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

The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted.

→ first; → dredge-up.

The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system.

→ first; → law; → thermodynamics.

The first astronomical observation done with a major newly built telescope.

→ first; → light.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

→ first; → point; → Aries.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon's disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.

→ first; → quarter.

A member of the → Population III → massive stars that formed some 500 Myr after the → Big Bang. First stars had a simple chemical composition consisting only of H, He, and traces of ⁷Li and were very short-lived. They are the most likely sources of → reionization of the Universe, which put an end to the → Cosmic Dark Age.

→ first; → star.

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

→ first; → order; → differential; → equation.

A system of → formal logic that is an extension of → propositional logic. It is also known as → first-order predicate calculus and → predicate logic. FOL eliminates deficiencies of propositional logic by: representing → objects (their properties, relations and statements about them), introducing → variables, and introducing → quantifiers.

→ first; → order; → predicate; → logic.

For a → function with several → variables, the derivative, done only once, with respect to one of the variables. For example ∂u/∂x for u = u(x,y).

→ first; → order; → partial; → derivative.

Same as → first-order logic.

→ first; → order; → predicate; → calculus.

An optical spectrum, produced by a diffraction grating, in which the difference in path length of light from adjacent slits is one wavelength.

→ first; → order; → spectrum.

A limbless cold-blooded vertebrate animal with gills and fins living wholly in water (OxfordDictionaries.com).

M.E. fis(c)h, fyssh, O.E. fisc; cognate with Du. vis, Ger. Fisch, O.Norse fiskr, Goth. fisks; akin to L. piscis; PIE root *pisk- "a fish."

Mâhi "fish," from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-.

Any material that is capable of undergoing → nuclear fission by → thermal neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239. Although sometimes used as a synonym for → fissionable material, this term has acquired a more restricted meaning.

From L. fissilis, from fiss(us), + -illis a suffix of adjectives expressing capability, susceptibility, liability, aptitude, etc.

Šekâftani, from šekâft, → fission + -i a suffix expressing capability, aptitude, etc.

An isotope that is capable of undergoing nuclear fission after capturing either fast neutron or thermal neutron. Typical fissionable isotopes: ²³⁸U, ²⁴⁰Pu, but also ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu

→ fissile; → isotope .

1) The act or process of splitting or breaking into parts.
2) Splitting of the nucleus of an atom into two or more fragments of comparable size, usually as the result of the impact of a neutron on the nucleus. Same as → nuclear fission.

Fission, from L. fissionem "a breaking up, cleaving," from root of findere "to split."

Šekâft, stem of šekâftan "to split, break, tear," akin to kaftan, kâftan "to split; to dig," Parthian Mid.Pers. q'f- "to split;" Sogdian kβ "to split;" Chorasmian kf- "to split, be split;" Proto-Iranian *kap-, *kaf- "to split."

Nuclides generated by the fission of higher mass elements or by subsequent radioactive decay of nuclides directly generated by fission.

→ fission; → product.

A theory that suggests the Moon was formed at the same time as Earth. A spinning Earth ejected a large piece of its material into space which then developed into the shape and orbit of the Moon. This event was also thought to be at the origin of the Pacific Ocean. This first modern idea about the formation of the Moon is due to George Darwin, the son of the great naturalist Charles Darwin. The fission theory explained the lack of volatile substances on the Earth. The volatile materials on the Earth would have been thrown out into space The fission theory is almost completely abandoned today. The analysis of lunar rocks brought to Earth by NASA astronauts showed that the Moon rocks are older than the rocks at the bottom of the Pacific Ocean. Moreover, modern → plate tectonics gives a better explanation of the origin of the Pacific Ocean. See also → giant impact hypothesis, → capture theory, → co-formation theory.

→ fission; → theory.

The material that can be fissioned by fast neutrons, such as uranium-238. Commonly used as a synonym for → fissile material.

From → fission + → -able.

1) (v.) To pass a mathematical line or curve through data points.
2) (n.) The act of fitting.

From M.E. fitten; akin to M.Du. vitten "to befit."

Saz, from sazidan "to suit, fit, be worthy," sazâ "suitable, agreeing with, congruous, deserving of," Mid.Pers. sacitan/sazidan "to fit," sazešn "fitness," sazâg "fitting, worth;" Av. ¹sak- "to understand or know a thing; to mark;" cf. Skt. śak- "to be able, powerful" śakta- "able, competent," śakti- "ability, power;" alternatively from Av. ²sak- "to go by, pass, pass away; to be up or over (of time)."