Ch. 27 Sec. 3

1. Ch. 27 Sec. 3 The Sun-Earth-Moon System

2. Motions of the Sun-Earth-Moon system define Earth’s day, month, and year. Review Vocabulary revolution: the time it takes for a planetary body to make one orbit around another, larger body

3. I. Daily Motions The Sun, the Moon, planets, and stars do not orbit around Earth every day. It appears that way because we observe the sky from a planet that rotates.

4. A. Earth’s rotation • Counter-clockwise • Objects appears to rise in east & set in west a. Sun b. Moon c. Stars d. Planets

5. B. Solar Day - the time period from one noon to the next 1. As Earth rotates, it moves in its orbit and has to turn a little farther each day to align with the Sun 2. 23 hr 56 min 4.1 sec

6. II. Annual Motions A. Ecliptic plane Fig. 27. 15 page 776

7. B. The effects of Earth’s tilt • Earth’s axis is tilted relative to the ecliptic at approximately 23.5 • As Earth orbits the Sun, the orientation of Earth’s axis remains fixed Fig. 27.17 page 777

8. 3. Earth’s tilt and orbital motion around the Sun result in a cycle of the seasons. 4. Another effect is the changing angle of the Sun above the horizon from summer to winter.

9. C. Solstice - Sun is overhead at its farthest distance either north or south of the equator The lines of latitude that correspond to these positions on Earth have been identified as the Tropic of Cancer and the Tropic of Capricorn.

10. 1. The summer solstice occurs in the northern hemisphere when the Sun is directly overhead at the Tropic of Cancer, 23.5° north latitude. Fig. 27.17 page 777

11. Summer – N. Hemisphere is tilted toward the sun a. North Pole has 24 hours daylight b. Approx. June 21

12. 2. The winter solstice occurs in the northern hemisphere when the Sun is directly overhead at the Tropic of Capricorn, 23.5° south latitude. Fig. 27.17 page 777

13. Winter – N. Hemisphere is tilted away from the sun a. N. Pole has no daylight b. Approx. Dec. 21

14. D. Equinox (equal night) Earth’s axis is perpendicular to the Sun’s rays and at noon the Sun is directly overhead at the equator

15. E. Changes in Altitude • The Sun’s maximum height, called its zenith, varies throughout the year depending on the viewer’s location. • For a person standing on the 23.5° north latitude, the Sun would be at zenith on the summer solstice. It would be at its lowest position at the winter solstice. Fig 27.17 page 778

16. III. Phases of the Moon • Due to positions of moon, sun, & Earth • Waxing – getting larger • Waning – getting smaller • Gibbous – more than half

18. Page 779 – figure 27.18

19. Visualizing Phases of the Moon

20. B. Synchronous rotation • Moon’s revolution = rotational period • Always see the same side of the moon

21. IV. Lunar Motions A. The length of a lunar month is about 29.5 days, which is longer than the 27.3 days it takes for one revolution, or orbit, around Earth.

22. B. Tides - Moon’s gravity pulls on Earth • Spring tides • a. Sun-Earth-Moon align • b. Higher tides • 2. Neap tides • a. Sun-Earth-Moon at 90o angle • b. Lower tides than normal

23. D. Eclipses • Lunar – Earth shadow on moon • Solar – moon blocks sun a. Annular – moon appears smaller than sun 3. Lunar orbit a. prevents monthly eclipses b. Tilts 5º

24. V. Solar Eclipses A solar eclipse occurs when the Moon passes directly between the Sun and Earth and blocks the Sun from view.

25. Moon casts a shadow on Earth 1. Those on Earth within the darkest part of the Moon’s shadow (umbra) see a total eclipse. 2. Those within the lighter part, or penumbral shadow, see only a partial eclipse. Fig. 27.22 page 782

26. B. Effects of tilted orbits Lunar orbit is tilted 5O relative to ecliptic 2. Solar eclipses can happen only when the Moon’s orbital plane and ecliptic plane intersect along the Sun-Earth line

27. C. Annular eclipses 1. The closest point in the Moon’s orbit to Earth is called perigee 2. The farthest point is called apogee An annular eclipse takes place when the Moon is too far away for its umbral shadow to reach Earth.

28. VI. Lunar Eclipses • When the Moon passes through Earth’s shadow • Red color due to refraction Fig 27.25 page 784

29. How is a lunar eclipse different from a solar eclipse?

30. Answer: A lunar eclipse is an eclipse of the Moon. During a lunar eclipse, Earth blocks sunlight from striking all or part of the Moon’s surface. The eclipse can be seen by anyone on Earth who can see the Moon. A solar eclipse is an eclipse of the Sun. During a solar eclipse, the Moon blocks some or all sunlight from reaching a portion of Earth. The eclipse can be seen by only the people in the affected region.

31. Why are lunar phases seen from Earth? Answer: Half of the Moon is always lighted by sunlight. As the Moon revolves around Earth, different amounts of the near side of the Moon, which always faces Earth, are illuminated.

32. Motions of the Sun-Earth-Moon system define Earth’s day, month, and year. • Earth’s rotation defines one day, and Earth’s revolution around the Sun defines one year. • Seasons are caused by the tilt of Earth’s spin axis relative to the ecliptic plane.

33. The gravitational attraction of both the Sun and the Moon causes tides. • The Moon’s phases result from our view of its lighted side as it orbits Earth. • Solar and lunar eclipses occur when the Sun’s light is blocked.