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

A → number system in which the value of each digit is determined by which place it appears in the full number. The lowest place value is the rightmost position, and each successive position to the left has a higher place value. In the → number system conversion, the rightmost position represents the "ones" column, the next position represents the "tens" column, the next position represents "hundreds", etc. The values of each position correspond to powers of the → base of the number system. For example, in the usual decimal number system, which uses base 10, the place values correspond to powers of 10. Same as → place-value notation and → positional notation. See also → number system conversion.

→ positional; → number; → system.

An empirical model developed by Ptolemy about 150 A.D., in which a motionless Earth was the center of the Universe. The Sun, Moon, and planets revolved around the Earth in → eccentric circles and → epicycles. The fixed stars were attached to an outer sphere concentric with Earth. The Ptolemaic system gave the positions of the planets accurately enough for naked-eye observations, although it also had serious defects. As an extreme example, according to Ptolemy's model for the Moon, our satellite should appear to be almost twice as large when it is full than it is at quadrature, which is an absurdity since it is not seen as such.

Claudius Ptolemaeus was a mathematician, geographer, astronomer, and astrologer. The most influential of Greek astronomers, he lived in Roman Egypt, and was probably born there; he died in Alexandria in 165 A.D.; → system.

A stellar system consisting of four stars orbiting around a common → center of mass.

→ quadruple; → system.

A → physical system that is specified by a → wave function.

→ quantum; → mechanical; → system.

A subsystem if its intrinsic energy is large, on the average, with respect to the energy of its interaction with other portions of the → closed system.

→ quasi-; → closed; → subsystem.

An optical system that is corrected for → distortion and → spherical aberration and therefore forms the image of a straight line as a straight line.

→ rectilinear; → system.

A database management system in which a relational data structure is used.

→ relational; → system.

A → dynamical system that has regained its → equilibrium. Especially a system in which the kinetic and potential energies obey a specific relationship known as the → virial theorem.

P.p. from relax, → relaxation.

→ planetary ring system.

→ ring; → system.

A → number system in which letters represent numbers, still used occasionally today. The cardinal numbers are expressed by the following seven letters: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000). If a numeral with smaller value is written on right of greater value then smaller value is added to the greater one. If it is preceded by one of lower value, the smaller numeral is subtracted from the greater. Thus VI = 6 (V + I), but IV = 4 (V - I). Other examples are XC (90), CL (150), XXII (22), XCVII (97), CCCXCV (395). If symbol is repeated then its value is added. The symbols I, X, C and M can be repeated maximum 3 times. A dash line over a numeral multiplies the value by 1,000. For example V^- = 5000, X^- = 10,000, C^- = 100,000, and DLIX^- = 559,000.

→ numeral; → system.

Same as → semidetached binary.

→ semi-; detached, p.p. of detach, from O.Fr. destachier (Fr. détacher), from des- "apart," + -tachier (as in atachier "to attach"); → system.

Râžmân, → system; nim-jodâ, from nim-→ semi- + jodâ "separate," from Mid.Pers. yut "separate, different;" Av. yuta- "separate, apart."

A number system whose base is 60. It originated with the ancient Sumerians around 2000 B.C., was transmitted to the Babylonians, and is still used in modified form for measuring time, angles, and geographic coordinates.

→ sexagesimal; → system.

The system of → SI units.

→ SI units.

A term introduced by the → International Astronomical Union (August 2006) to name the → solar system bodies other than → planets and → dwarf planets. Small solar system bodies include → asteroids, → comets, and → meteoroids.

→ small; → solar; → system; → body.

The collective name for the Sun and all objects gravitationally bound to it. These objects are the eight planets, their 166 known moons, five dwarf planets, and billions of small bodies. The small bodies include asteroids, icy Kuiper belt objects, comets, meteoroids, and interplanetary dust. The solar system is roughly a sphere with a radius greater than 100,000 AU. Planets, satellites, and all interplanetary material together comprise only about 1/750 of the total mass. Geochemical dating methods show that the solar system chemically isolated itself from the rest of the Galaxy (4.7 ± 0.1) × 10⁹ years ago.

→ solar; → system.

Same as → protosolar abundance.

→ solar system; → abundance.

Photometric system used as a reference.

→ standard; → system.

Same as → stellar system.

→ stellar; → system.

A system comprised of a group of stars bound by → gravitational attraction. Same as → star system.

→ stellar; → system.

A → photometric system, also called the → uvby system.

→ Stromgren sphere; → system.