1.8 Precession
Pre-Lecture Reading 1.8
Video Lecture
Supplementary Notes
Conditions for Top Precession
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Top must be non-spherical
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Top must be under the influence of an external force
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The rotation axis of the top cannot be parallel or perpendicular to the force (i.e., the top must be tipped).
Earth's and the Celestial Sphere's Long-Term Motion
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Earth is only very mildly non-spherical, so it precesses very slowly compared to its rotation.
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Earth rotates 360° every sidereal day.
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Earth's rotation axis precesses 360° every 26,000 tropical years.
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Because of precession, Earth's rotation axis points toward the sun 20 minutes earlier in its orbit each year.
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Consequently, Earth's seasons, and hence its tropical year, begin 20 minutes earlier than they would if there was no precession (e.g., if Earth were perfectly spherical).
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Consequently, after 13,000 years, it is, e.g., northern hemisphere winter where it used to be northern hemisphere summer in Earth's orbit.
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Because of precession of Earth's rotation axis, the north celestial pole moves, by 20 arcseconds each year.
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Consequently, Polaris is not always close to the north celestial pole, and hence is not always the pole star.
Assignment 1
Do Question 7.