An Etymological Dictionary of Astronomy and Astrophysics
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The setting of a star at nightfall. → heliacal setting.

→ acronical; → setting.

A set → if and only if (iff) for any two points in the → set, the line through them is contained in the set. In other words, for any two points in the set, their → affine combinations are in the set itself.

→ affine; → set.

A short distance from the target, in right ascension, where the telescope is pointed for various purposes.

→ alpha; → offset.

The instant of time when the object is in the West and the geometric → zenith distance is equal to 90° plus the → horizontal refraction plus the semidiameter minus the → parallax.

→ apparent; → setting.

In → CCD detectors, same as → bias and → offset.

→ bias; → offset.

Quantum mechanics: A set of states such that any state can be represented as a superposition of them.

→ complete; → set.

A classical set that allows only full membership or no membership at all for its elements, as contrasted with a → fuzzy set. Crisp set can be viewed as a restricted case of the more general fuzzy set concept.

A short distance from the target, in → declination, where the → telescope is pointed for various purposes.

→ delta; → offset.

The process whereby a → galaxy evolves from a disturbed to an ordered system, as it develops into a → rotation dominated → settled disk.

→ disk; → settling.

A process occurring in → protoplanetary disks whereby relatively large → dust grains settle to the disk midplane. These grains, with sizes from → micron to → millimeter, grow from smaller dust grains and will eventually give rise to a planetary system.

→ dust; → settling.

A set containing no → elements.

→ empty; → set.

A → collection of → subsets of a set.

→ family; → set.

A → set whose elements can be numbered from 1 to n, for some positive integer n.

→ finite; → set.

A set of → truth values in → fuzzy logic that does not have sharp boundaries. Instead, set members have degrees of membership. If the value of 1 is assigned to objects entirely within the set and a 0 is assigned to objects outside of the set, then any object partially in the set will have a value between 0 and 1. This contrast with → crisp sets in → classical logic where members assume a precise value of 1 or 0. Fuzzy sets were first introduced by Lotfi A. Zadeh (1965) and defined as follows. Let X be a space of points, with a generic element of X denoted by x. Thus X = {x}. A fuzzy set A in X is characterized by a → membership function  f_A(x) which associates with each point in X a real number in the interval [0,1], with the values of f_A(x) at x representing the "grade of membership" of x in A. Thus, the nearer the value of f_A(x) to unity, the higher the grade of membership of x in A. Generally, the intersection operations of fuzzy sets are the expansion of that operation on → nonfuzzy sets. In other words, operations on nonfuzzy sets are a particular case of operations on fuzzy sets.

→ fuzzy; → set.

A physical process occurring in → stellar atmospheres whereby in a very stable atmosphere → heavy elements are gravitationally pulled down preferentially. If such an atmosphere is stable for long periods of time, the → absorption lines of heavy elements may therefore become very weak. Observationally, the star seems to contain only → hydrogen and → helium. Gravitational settling takes place in the Sun at the bottom of the outer → convective zone where helium is dragged down, leading to a surface He abundant smaller than the cosmic value. It occurs also in the atmospheres of → brown dwarfs and → planets. See also → radiative levitation, → element diffusion, → thermal diffusion.

→ gravitational; → settling.

The last visible setting of a star below the western horizon just after sunset entering into a conjunction with the Sun.

→ heliacal; → setting.

A set which can be put in a one-to-one correspondence with part of itself.

→ infinite; → set.

A set of points in the complex plane, the boundary of which forms a fractal with varying shapes at different magnifications. Mathematically, it is the set of all C values for which the iteration z_n+1 = z_n² + C, starting from z₀ = 0, does not diverge to infinity.

Discovered by Benoît Mandelbrot (1924-) a Polish-born French mathematician, best known as the "father of fractal geometry;" → set.

The crossing of the visible horizon by the upper limb of the descending Moon.

→ moon; → set.

The delay between → sunset and → moonset.

→ moonset; → lag.