lecture04spring2009 - Astronomy100Dr.Rhodes Lecture Chapter...

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Astronomy 100 - Dr. Rhodes Lecture # 4 - 1/21/2009
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Chapter 2: From an Earth-Centered to a Sun-Centered System Back to Geocentrism The man considered to be the greatest astronomer of the pre-Christian era was Hipparchus . Hipparchus lived after Aristarchus and worked from 160 to 127 B.C. Hipparchus created an island observatory on the Island of Rhodes in the eastern Mediterranean Sea. He added epicycles to the geocentric model. He also discovered the precession of the Earth’s spin axis. He also introduced the idea of stellar magnitudes .
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Chapter 2: From An Earth-Centered to a Sun-Centered System Post-Christian Greek Astronomy The Greek astronomer who was considered to be the greatest of the post-Christian era was known as Claudius Ptolemy , or also simply as Ptolemy . By 150 A.D. Ptolemy had developed the most comprehensive and complicated geocentric model yet to be devised by any of the Greeks.
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Chapter 2: From an Earth-Centered to a Sun-Centered System The Greek Geocentric Celestial Model Ptolemy’s model, presented in his book called the Almagest , held sway for more than 1,300 years. Because the heavens were viewed as perfect, the use of the symmetrical circle to model the motions of celestial objects was thought to be reasonable by Ptolemy. The Ptolemaic model could explain the motion of the nearby Moon quite well.
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Chapter 1: The Quest Ahead Observations of Planetary Motion Five planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter, Saturn. These planets lack the simple, uniform motion of the Sun and Moon. These planets always stay near the ecliptic. Mercury and Venus never appear very far from the position of the Sun in the sky. Thus their elongation angles remain small. A planet’s elongation angle is its angle relative to the position of the Sun in the sky.
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Chapter 1: The Quest Ahead Observations of Planetary Motion (cont.) The planets sometimes stop their eastward motion and move westward against the background of stars. This is called retrograde motion, which we will demonstrate in the next three slides. Retrograde motion was so unusual that the Babylonians and the Greeks thought that the planets had god-like powers. This idea of god-like powers of the planets is what led both the Babylonians and the Greeks to develop astrology. Today we know that there is no scientific support for the ideas of astrology.
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QuickTimeª and a Video Format cvid decompressor are needed to see this picture. Animation to demonstrate retrograde motion
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QuickTimeª and a Sorenson Video decompressor are needed to see this picture. Additional example of retrograde motion
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Chapter 2: From an Earth-Centered to a Sun-Centered System A Model of Planetery Motion: Epicycles Hipparchus and Ptolemy had to introduce the notion of epicycles to explain retrograde motion in their geocentric models.
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