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| Earth-centered systems (promoted by Ptolemy, Aristotle, etc.) |
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| A closed loop that is not circular; the planet’s orbits are elliptical |
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| The minimum velocity needed for an object to “jump off” of the surface of an object. The greater the gravitational attraction of the planet the larger this escape velocity will be. |
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| The apparent doubling back of an object (typically a planet) against the background stars. |
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| Kepler's Three Laws of Motion |
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| #1 Planets Travel in ellipses with the Sun at one focal point. #2 The planets sweep out equal areas in equal times. #3 P2 = K A3. |
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| either the Summer or Winter moment When the gap between the Sun, on the ecliptic, and the celestial equator is greatest. |
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| Sun-centered systems (first proposed by Copernicus) |
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| The set of constellations that lie on the ecliptic line; the Sun passes through 1 each month (or so) |
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| An angle used on Earth to measure the position ALONG THE HORIZON of an object. North = 0, East = 90, and so on. |
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| The smaller of two circles used by Ptolemy to explain why planets undergo retrograde motion. The epicycle is a small circle whose center lies on the deferent. |
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| The line between the shadowed and lit portions of a planet or moon |
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| Either the Autmnal or Vernal (spring) moment when the Sun, on the ecliptic, crosses the celestial equator. |
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| The apparent path of the Sun; most of the planets’ orbits lie within a few degrees of it |
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| The angular separation between the Sun and an inner planet as seen from an outer planet’s perspective. When it is the largest value, we call that the ‘greatest elongation.’ For example, the greatest elongation of Venus, as seen from the Earth, is usually around 45 degrees. |
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| An angle used to measure an object’s position above/below the horizon. |
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| The larger of the two circles used by Ptolemy to explain why planets undergo retrograde motion. The center of the deferent was the Earth. |
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| A “stretching” force that is the result of the difference in gravitational attraction between one side of an object and the other. While there is a small difference in gravity between your head and your toes, it is insufficient to truly stretch you. On the other hand, near a black hole this difference would be enormous and, shall we say, uncomfortable! (see question #27 for additional insight) |
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| A measure of how fast an object is moving in space by measuring its position changes in terms of degrees (or fractions of degrees). Typical units are degrees/day. |
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| Greek philosopher who proposed Geocentrism and the perfection of the heavens |
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| First century AD astronomer who proposed geocentrism, epicycles and deferents to describe retrograde motion. He also produced tables of predictions for planetary positions which were accurate for many years, but which would need updating every 20 years or so. |
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| In 1543, he published “De Revoltionibus” which proposed that the Sun was the center of the solar system. The motion of the heavens, he said, was largely due to the Earth’s own motions. |
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| Late 1500s Danish nobleman who manufactured the best instruments for measuring positions of the planets. His data eventually led Kepler to the understanding that the planets moved in ellipses, not circles. |
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| Proposed Three Laws of Planetary Motion, which all boiled down to the idea that planets moved in ellipses. He pursued a small discrepancy between the circular predictions of planetary positions versus the observations of Tycho and concluded that Tycho was right and the circular model was wrong. |
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| His observations (see question #1) led to the eventual downfall of Aristotelian teachings: the heavens were NOT perfect or immutable, the Earth was NOT the center of all motion, the Earth itself could be in motion and still have a satellite, etc. |
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| Linked the idea of gravity on Earth to gravity throughout space. His Law of Universal Gravitation allowed astronomers to calculate the gravitational force between any two bodies in the Universe. |
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| With his discovery of Uranus, he doubled the size of the solar system from 9.5 AUs (the distance to Saturn) to 19.0 AUs (the distance to Uranus). |
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| Co-discoverers (or possibly not, according to recent research) of the planet Neptune. Each man used Newton’s Law of Gravity to predict the existence of a planet beyond Uranus. They were spurred to do so based on un-explained disturbances in Uranus’s orbit. |
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