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### 17 Cards in this Set

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 archaeostronomy the study of the astronomy of ancient cultures uniform circular motion the classical belief that the perfect heavens could move only by the combination of constant motion along circular orbits geocentric universe a model universe with Earth at the center, such as the Ptolemaic universe eccentric in astronomy, an off-center circular path heliocentric universe a model of the universe with the sun at the center, such as the Copernican universe parallax the apparent change in the position of an object due to a change in the location of the observer. Astronomical ____ is measured in seconds of arc. retrograde motion the apparent backward (westward) motion of planets as seen against the background of stars epicycle the small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (deferent) around Earth deferent In the Ptolemaic theory, the large circle around Earth along which the center of the epicycle moved equant the point off-center in the deferent from which the center of the epicycle appears to move uniformly paradigm a commonly accepted set of scientific ideas and assumptions ellipse a closed curve enclosing two points (foci) such that the total distance from one focus to any point on the curve back to the other focus equals a constant semimajor axis (a) Half of the longest axis of an ellipse eccentricity (e) A measure of the flattening of an ellipse. An ellipse of e=0 is circular. The closer to l e becomes, the more flattened the ellipse hypothesis a conjecture, subject to further tests, that accounts for a set of facts theory a system of assumptions and principles applicable to a wide range of phenomena that have been repeatedly verified natural law A conjecture about how nature works in which scientists have overwhelming confidence