Aberration The apparent displacement of a star owing to the orbital motion of earth and the bending of light rays from the star. As the Earth travels around the sun, the aberration causes the star to appear to trace an ellipse about its true position. Absorption Nebula A nebula seen in silhouette because I is absorbing or blocking light from behind. It is also called a dark nebula. Accretion The process where small particles coalesce by collisions or mutual gravitational pull, creating larger bodies. Accretion is suspected as a major process in the formation of the planets and satellites in any solar systems. Albedo The proportion of light reflected from a celestial body. The moon reflects only about 7 percent of the sunlight falling on it, whereas the albedo of Venus is more than 70 percent owing to its heavy cloud cover, which reflects as greater proportion of light. Altitude Number of degrees above the horizon of an object on the celestial plane. Aphelion The point in an object's orbit that is farthest from the sun. See also perihelion. Apogee The point in the Moon's orbit (or any other orbiting body, such as an artificial satellite) when it is farthest from the Earth. See also Perigee. Asterism A pattern of stars that does not constitute one of the 88 official constellations. For example, the Big Dipper in the constellation of Ursa Major is an asterism. Asteroid A small, rocky object or minor planet that orbits the sun. Most asteroids have orbits in the asteroid belt, located between the orbits of Mars and Jupiter. More that 5,000 of these objects have been identified, and it is like that thousands more exist undetected. Astrometry The measure of the positions and apparent motions of celestial objects and the attempt to understand the factors that influence such movements. Astronomical Unit An astronomical distance, equal to the average distance from the Earth to the Sun, or about 93,000,000 (150,000,000 kilometers). Big Bang Model A theory that describes the beginning of our universe as a titanic explosion. This explosion did not occur at a particular point in space, according to the theory, but rather was a transition from enormous density and temperature throughout all space to conditions of even lower density and lower temperature as space itself expanded. After the hypothetical explosion, the universe was swamped with energy in the form of radiant energy and various atomic particles. This phase was followed by a cooling and thinning out of the universe. It is believed that the universe is still expanding at this time. Binary Star Two stars that are gravitationally attracted to each other. See also double star. Celestial Sphere An imaginary sphere to locate the positions and track the motions of all astronomical objects. Comet A small object composed of rock, ice, and gases moving about the sun in an elliptical orbit. A comet has three distinct components: the nucleus, made up of rock and ice; the coma, consisting of gases and dust; and the tail, formed when gases and dust spread out from the nucleus or coma. Short period comets complete their orbits in less than 200 years; long-period comets make take thousands of years to revolve around the Sun, or may never return at all (parabolic orbit). Some astronomers believe that comets originate in the Oort cloud, a hypothetical region or space that lies outside the solar system. Corona The outer envelope, or "atmosphere," of gas surrounding the sun, possibly extending to the orbit of Earth. During an eclipse of the Sun, the corona may be visible around the edges of the moon. It has a density that is about one-millionth that of the atmosphere of Earth. Cosmogony The study of how the universe was formed. Cosmology The study of the universe at large, of the distribution and behavior of the matter and energy in it, of the laws governing these factors, and of its origin and evolution. Dark Matter Matter that is thought to exist in the universe but has not yet been observed. It is based on measurements of unexplained gravitational effects on visible matter. Declination On the celestial sphere, the coordinate analogous to latitude on the Earth. Declination is measured in degrees, minutes, and seconds of arc north (positive above the celestial equator) or south (negative below the celestial equator). Doppler Effect The phenomenon in which, as a source of waves (i.e., sound or light) and the observer move relative to each other, the emitted wavelength appears to change. In astronomy, Doppler shifts are used to determine the velocity and direction of distant objects. For example, light from a galaxy shifts to the red on the electromagnetic spectrum if the galaxy is moving away from the observer (red shift) and to the blue if the galaxy is moving toward the observer (blue shift). Double star Two stars that appear close together along a line of sight. Double stars may be an optical double, which are stars that just appear to be close as seen from Earth but are physically quite distant from one another; or true binaries, stars that are gravitationally bound to one another. Ecliptic The apparent path of the Sun in the sky as seen from Earth. Fireball A bright meteor, with an apparent magnitude ranging from about-5 to-20 (to compare, the Sun has an apparent magnitude of -26.7). Fireballs are sometimes seen during the day. Galaxy A large system of stars, usually containing between 1 million and 1 trillion stars, along with clouds of gas and dust. Galaxies are sometimes classified according to their shapes as spiral, elliptical, or irregular. Globular Cluster a nearly spherical, dense cluster of hundreds of thousands to millions of stars. Gravitational Collapse The contraction of a star when the pressure of thermonuclear reactions can no longer sustain the force of self-gravitation. Collapse occurs at the end of a star's life when its fuel of hydrogen and other elements is depleted. Depending on its original mass, the star may evolve into a white dwarf, a neutron star, or a black hole, or it may explode as a supernova. Inferior Conjunction The passage of Mercury or Venus between the Earth and the Sun. Libration The effect