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satellite A body that revolves about a planet.
Schwarzschild radius  See event horizon.
scientific method The procedure scientists follow to understand the natural world: (1) the observation of phenomena or the results of experiments; (2) the formulation of hypotheses that describe these phenomena and that are consistent with the body of knowledge available; (3) the testing of these hypotheses by noting whether or not they adequately predict and describe new phenomena or the results of new experiments; (4) the modification or rejection of hypotheses that are not confirmed by observations or experiment.
seeing The unsteadiness of the Earth's atmosphere, which blurs telescopic images. Good seeing means the atmosphere is steady.
semimajor axis Half the major axis of a conic section, such as an ellipse.
SETI The search for extraterrestrial intelligence, usually ap-plied to searches for radio signals from other civilizations.
Seyfert galaxy A galaxy belonging to the class of those with active galactic nuclei; one whose nucleus shows bright emission lines; one of a class of galaxies first described by C. Seyfert.
shepherd satellite Informal term for a satellite that is thought to maintain the structure of a planetary ring through its close gravitational influence.
sidereal period The period of revolution of one body about another measured with respect to the stars.
sidereal time Time on Earth measured with respect to the stars, rather than the Sun; the local hour angle of the vernal equinox.
sidereal year Period of the Earth's revolution about the Sun with respect to the stars.
sign (of zodiac) Astrological term for any of 12 equal sections along the ecliptic, each of length 30 degrees. Because of precession, these signs today are no longer lined up with the constellations from which they received their names.
singularity A theoretical point of zero volume and infinite density to which any object that becomes a black hole must collapse, according to the general theory of relativity.
SNC meteorite One of a class of basaltic meteorites now believed by many planetary scientists to be impact-ejected fragments from Mars.
solar activity Phenomena of the solar atmosphere: sunspots, plages, and related phenomena.
solar antapex Direction away from which the Sun is moving with respect to the local standard of rest.
solar apex The direction toward which the Sun is moving with respect to the local standard of rest.
solar eclipse An eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun. Solar eclipses can occur only at the time of new moon.
solar motion Motion of the Sun, or the velocity of the Sun, with respect to the local standard of rest.
solar nebula The cloud of gas and dust from which the solar system formed. 
solar seismology The study of pulsations or oscillations of the Sun in order to determine the characteristics of the solar interior. 
solar system The system of the Sun and the planets, their satellites, the minor planets, comets, meteoroids, and other objects revolving around the Sun.
solar time A time based on the Sun; usually the hour angle of the Sun plus 12 hours.
solar wind A flow of hot charged particles leaving the Sun.
solstice Either of two points on the celestial sphere where the Sun reaches its maximum distances north and south of the celestial equator; time of the year when the daylight is the longest or the shortest.
spacetime A system of one time and three spatial coordinates, with respect to which the time and place of an event can be specified. 
space velocity or space motion  The velocity of a star with respect to the Sun.
spectral class (or type)  The classification of stars according to their temperatures using the characteristics of their spectra; the types are 0 B A F G K M.
spectral line  Radiation at a particular wavelength of light produced by the emission or absorption of energy by an atom. 
spectral sequence The sequence of spectral classes of stars arranged in order of decreasing temperatures of stars of those classes. 
spectrometer An instrument for obtaining a spectrum; in astronomy, usually attached to a telescope to record the spectrum of a star, galaxy, or other astronomical object.
spectroscopic binary star  A binary star in which the components are not resolved optically, but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion.
spectroscopic parallax A parallax (or distance) of a star that is derived by comparing the apparent magnitude of the star with its absolute magnitude as deduced from its spectral characteristics.
spectroscopy The study of spectra.
spectrum The array of colors or wavelengths obtained when light (or other radiation) from a source is dispersed, as in passing it through a prism or grating. 
spicule A jet of rising material in the solar chromosphere.
spiral arms  Arms (or long denser regions) of interstellar mate-rial and young stars that wind out in a plane from the central nucleus of a spiral galaxy.
spiral density wave A mechanism for the generation of spiral structure in galaxies; a density wave interacts with interstellar matter and triggers the formation of stars. Spiral density waves are also seen in the rings of Saturn.
spiral galaxy A flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars winding out from its nucleus. 
standard bulb An astronomical object of known luminosity; such an object can be used to determine distances.
star cluster An assemblage of stars held together by their mutual gravity.
Stefan-Boltzmann law  A formula from which the rate at which a blackbody radiates energy can be computed; the total rate of energy emission from a unit area of a blackbody is proportional to the fourth power of its absolute temperature.
stellar evolution The changes that take place in the characteristics of stars as they age.
stellar model  The result of a theoretical calculation of the physical conditions in the different layers of a star's interior.
stellar parallax  See parallax.
stellar wind The outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star.
stony-iron meteorite A type of meteorite that is a blend of nickel-iron and silicate materials.
stony meteorite A meteorite composed mostly of stony material.
summer solstice The point on the celestial sphere where the Sun reaches its greatest distance north of the celestial equator; the day with the longest amount of daylight.
sunspot A temporary cool region in the solar photosphere that appears dark by contrast against the surrounding hotter photosphere.
sunspot cycle The semiregular 11-year period with which the frequency of sunspots fluctuates.
supercluster A large region of space (more than 100 million LY across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies.
supergiant A star of very high luminosity and relatively low temperature.
supernova An explosion that marks the final stage of evolution of a star. A Type I supemova occurs when a white dwarf accretes enough matter to exceed the Chandrasekhar limit, collapses, and explodes. A Type 11 supemova marks the final collapse of a massive star.
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