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Vocabulary Word Definition CHAPTER 12 the branch of science that involves observations and explanations of Astronomy events and objects that occur beyond Earth and its atmosphere a scientist who studies astronomy; different astronomers study Astronomer different aspects of astronomy Rotation the spinning of an object around an imaginary line called an axis the motion of an object in a circular or elliptical path (usually around Revolution another object) Photoperiod the number of hours of daylight between sunrise and sunset Planet a large object that orbits the Sun the apparent slowing, reversal, and then Retrograde motion looping of a planet in its path across the sky Precession the changing direction of Earth’s axis the point at which Earth is closest to the Sun; occurs around Perihelion January 3 the point at which Earth is farthest from the Sun; Aphelion occurs around July 4 Constellation a group of stars that form a pattern an imaginary rotating sphere upon which lie the all the objects Celestial sphere in the universe a circle on the celestial sphere; the extension of Earth’s Celestial equator equator projected into the universe the apparent path the Sun takes through the Plane of the ecliptic sky, as marked by the 12 constellations of the zodiac the two dates when the Sun, on the ecliptic, crosses the celestial equator; occurring around March 21 and September 22, and known as the Equinoxes spring and fall equinoxes, respectively. On these days, the number of hours the Sun is above the horizon and below the horizon is equal. occur when the Sun reaches its highest and lowest positions in the sky, when Earth is tilted closer to or farther away from the Sun (due to the Solstices 23.5° axis tilt). The summer solstice usually occurs on June 21, marking the first day of summer and the longest day of the year. The winter solstice usually occurstoonconstellations December 21around and marks the beginning of winter and that the shortest refers the pole; refers to constellations Circumpolar never day of disappear the year. below the horizon as Earth rotates refers to the Moon’s phases, which can be divided into eight distinct stages results during a new moon, when the Moon, at one of its nodes, is Solar eclipse directly between Earth and the Sun Lunar cycle Lunar eclipse Nodes Tides Nebula results when Earth is positioned between the Sun and the Moon such that Earth casts its shadow on the surface of the Moon; the Moon must be at a node the points at which the Moon’s orbital plane intersects Earth’s orbital plane; eclipses occur only at lunar nodes the alternate rising and falling of the surface of large bodies of water; caused by the interaction between Earth, the Moon, and the Sun refers to a collection of dust and gases, consisting mainly of hydrogen and helium gases and, to a lesser extent, grains of solid matter such as iron, rock, and ice small clumps of matter, forced together by gravity, that condense to form a planet the name given to the four planets beyond Mars: Jupiter, Saturn, Outer planets Uranus, and Neptune; also known as the outer planets; their atmospheres consist mainly of hydrogen and helium gases Protoplanet the name given to the four planets beyond Mars: Jupiter, Saturn, Gas giants Uranus, and Neptune; also known as the outer planets; their atmospheres consist mainly of hydrogen and helium gases the name given to the four planets closest to the Sun: Mercury, Venus, Earth, and Mars; also known as the terrestrial planets. They Inner planets resemble Earth in that they are small and have densities similar to most rocks, about 5 g/cm3 the name given to the four planets closest to the Sun: Mercury, Venus, Earth, and Mars; also known as the inner planets. They Terrestrial planets resemble Earth in that they are small and have densities similar to most rocks, about 5 g/cm3 refers to the non-planetary matter in the solar system; includes asteroids, meteoroids (meteors and meteorites), and comets rocky and metallic object that revolves around the Sun but is too small Asteroid to be called a planet; sometimes called minor planets, most are settled in the asteroid belt Meteoroid lump of rock or metal that encounters Earth as it travels through space smaller chunk of solid matter that burns up in Earth’s atmosphere due to Meteor friction, creating a bright streak of light; often called a shooting star Minor bodies Meteorite larger meteor that actually reaches Earth’s surface chunk of frozen matter that orbits the Sun in very long elliptical paths; Comet some visit the Sun once; come from the Oort Cloud and the Kuiper Belt the current theory that explains the formation of the Moon; it states that, approximately 4.5 billion years ago, the Moon was formed as a Big Splash Theory result of an object approximately the size of Mars colliding with Earth, causing less dense portions to be blasted into space Astronomical Unit defined as the distance from Earth to the Sun, 150 million kilometres Astrolabe a historical instrument used by astronomers to determine the distance to stars; also used as a navigation tool Earth-centred; refers to the Earth-centred model of the universe, Geocentric which places a motionless Earth at the centre with all the planets and stars at fixed positions within eight concentric spheres that spin in circles called orbits Sun-centred; refers to the Sun-centred model of our solar system, Heliocentric in which all planets revolve around the Sun three laws that state: (1) the planets travel in elliptical orbits around an off-centre Sun, with the Sun Kepler’s Laws of Planetary at one focal point; Motion (2) the speed of a planet’s orbit depends on its distance from the Sun; (3) the farther a planet (or dwarf planet) is from the Sun, the longer its orbit.