889-1976), a Swedish-born American physicist, who made important contributions to information theory, and Claude Elwood Shannon (1916-2001), an American mathematician and pioneer of information theory; → theorem.

The celestial sphere when the circles parallel to the equator are cut obliquely by the horizon plane, which divides them into two unequal parts. In other words, the sphere when its axis is oblique to the horizon of the place.

→ oblique; → sphere.

1a) Lacking in light or illumination; dark; dim; murky.
1b) Indistinct to the sight or any other sense.
1c) Not clear to the understanding.
2) To make dark, dim, indistinct, etc.

From O.Fr. obscur "dark, dim, not clear," from L. obscurus "covered over, dark, obscure," from ob "over" + -scurus "covered," from PIE *(s)keu- "to cover, conceal;" from which derives also the term → sky.

Tiré, from Mid.Pers. târag "dark, turbid," related to târik "dark," Mid.Pers. târig "dark," târ "darkness," Av. taθra- "darkness," taθrya- "dark," cf. Skt. támisrâ- "darkness, dark night," L. tenebrae "darkness," Hittite taš(u)uant- "blind," O.H.G. demar "twilight."
Tiré kardan compound verb with kardan "to do," → -ize.

A written report provided by a visiting astronomer to the observatory describing how his/her observing run has been carried out.

Observing, noun of → observe; → report.

1) The action, fact, or instance of occurring.
2) Something that happens; event; incident (Dictionary.com). 3) Logic: → bound occurrence; → free occurrence.

Verbal noun of → occur.

Any of various natural clays rich in ferric oxides (Fe₂O₃) and containing silica and alumina. Used as a red (→ hematite) or yellow (→ limonite) pigment.

M.E. oker, O.Fr. ocre, from L. ochra, from Gk. okhra, from okhros "pale yellow."

Oxrâ, loan from Gk.

Same as → ubiquitous.

→ omni-; → present.

Preliminary reduction of observational data at a telescope simultaneously with their acquisition.

→ on-line; → reduction.

A relationship between two distinct sets of elements such that every member of the first set can be paired with a unique element in the second set; and every member of the second set can be paired with a unique element in the first set. Two sets so related are said to be isomorphic.

→ one; → correspondence.

To burden with cruel or unjust impositions or restraints; subject to a burdensome or harsh exercise of authority or power (Dictionary.com).

M.E. oppressen, from O.Fr. opresser "oppress, torment, smother," from M.L. oppressare, from L. opprimere "press against, press together, press down; subdue, prosecute relentlessly," from op variant of ob "against" + premere "to press, hold fast."

Infinitive from setam, → oppression.

1) The exercise of authority or power in a burdensome, cruel, or unjust manner.
2) An act or instance of oppressing or subjecting to cruel or unjust impositions or restraints.
3) The state of being oppressed (Dictionary.com).

M.E. oppressioun, from O.Fr. opresser "oppress; torment," from M.L. oppressare, from L. opprimere "press against, press down;" from op, variant of ob "against" + premere "to press, hold fast."

Setam, from Mid.Pers. sthmbk / stambag / "oppressive; obstinate," related to sitabr "strong, firm," staft "hard; firm, strong; fierce," Pers. seft "firm, hard, tight;" sitanbah "strong, robust, bold;" Av. aša.stəmbana- "having the support/firmness of aša;" Lith stembti "to oppose;" Gk. astemphes "unshakable."

An instrument used to test lenses by utilizing the → optical transfer function. It consists of a HeNe laser, a beamsplitter and two mirrors.

→ optical; → autocorrelator.

→ orbital maneuver.

→ orbital; → manoeuvre.

Same as → relativistic precession.

→ orbital; → precession.

The situation in which two orbiting objects exert a regular, periodic gravitational influence on each other and therefore their orbital frequencies are related by a ratio of two small → integers. Orbital resonance often results in an unstable interaction in which bodies exchange momentum and shift orbits until the resonance disappears. The resonance increases the eccentricity until a body approaches a planet too closely and the body is slung away.

→ orbital; → resonance.

A whole number which characterizes a particular position of an interference fringe according to whether there is interference arising from one, two, three, etc. wavelength difference of path. Same as → interference order

→ order; → interference.

Characterized by → order; arranged according to a rule. → ordered tree.

→ order + -ed.

In → graph theory, a → tree in which the → children of each → vertex are ordered.

→ ordered; → tree.

A → differential equation in which the unknown function depends on only one → independent variable, as contrasted with a → partial differential equation.

→ ordinary; → differential; → equation.