An Etymological Dictionary of Astronomy and Astrophysics
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1) A transformation that preserves angles and changes all distances in the same ratio.
2) A transformation of the form B = X^-1AX relating two → square matrices A and B.

→ similarity; → transformation.

A fraction in which the → numerator and → denominator are positive → integer numbers. Compare → compound fraction.

→ simple; → fraction.

The motion of a body subjected to a restraining force which is directly proportional to the displacement from a fixed point in the line of motion. The equation of simple harmonic motion is given by x = A sin(ωt + θ₀), where x is the body's displacement from equilibrium position, A is the → amplitude, or the magnitude of harmonic oscillations, ω is the → angular frequency, t is the time elapsed, and θ₀ is the → initial phase angle.

→ simple; → harmonic; → motion.

An oscillator whose force is proportional to its extension, according to → Hooke's law. The way the oscillator moves is called → simple harmonic motion.

→ simple; → harmonic; → oscillator.

A set of stars resulting from a spatially (≤ few pc) and temporally (≤ Myr) correlated star formation event.

→ simple; → population.

The act of simplifying, or the fact of being simplified.

Verbal noun of → simplify.

The construction of a mathematical model to reproduce the characteristics of a phenomenon, system, or process, often using a computer, in order to infer information or solve problems.

Verbal noun of → simulate.

A radio astronomical observation which uses only one antenna, in contrast to interferometric observations.

→ single; → dish; → observation.

An atom that has lost one electron and has become a positive ion.

→ single; → ionized; → atom.

A carbon atom → singly ionized by a photon of energy 11.3 eV. The ion C⁺ emits a → fine-structure line (²P_3/2→ ²P_1/2) at 157.7 μm when excitation conditions are satisfied (critical density ~ 3 x 10³ cm^-3). In → photodissociation regions, [C II] 157.7 μm is a major cooling line for regions exposed to significant → far ultraviolet (FUV) photon fluxes. In Galactic → H II regions, as well as in the central regions of external galaxies, the luminosity of the [C II] line is typically ~ 0.05-0.5% of the FUV luminosity and correlates well with → carbon monoxide (CO) line intensities.

→ single; → ionized; → carbon.

Of partial differential equations, the solution which cannot be obtained from the general solution by particular choice of arbitrary functions. → general solution; → particular solution.

→ singular; → solution.

A ∩-shaped tube with unequal arms that is used to move a liquid from one level to a lower level via a third level higher than either. Once the short arm is filled, for example, by suction, the liquid flows down in the long arm under the action of gravity due to mass excess in it.

From Fr. siphon, from L. sipho (genitive siphonis), from Gk. siphon "pipe, tube," of unknown origin.

The process of choosing a site for an astronomical observatory based on meteorology, seeing conditions, and access to the site.

→ site; → selection.

1) The manner of being placed with respect to surroundings.
2) Momentary state; a set of circumstances.

Verbal noun of → situate.

The act or instance of removing the contribution of non-related, intervening foreground light to the object.

→ sky; → subtraction.

In → supersymmetry theories, a hypothetical → boson super-partner of a → lepton. See also → squark

s from → supersymmetry; → lepton.

A neutron whose kinetic energy does not exceed about 10 electron-volts. Also called → thermal neutron.

→ slow; → neutron.

An asteroid taxonomy built on the → Tholen classification but based on the presence or absence of → absorption features in the visible part of the spectrum. In many cases the two classifications are the same, but the Tholen C and S classes are subdivided in the SMASS classification.

SMASS stands for the Small Main-belt Asteroid Spectroscopic Survey, → small; → main belt; → asteroid; → spectroscopic; → survey; → classification.

A method allowing for a simplified solution to the → radiative transfer equation at frequencies of spectral lines in media moving with a high velocity gradient. This method assumes that the macroscopic velocity gradients are more important than local random variations of thermal line width: dv/dr > v_th/l, where dv/dr is the velocity gradient, v_th is the thermal broadening of the line, and l the length scale. The Sobolev approximation is only valid if the conditions of the gas do not change over the → Sobolev length. Under the Sobolev approximation, each point in the medium is isolated from other points, and the → radiative transfer problem becomes a local one and therefore much easier to solve.

Named after the Russian astronomer Viktor Viktorovich Sobolev, Moving Envelopes of Stars [in Russian], Leningr. Gos. Univ., Leningrad (1947) [translated by S. Gaposchkin, Harvard Univ. Press, Cambridge, Mass. (1960)]; → approximation.

A continuing process whereby an individual acquires a personal identity and learns the norms, values, behavior, and social skills appropriate to his or her social position (Dictionary.com).

→ socialize; → -tion.