Yearly Motion

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1. ©2016 W.W. Norton & Company, Inc. The Yearly Motion of the Sky 2. ©2016 W.W. Norton & Company, Inc. The Ecliptic and the Zodiac As the earth revolves the Sun…
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  • 1. ©2016 W.W. Norton & Company, Inc. The Yearly Motion of the Sky
  • 2. ©2016 W.W. Norton & Company, Inc. The Ecliptic and the Zodiac As the earth revolves the Sun is projected in front of different constellations at different times of year. The path the Sun takes across heavens is called the ecliptic. The constellations through which the Sun seems to pass are called the Zodiac.
  • 3. ©2016 W.W. Norton & Company, Inc. How long does it take the Sun to complete one trip around the ecliptic? (i.e. if the Sun is lined up with a certain star, how long until it is lined up again?) A 24 hours B 29 days C 365 days D 26,000 years
  • 4. ©2016 W.W. Norton & Company, Inc. If I can see the constellation Taurus in tonight’s sky, will I be able to see it at night six months from now? A Ye s
  • 5. ©2016 W.W. Norton & Company, Inc. Ecliptic The ecliptic also defines the plane of Earth’s orbit projected on the sky. 1 year = 1 orbit 1 year = ~365.25 days The other planets in the Solar System orbit the Sun in similar planes, so they are always found near the ecliptic.
  • 6. ©2016 W.W. Norton & Company, Inc. Ecliptic Equinoxes Solstice Solstice The equinoxes are where the ecliptic crosses the celestial equator. The solstices are the points the Sun’s position is the farthest from the celestial equator.
  • 7. ©2016 W.W. Norton & Company, Inc. Precession In addition to its rotation and revolution, the earth’s axis also precesses (wobbles) like a top. The angle between the ecliptic and the equator remains at 23.5°, but the direction changes. The period of this precession of the equinoxes is about 26,000 years.
  • 8. ©2016 W.W. Norton & Company, Inc. Polaris won’t always be the North Star The season during which a constellation appears at night changes very slowly over time Because of precession:
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