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438-6000_D TXT_150229 D TXT 1/7/13 10:07 AM Page 117 Solar System Excerpt from DSM Solar System Teacher’s Guide, © Copyright by Delta Education, a member of the School Specialty Family. Not for resale, redistribution, or use other than classroom use without further permission. G LO S S A RY 117 Alpha Centauri A three star system, the closest to Earth. aphelion The point along a planet’s orbit at which it is farthest from the Sun. asteroid One of millions of very small planets that orbit the Sun. asteroid belt The area between the orbits of Mars and Jupiter where most of the asteroids in our Solar System are concentrated. average distance A measurement used to calculate the difference in distance between planets. axis An imaginary line, or axle, through the center of an object, about which the object rotates. Big Dipper A part of the constellation Ursa Major, the Great Bear. Cassiopeia The W-shaped constellation in the Northern Hemisphere. centimeter A unit of length equal to one hundredth of a meter. Cepheus A constellation near Cassiopeia and Draco. circle A closed curved line on which all points are equidistant from its center. circumference The length of the closed curve of a circle. comet A mass of ice, dust, and rock that orbits the Sun or a planet. constellation An area of the sky containing stars that seem to outline a figure, as viewed from Earth. crater The bowl-shaped depression in the surface of a satellite caused by the impact of an asteroid or comet. day The length of time a planet takes to make one complete rotation about its axis. diameter The distance from one side of a circle, through the focus, to the other side. drawing compass A mechanical device used to draw circles. dwarf planet A spherical satellite of a star that is not massive enough to have cleared the neighborhood of its orbit. Earth The third planet from the Sun in our Solar System. ellipse A shape like a stretched-out circle; an oval. focus (plural: foci) The point at the center of a circle, or the two points inside an ellipse. force A push or pull on an object. friction The force that creates drag on two substances rubbing together. galaxy One of countless swirling, massive clusters of solar systems in the universe. gravitational attraction The force of gravity that causes objects to be attracted to one another. gravity The attractive force that exists between objects. Jupiter The fifth planet from the Sun in our Solar System. light-year A unit of distance, equal to the distance light travels in a year—about 9.46 trillion km. map A scale drawing used to represent geographic locations and their relative distances from one another. Mars The fourth planet from the Sun in our Solar System. mass The measure of the amount of material an object contains. delta science modules Solar System 117 438-6000_D TXT_150229 D TXT 1/7/13 10:07 AM Page 118 Mercury The planet closest to the Sun in our Solar System. meteor A meteoroid falling through a planet’s atmosphere and burning brightly because of friction. meteorite The part of a meteor that crashes into the surface of a satellite. meteoroid One of countless bits of rock and metal, smaller than asteroids, that orbit the Sun or a planet. meter The international standard unit of length. Milky Way The galaxy of which our Solar System is a member. millimeter A unit of length equal to one thousandth of a meter. mnemonic device A memory aid, such as a made-up sentence or rhyme, that helps recall the order of items in a list. moon A satellite of a planet. mythology The study of myths. Neptune The eighth planet from the Sun in our Solar System. North Star (Polaris) The star almost directly above the north pole of the Earth. orbit The elliptical path that a satellite travels as it moves around a star or a planet. Orion A constellation near Gemini and Taurus. perihelion The point along a planet’s orbit at which it is nearest to the Sun. planet A large, spherical satellite of a star. Pluto Once called the ninth planet from the Sun in our Solar System; now classified as a dwarf planet. radius (plural: radii) The distance on a circle from the focus to the circumference. ratio A numerical representation of a relationship between two measurements, as in a scale for a drawing or a model. relative brightness The brightness of an object when compared with the brightness of surrounding objects. relative distance The distance between objects when compared with the distance between surrounding objects. relative size The size of an object when compared with the size of surrounding objects. revolution One complete orbit of a satellite. revolve To travel around another object. rotation One complete turn of a satellite about its axis. satellite An object in orbit around a larger object. Saturn The sixth planet from the Sun in our Solar System. scale A proportion used in determining the relationship between two measurements of size or distance. scale drawing A two-dimensional scaled representation of a smaller or larger object. solar system A star and all the satellites that orbit it. star An extremely large, hot ball of glowing gas that radiates light and heat. Sun The star of our Solar System. system An interdependent group of items that form a unified whole. universe The vast space that surrounds and includes all the galaxies as well as all other existing matter and energy. Uranus The seventh planet from the Sun in our Solar System. Venus The second planet from the Sun in our Solar System. year The length of time a planet takes to make one complete revolution around the Sun. Note: The Delta Science Reader includes its own glossary of terms. 118 glossary 118