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AST 111 Lecture 7

AST 111 Lecture 7. Eclipses, Solar and Sidereal Days, Precession. Eclipses. An eclipse is when one celestial object passes in front of another. Eclipses. Eclipses. Transit: Small object in front of large Occultation: Large object in front of small. Eclipses. Eclipses. Lunar Eclipses.

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AST 111 Lecture 7

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  1. AST 111 Lecture 7 Eclipses, Solar and Sidereal Days, Precession

  2. Eclipses An eclipse is when one celestial object passes in front of another.

  3. Eclipses

  4. Eclipses Transit: Small object in front of large Occultation: Large object in front of small

  5. Eclipses

  6. Eclipses

  7. Lunar Eclipses • Sun, Earth, and Moon in a straight line • The Earth gets between the Sun and the Moon • Must be a Full Moon

  8. Lunar Eclipses • Why don’t we see a lunar eclipse during every full moon?

  9. Lunar Eclipses • The plane of the Moon’s orbit is inclined by 5 degrees to the ecliptic • If Earth orbits the Sun in a pond, the moon spends half its time above and half its time below the pond’s surface

  10. Lunar Eclipses

  11. Lunar Eclipses

  12. Lunar Eclipses • Moon in umbra • Earth’s atmosphere “lenses” light onto the moon

  13. Lunar Eclipses • Partially in umbra, partially in penumbra

  14. Lunar Eclipses • Moon in penumbra • Almost can’t tell it’s an eclipse

  15. Solar Eclipses • Sun, Moon, Earth in a straight line • The Moon gets between the Sun and the Earth • Must be a New Moon

  16. Solar Eclipses

  17. Solar Eclipses

  18. Solar vs. Sidereal Day • Imagine you’re where Earth is – but there’s no Earth. • You’re rotating in place. You see the Celestial Sphere rotating. • How many degrees do you need to rotate through to get back to the same view? • Yes, this is as simple as you think it is!

  19. The Sidereal Day • The length of time for Earth to complete one full rotation about its axis • Also equal to the length of time it takes for a star (not the Sun) to come back to the same position in the sky • 23 hours 56 minutes

  20. The Solar Day • Say it’s noon, and the Sun is on the meridian. • If Earth rotates 360 degrees: • Is the Sun back on the meridian? • Why or why not?

  21. The Solar Day • The length of time for the Sun to start at the Meridian and return to the Meridian • 24 hours on average

  22. The Sidereal Day

  23. The Solar Day

  24. Solar and Sidereal Days • So… why are they different? • Earth’s orbit around the Sun causes the Sun to move in the sky • Earth must rotate a little extra to bring the Sun to the Meridian

  25. Solar and Sidereal Days • If Earth did not orbit the Sun (just stayed stationary), would the solar day equal the sidereal day?

  26. Solar and Sidereal Days • Length of solar day varies throughout a year • This is due to the ellipticity of Earth’s orbit • This causes the horizontal motion of the analemma • Length of sidereal day does not • The time it takes Earth to rotate once does not vary

  27. Earth’s axis precesses. • Just like a wobbling, spinning top • Every 26,000 years

  28. The North Star • Precession of Earth’s axis causes the North Star to change after long periods of time • Current North Star: Polaris • Vega was the north star in 12000 BC • We will have a new north star in AD 3000

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