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Patterns in the Sky

Patterns in the Sky. Solar System Astronomy Chapter 2. Motions in the Sky. The Sun and stars show regular patterns of motion which reflect the motion of the Earth. are clues to how the Solar System works. Our focus: Daily rotation of the Earth Annual orbit of the Earth

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Patterns in the Sky

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  1. Patterns in the Sky Solar System Astronomy Chapter 2

  2. Motions in the Sky • The Sun and stars show regular patterns of motion which • reflect the motion of the Earth. • are clues to how the Solar System works. • Our focus: • Daily rotation of the Earth • Annual orbit of the Earth • Monthly orbit of the Moon

  3. Frames of Reference • An observer sees things from a frame of reference. • On the Earth, different observers see • different parts of the sky. • different motions of the stars. • This shows the Earth is a sphere and rotates daily.

  4. Constellations • Constellations are arbitrary patterns of stars in the sky. • During the year, the Sun passes in front of some constellations. • The annual path of the Sun is called the ecliptic. • The constellations along the ecliptic are called the zodiac.

  5. Constellations & such… Sagittarius Scorpius Pisces Aries Pegasus Boötes Libra Andromeda Leo Orion Ursa Major Canis Major Taurus SummerTriangle Argo Crux Centaurus

  6. The Celestial Sphere • It helps to think of the sky as the celestial sphere. • The stars, Sun, and planets are actually at different distances. • The celestial sphere rotates around the north and south celestial poles each day. • Midway between the poles is the celestial equator.

  7. Celestial Sphere and Ecliptic

  8. Celestial Sphere, Ecliptic, &c

  9. At the North Pole • An observer would see half the celestial sphere. • The north celestial pole is overhead. • The stars rotate counterclockwise in about 24 hours. • No star rises or sets: all are circumpolar.

  10. At the Equator • All stars rise and set. • The celestial poles are on the northern and southern horizons. • Observers can see the whole celestial sphere as it rotates.

  11. In Between (like Radford) • One pole is above the horizon. • The angle to the horizon equals the latitude. • Some stars are circumpolar; others rise and set. • Some of the celestial sphere is never visible.

  12. Rotation of the Earth • The motion of the celestial sphere reflects the daily spin of the Earth. • The rotation of the Earth sets the spin of hurricanes (Coriolis effect). • Storms spin counterclockwise in northern hemisphere, clockwise in the southern.

  13. Annual Motion of the Sky • The changing position of the Sun reflects the Earth’s orbit around the Sun. • As the Earth’s position changes, the Sun is seen against different constellations.

  14. The Earth’s Axis & Rotation • The Earth’s axis is not perpendicular to the ecliptic plane. • 23.5˚ • Sun is alternately north and south of the celestial equator during the year. • Produces seasons • receive more direct sunlight in the summer, and Sun is up longer

  15. The Earth’s Axis & Rotation

  16. Special Days of the Year • Apparent Motion of the Sun • Summer solstice: Sun farthest north. • Autumnal equinox: Sun on the equator, moving southward. • Winter solstice: Sun farthest south. • Spring (vernal) equinox: Sun on the equator, moving northward.

  17. The Earth Spins and Revolves

  18. Precession • Gyroscopic Effect • The location of the celestial poles slowly shifts. • Currently the north celestial pole is near the bright star Polaris. • This reflects a 26,000-year wobble in the Earth’s spin axis.

  19. Precession • Gyroscopic Effect • The location of the celestial poles slowly shifts. • Currently the north celestial pole is near the bright star Polaris. • This reflects a 26,000-year wobble in the Earth’s spin axis.

  20. the poor, dull-witted astrologers… • Astrology developed in Babylonia • ~2500 years ago • Precession of the equinoxes unknown • Aries if born March 21 to April 20 • Sun is in Aries April 19 to May 13

  21. Moon’s Orbit: Eclipses & Phases

  22. Orbital Period of the Moon

  23. Orbital Period of the Moon • Moon takes 27.3 days to orbit once with respect to the stars • sidereal period • Lunar phases repeat every 29.5 days • synodic period • The Moon’s rotation period equals its sidereal period • As a result, we always see the same hemisphere of the Moon • Or do we??

  24. Libration

  25. Eclipses • Types: • Solar eclipse: Moon passes in front of Sun. • Lunar eclipse: Moon enters Earth’s shadow. • Solar eclipses happen at new moon. • Lunar eclipses happen at full moon. • We don’t get eclipses every month: Moon’s orbital plane is at an angle to the ecliptic plane.

  26. Eclipses

  27. Lunar Eclipse

  28. Lunar Eclipse

  29. Lunar Eclipse • Can last up to 1 hour 40 min • Red?

  30. Lunar Eclipse • Typically 1-2 each year • Fairly common

  31. Solar Eclipse

  32. Solar Eclipse • Sun & Moon are same angular size…

  33. Solar Eclipse

  34. Solar Eclipse • About 1/year – rare. Applause

  35. Last look at lunar cycle

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