An Etymological Dictionary of Astronomy and Astrophysics
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The theory formulated by A. Einstein in 1905, which is based on the following two → postulates:
1) → Principle of relativity: The laws of physical phenomena are the same when studied in terms of two reference systems moving at a constant velocity relative to each other.
2) → Principle of constancy: The → velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer.
The term "special theory of relativity" refers to the restriction in the first postulate to reference systems moving at a constant velocity relative to each other (→ inertial reference frame). See also → general relativity.

→ special; → relativity.

An emission or absorption mark in the spectrum of an astronomical object, of known or unknown origin, usually with complex structure.

→ spectral; → feature.

An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.

→ spectral; → region.

The capacity of a spectrograph to separate two adjacent spectral lines. The theoretical spectral resolution depends on the grating dispersion, grating position, pixel size, collimator and camera focal length, and the entrance slit-width.

→ spectral; → resolution.

Domain of the electromagnetic spectrum over which a detector is sensitive. Same as spectral sensitivity.

→ spectral; → response.

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm). SPHERE is capable of obtaining → diffraction-limited images at 0''.02 to 0''.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

→ spectro-; → polarimetric; → high; → contrast; → exoplanet.

The reflection of light waves in which the reflected waves travel in a definite direction, and the directions of the incident and reflected waves make equal angles with a line perpendicular to the reflecting surface. Same as → regular reflection; opposite of → diffuse reflection.

From L. specularis, from speculum "mirror;" → reflection.

Bâztâb, → reflection; âyenevâr "mirror-like," from âyené, → mirror + -vâr similarity suffix.

A solid geometric figure generated by the revolution of a semicircle about its diameter; equation: x² + y² + z² = r².

M.E. spere, from O.Fr. espere, from L. sphæra "globe, ball, celestial sphere," from Gk. sphaira "globe, ball," of unknown origin.

Koré, loan from Ar. kurat.
Sepehr "sphere, celestial globe, heavens, sky;" Mid.Pers. spihr "sphere, sky, firmament, fate;" Av. spiti- in compounds: spiti-dôiθra- "with clear eyes;" Proto-Iranian *spiθra- (in proper name); cf. Skt. śvitrá- "white, whitish."

The region of space around one of the bodies in a system of two celestial bodies where a third body of much smaller mass is influenced by the gravitational field of that body. The sphere of influence of a planet with respect to the Sun has a radius given by: R = R_P(M_P/M_S)^2/3, where R_P is the radius of the planet's orbit around the Sun, M_P is the mass of the planet, and M_S is the solar mass. The sphere of influence of the Earth has a radius of about 927,000 km or slightly under 150 Earth radii. Beyond this limit, a space probe will come under the influence of the Sun.

→ sphere; → influence.

A series of spheres with varying radii centred on the Earth, each rotating uniformly about an axis fixed with respect to the surface of the next larger sphere, all comprising a model in Greek astronomy to describe the motions of the heavenly bodies. The spheres turned with different speeds about axes with different orientations. The fixed stars revolved around the Earth by the motion of the most distant sphere to which the stars were thought to be attached. Each of the five planets' (Mercury, Venus, Mars, Jupiter, and Saturn) motion could be described using four spheres. The Sun and the Moon required three spheres each to explain their motions. Therefore, a total of 27 spheres described the behavior of the heavenly bodies in terms of circular motion. Eudoxus was the first person to devise a model that could explain the → retrograde motion of the planets in the sky along a looped curve known as the → hippopede.

→ sphere; Eudoxus (Ευδοξοσ) of Cnidus (c 408 BC - c 355 BC), Greek astronomer and mathematician.

The → excitation temperature of the → hyperfine structure levels of the → neutral hydrogen→ 21-centimeter line.

→ spin; → temperature.

The morphology of a galaxy which displays → spiral arms.

→ spiral; → structure.