that allows an observer on Earth to see about 59 percent of the Moon's surface, slightly more than would otherwise be visible. Because the Moon's rotation and orbital period are equal (on average), the Moon always keeps the same face to the Earth. Libration occurs because the Moon's elliptical orbit speed is not constant and its orbit is slightly tilted. Meteor A meteoroid that produces a streak of light as it enters the Earth's atmosphere and is vaporized by the resulting friction. This phenomenon is quiet common when the Earth passes through swarms of meteoroids. The resulting meteor showers are usually associated with a specific constellation and time of year. Meteors are commonly called shooting or falling stars. Meteorite A meteor that passes through the outer layers of the Earth's atmosphere and strikes the planet's surface. The resulting explosive impact buries or disperses the meteorite, leaving a crater behind. Meteorites are classified as siderites (containing only metals, chiefly nickel and iron), aerolites (stony objects consisting of a variety of mineral elements), and siderolites (meteorites composed of both metal and stone). Milky Way The spiral galaxy in which our solar system is located. It contains about 150 billion stars, has a diameter of 500,000 light-years, and is about 12 billion years old. Nadir The point directly below the observer, or 90 degrees below the horizon. See also Zenith. Nebula A concentration of gas and dust in the galaxy. Oblate The shape of a planet or natural satellite that is not completely spherical but bulges in the center and is flattened at the poles. The shape is usually caused by rapid spinning or the gravitational pull form an accompanying moon. For example, rapidly rotating Jupiter is an oblate spheroid. Occulation The crossing of one body in front of another, such as the Moon in front of a star, relative to an observer. Opposition The point in a planet's orbit when it is 180 degrees from the Sun, usually as observed from Earth. Orbit The path of an object around a central body; gravitational attraction keeps the bodies in orbit. Parallax The change in the relative position of an object when viewed from different places; in astronomy, the closer the object the greater the parallax. Perigee The point where the Moon (or any other orbiting body, such as an artificial satellite) is closer in its orbit to the Earth. See also apogee. Perihelion The point in the orbit of an object when it is closer to the Sun. See also aphelion. Perturbation A local gravitational disturbance in the uniform motion of a body because of the gravitational influence of another object. For example, a comet orbiting the Sun can be perturbed by a close encounter with Jupiter, the solar system's largest planet, which influences the orbit of the comet. Planetoid A mostly obsolete term, usually used to describe the larger remnants of rocks left over from the formation of the solar system. Plasma Matter in the form of electrically charged particles; the state in which most of the universe exists. Proper motion The apparent angular motion of an object across the sky, determined as change in position with respect to the background star; caused by the star's true motion and the relative motion of the solar system. Quasar A contraction of the word quasi-stellar used to describe celestial objects with a starlike appearance. Quasars are the most distant objects known. They have large red shifts indicating great recessional velocities and emit energy that is more than a thousand times that of an average galaxy. Red Shift The shift of a spectrum of light toward long, red wavelengths owing to the Doppler effect of recession of a star. He faster an object recedes from Earth, the greater the shift of its light toward the red end of the spectrum. Revolution The movement of an object around a central body. Right ascension The angle of an object eastward from the vernal equinox, along the celestial equator; right ascension is measured in hours, minutes, and seconds. Rotation The movement of a body as it turns on its axis. Sidereal Time Time that is measured by the rotation of Earth with respect to the stars as distinguished from solar time, which is based on the rotation of Earth with respect to the Sun. Solar Wind A stream of particles, primarily proton and electrons, that constantly flows outward from the Sun. Space-time A four-dimensional way of describing events and locations with three units of distance and one of time. Under he influence of gravity, space-time can actually warp and bend. Spectrum Radiation (usually visible light) broken into its component wavelengths. Spectroscope An instrument used to determine the spectrum or wavelength of a ray of light emanating from an object. A spectroscope is often used in astronomy. Star A spherical celestial body consisting of a large mass of hot gas held together by its own gravity. It is self-luminating because of extensive internal nuclear reactions. Our Sun is a typical star. Superior Conjunction For a planet (Mercury of Venus) inside the Earth's orbit, the condition when the planet is behind the Sun, relative to the Earth. Syzygy The condition when three celestial bodies are arranged in a straight line. Syzygy occurs during solar and lunar eclipses, when the Sun, Moon, and Earth are aligned. Terminator The line separating sunlight and darkness on a planet or moon. Transit The movement of a smaller object across the lighted face of a larger object, such as the movement of Mercury across the face of the Sun, or the moon Io across the face of the planet Jupiter. Universe The entirety of all that is known to exist. The size of the observable universe is limited to the distance light has traveled since the Big Bang. Zenith The point directly overhead from the observer, or 90 degrees above the horizon. See Also nadir. Zodiacal Light A faint cone of light seen along the ecliptic at sunset, usually around the time of the equinox. It is caused by sunlight scattering small dust particles that possibly have an interplanetary origin.     COPYRIGHT©2005 VICTORIA PACKING CORP. ALL RIGHTS RESERVED ( CTR-110505-ART)