lecture08spring2009

# lecture08spring2009 - Astronomy100Dr.Rhodes...

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Astronomy 100 - Dr. Rhodes Lecture # 8 - 1/30/2009 (Last Day to Drop Without a Mark of W) (Last Day to Opt for Pass/NoPass)

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Chapter 6: The Earth-Moon System Non-Spherical Shape of the Earth The Earth is not a perfect sphere; rather, the Earth’s rotation about its axis causes its equatorial diameter to slightly exceed its polar diameter by a distance of about 26 miles. This distortion in the Earth’s shape is due to the centrifugal force of the rotating Earth Oblateness is a measure of the “flattening” of the Earth’s shape; the Earth is said to be an oblate spheroid.
Chapter 6: The Earth-Moon System Precession of the Earth Oblateness is calculated by dividing the difference between the equatorial and polar diameters by the equatorial diameter. Because the Earth has an oblate shape, both the Sun and the Moon try to tip the spinning Earth so that its axis is normal to the ecliptic plane; however, instead of decreasing its obliquity to zero, the Earth responds by having its spin axis move very slowly on the surface of a cone; that is, the Earth’s spin axis is said to precess .

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Chapter 6: The Earth-Moon System Precession of the Earth (cont.) Precession was discovered by the Greek astronomer Hipparchus, who was clever enough not to decribe the apparent change in location of the celestial pole among the stars as being due solely to observational errors made by the earlier astronomers.
Demonstration of the Gravitational Attraction of the Moon on the Earth’s Center and on the Parts of Its Equatorial Bulge

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The Effects of the Precession of the Earth’s Spin Axis: Polaris will not remain the Pole Star It takes a total of 26,000 years for the Earth’s spin axis to precess around this cone once. Because of the slow precession of the Earth’s spin axis, Polaris will not always be the Pole Star; instead in the future there will be times when there will be no single star which will be close to the position of the North Celestial Pole; at other times other stars will replace Polaris as the Pole Star.

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Comparison of the Precession of the Earth’s Spin Axis and the Precession of the Rotational Axis of a Spinning Top on a Table Under the Influence of Gravity

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• Spring '08
• RHODES
• Astronomy, Light, Electric charge, Fundamental physics concepts, Neptune

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