An Etymological Dictionary of Astronomy and Astrophysics
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The postulate that the concentrations of matter in the Universe follow a → fractal structure over a wide range of scales.

→ fractal; → cosmology.

A mathematical logic that recognizes more than simple → true and → false → propositions. With fuzzy logic, propositions can be represented with degrees of truthfulness and falsehood. In this system, → truth values are → fuzzy sets without sharp boundaries (→ crisp set) in contrast with → classical logic. Fuzzy logic is applied to a wide range of problems including: industrial control, domestic goods, decision making, robotics, intelligent machines, and image processing in medicine.

→ fuzzy; → logic.

An engineering system which uses → fuzzy logic. It generally consists of four main components: → fuzzification interface (fuzzifier), → fuzzy rule base, → fuzzy inferencing unit, and → defuzzification interface (difuzzifier). Also called → fuzzy inference system.

→ fuzzy; → logic; → system.

A model of the Universe in which the Earth is centrally located and the Sun, planets, and stars revolve around the Earth.

→ geocentric; → cosmology.

Of, pertaining to, or based on → geology. Also geological.

From geolog(y), → geology, + → -ic.

The long span of time from the end of the formation of Earth during which our planet underwent its major transformations.

→ geologic; → time.

The scientific study of the composition, structure, and physical history of the Earth.

→ geo- + → -logy.

A compilation of all known stars within the solar neighborhood with accurately known distances. The first version, Catalogue of Nearby Stars, published in 1957, contained nearly 1000 stars located within 20 pc of Earth, listing their known properties. Gliese published an updated version in 1969, extending the range out to 22 pc. He published the second edition of the catalog in 1979 in collaboration with Hartmut Jahreiss. The combined catalog is now commonly referred to as the Gliese-Jahreiss catalog.

Wilhelm Gliese (1915-1993), a German astronomer who worked at the Heidelberg observatory; → catalog.

A member of a group of five chemical elements having closely related and similar properties. The halogens are: fluorine, chlorine, iodine, bromine, and astatine. They make up Group 17 of the → periodic table and can be found on the left-hand side of the → noble gases.

From Gk. halo- prefix from Gk. hals "salt" + → -gen.

A model of the Universe in which the Sun was centrally located.

→ heliocentric; → cosmology.

The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. See also → asteroseismology and → stellar pulsation. The surface of the Sun vibrates much like a bell. A piano has 88 keys or musical tones, whereas the Sun has millions of notes. These vibrations are the result of internal pressure waves that reflect off the → photosphere and repeatedly cross the solar interior. They are detected through the → Doppler shift of absorption lines formed in the photosphere. Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun's surface. The mean period of the vibrations is about five minutes, which corresponds to a speed of 0.5 km s^-1 or a frequency of about 3 mHz. See also → p mode.

From → helio- + → seismology.

A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.

→ hierarchical; → cosmology.

A three-dimensional image produced with the technique of → holography.

→ holo-; → -gram.

Of, relating to, or produced using → holography; three-dimensional.

→ holo- + → -graphic.

A → diffraction grating produced from a series of constructive → interference fringes. The fringes, whose intensities vary in a sinusoidal pattern, correspond to the grooves of the grating. They are recorded on a photosensitive substrate and subsequently treated using a chemical procedure. Since the grooves are created by the interference of light, such a grating is free from the random and periodic errors present in → ruled gratings.

→ holographic; → grating.

A technique for making three-dimensional images by recording → interference patterns from a split → laser beam on a medium such as photographic film. One of the → coherent beams irradiates the object, the second beam illuminates a recording medium. The two beams produce an interference pattern, called → hologram, on the film. The hologram contains information on both → phase and → amplitude of the object. However, this information is in a coded form, and the image must be reconstructed. When the object is removed and the hologram is illuminated by the laser from the original direction, a 3-dimensional image of the object appears where the object was originally, as if it were not removed. The visible object seems so real that the observer can detect → parallax by changing the position of one's head.

From → holo- "whole" + → -graphy. By using the term holography, Dennis Gabor (1900-1979), the Hungarian-British electrical engineer and inventor, wanted to stress that the technique records complete information about a wave, both about its amplitude and its phase, in contrast to the usual photography in which only the distribution of the amplitude is recorded.

The Clock. A faint constellation in the southern hemisphere, at about 3h right ascension, 55° south declination. Its brightest star, α Horologii, is of magnitude 3.9. Abbreviation: Hor; Genitive: Horologii.

Horologium "clock," from L., from Gk. horologion, from horolog(os) "timeteller," from horo-, combining form of hora "hour" (→ year) + -log-, stem of legein "to speak, tell" (+ -os adj. suffix) + -ion diminutive suffix.
Originally named Horologium Oscillitorium by Abbé Nicolas Louis de Lacaille (1713-1762) to honour the inventor of the pendulum clock, Christian Huygens (1629-1695).

Sâ'at "clock," from Ar.

The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas; often called the water cycle.

Hydrologic, pertaining to → hydrology; → cycle.

The study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface.

Hydrology, from → hydro-; + → -logy.

Either of two catalogues of non-stellar objects, which serve as supplements to the → New General Catalogue.

→ index; → catalog