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### 25 Cards in this Set

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 center of mass The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces. (p. 69) circular velocity The velocity an object needs to stay in orbit around another object. (p. 68) closed orbit An orbit that returns to the same starting point over and over. Either a circular orbit or an elliptical orbit. (p. 69) deferent In the Ptolemaic theory, the large circle around Earth along which the center of the epicycle was thought to move. (p. 51) eccentricity, e A number between 1 and 0 that describes the shape of an ellipse. The distance from one focus to the center of the ellipse divided by the semimajor axis. (p. 59) ellipse A closed curve around two points called the foci such that the total distance from one focus to the curve and back to the other focus remains constant. (p. 58) epicycle The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (the deferent) around Earth. (p. 51) equant In the Ptolemaic theory, the point off center in the deferent from which the center of the epicycle appears to move uniformly. (p. 51) escape velocity The initial velocity an object needs to escape from the surface of a celestial body. (p. 69) geocentric universe A model universe with Earth at the center, such as the Ptolemaic universe. (p. 50) geosynchronous satellite A satellite that orbits eastward around Earth with a period of 24 hours and remains above the same spot on Earth's surface. (p. 68) heliocentric universe A model of the universe with the sun at the center, such as the Copernican universe. (p. 49) hypothesis A conjecture, subject to further tests, that accounts for a set of facts. (p. 59) inverse square relation A rule that the strength of an effect (such as gravity) decreases in proportion as the distance squared increases. (p. 66) mass A measure of the amount of matter making up an object. (p. 66) natural law A theory that is almost universally accepted as true. (p. 59) neap tide Ocean tide of low amplitude occurring at first- and third-quarter moon. (p. 71) open orbit An orbit that carries an object away, never to return to its starting point. (p. 69) paradigm A commonly accepted set of scientific ideas and assumptions. (p. 54) parallax The apparent change in position of an object due to a change in the location of the observer. Astronomical parallax is measured in seconds of arc. (p. 50) retrograde motion The apparent backward (westward) motion of planets as seen against the background of stars. (p. 50) semimajor axis, a Half of the longest diameter of an ellipse. (p. 58) spring tide Ocean tide of high amplitude that occurs at full and new moon. (p. 71) theory A system of assumptions and principles applicable to a wide range of phenomena that have been repeatedly verified. (p. 59) uniform circular motion The classical belief that the perfect heavens could only move by the combination of uniform motion along circular orbits. (p. 50)