A physical phenomenon whereby a symmetric system becomes permanently asymmetric. A simple example is a ball lying on top of a hill in equilibrium. The hill-ball system is symmetric about the vertical axis through the top of the hill. Moreover, there is no preferred horizontal direction to the system. However, its state is unstable, since the slightest perturbing force will cause the ball to roll down the hill in some particular direction. The system becomes permanently asymmetric because the ball will not roll uphill by itself. Symmetry breaking is found in several fields of physics, for example in → magnetism (→ ferromagnetism), → thermodynamics (→ crystallization), and → particle physics, where it constitutes the basis of → electroweak interactions. In cosmology, according to the → Big Bang model, the fundamental forces of the Universe split off from one another in a form of spontaneous symmetry braking. If a single, unified force existed with a certain symmetry just after the Big Bang, if that symmetry were somehow broken so that the unified force were fractured, then the result might be several fundamental forces. See also → grand unified theory, → theory of everything, → phase transition.

→ spontaneous; → symmetry; → break.

A reproductive body in flowerless plants corresponding to the seeds of flowering ones.

From Modern L. spora, from Gk. spora "a seed, a sowing, seed-time," related to speirein "to sow, scatter."

Hâg, variant of xâg, → egg.

A period of low → solar activity that lasted from about A.D. 1420 to 1570. It occurred before → sunspots had been studied, and was discovered by analysis of the proportion of carbon-14 in tree rings, which is strongly correlated with solar activity.

Named for the German astronomer Gustav Spörer (1822-1895); → minimum.

A period of low → solar activity that lasted from about A.D. 1420 to 1570. It occurred before → sunspots had been studied, and was discovered by analysis of the proportion of carbon-14 in tree rings, which is strongly correlated with solar activity.

Named for the German astronomer Gustav Spörer (1822-1895); → minimum.

The empirical law that predicts the variation of → sunspot latitudes during a → solar cycle. At the start of a sunspot cycle, sunspots tend to appear around 30° to 45° latitude on the Sun's surface. As the cycle progresses, they appear at lower and lower latitudes, until 5° to 10°, at the end of the cycle. This tendency is revealed on a → butterfly diagram. Although named after Gustav Spörer, the "law" was first discovered by Richard Carrington.

→ Sporer minimum; → law.

The empirical law that predicts the variation of → sunspot latitudes during a → solar cycle. At the start of a sunspot cycle, sunspots tend to appear around 30° to 45° latitude on the Sun's surface. As the cycle progresses, they appear at lower and lower latitudes, until 5° to 10°, at the end of the cycle. This tendency is revealed on a → butterfly diagram. Although named after Gustav Spörer, the "law" was first discovered by Richard Carrington.

→ Sporer minimum; → law.

1a) To draw, stretch, or open out, especially over a flat surface, as something rolled or folded (often followed by out).
1b) To stretch out or unfurl in the air, as folded wings, a flag, etc.
2) An act or instance of spreading; expansion, extension, or diffusion (Dictionary.com).

M.E. spreden, from O.E. sprædan "to spread, extend," cf. Dan. sprede, O.Swed. spreda, M.Du. spreiden, O.H.G. and Ger. spreiten "to spread," from PIE root *sper- "to strew."

Gostardan "to spread; to diffuse, to expand," from Mid.Pers. wistardan "to extend; to spread;" Proto-Iranian *ui.star-; Av. vi- "apart, away from, out" (O.Pers. viy- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + Av. star- "to spread," starati "spreads" (cf. Skt. star- "to spread out, extend, strew," strnati "spreads;" Gk. stornumi "I spread out," strotos "spread, laid out;" L. sternere "to spread;" Ger. Strahlung "radiation," from strahlen "to radiate," from Strahl "ray;" from M.H.G. strāle; from O.H.G. strāla "arrow," stripe; PIE base *ster- "to spread").

1) A rectangle having all four sides of equal length.
2) The second power of a quantity, expressed as a² = a × a, where a is the quantity. → inverse square law.

M.E., from O.Fr. esquire "a square, squareness," from V.L. *exquadra, from *exquadrare "to square," from L. → ex- "out" + quadrare "make square," from quadrus "a square," from quattuor→ four.

Câruš, from Av. caθruša- "four sides (of a four-sided figure)", from caθru- "four," Mod.Pers. cahâr, câr "four" + uša- "angle," Mod.Pers. guš, gušé.