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geocentric a description of the solar system in which all of the planets revolve around Earth (p51) heliocentric a description of the solar system in which all of the planets revolve around the sun (p52) ellipse an elongated circle, or oval shape; the shape of the planets' orbits (p53) inertia the tendency of a moving object to continue in a straight line or a stationary object to remain in place (p53) nuclear fusion the process by which hydrogen atoms join together to form helium, releasing energy (p56) core the central part of the sun, where nuclear fusion occurs (p56) photosphere the inner layer of the sun's atmosphere (p57) chromosphere the middle layer of the sun's atmosphere (p57) corona the outer layer of the sun's atmosphere (p57) solar wind a stream of electrically charged particles produced by the sun's corona (p58) sunspot a dark area of gas on the sun that is cooler than surrounding gases (p58) prominence a loop of gas that protrudes from the sun's surface, linking parts of sunspot regions (p60) solar flare an explosion of hydrogen gas from the sun's surface that occurs when loops in sunspot regions suddenly connect (p60) terrestrial planets the name given to the four inner planets; Mercury, Venus, Earth, and Mars (p62) retrograde rotation the spinning motion of a planet from east to west, opposite to the direction of rotation of most planets and moons (p65) greenhouse effect the trapping of heat by a planet's atmosphere (p66) gas giant the name given to the first four outer planets; Jupiter, Saturn, Uranus, and Neptune (p70) comet a ball of ice and dust whose orbit is a long, narrow ellipse (p80) asteroid belt the region of the solar system between the orbits of Mars and Jupiter, where many asteroids are found (p82) meteor a streak of light in the sky produced by the burning of a meteoroid in Earth's atmosphere (p83) asteroids objects revolving around the sun that are too small and too numerous to be considered planets (p82) meteoroid a chunk of rock or dust in space (p83) meteorite a meteoroid that has hit Earth's surface (p83) extraterrestrial life life that arises outside of Earth (p84)