1. August 29, 2011: 10am Class On-line Quiz #1 on d2l: Please complete before class on Friday, Sept. 2 HW 2 on line (d2l and boojum): Create a Constellation Due Wednesday, Sept. 7, in class. Honors Section 3H: Please complete doodle poll (see Megan’s email and class web site) Today: Precession, Phases of the Moon, Eclipses, Seasons

2. Precession

3. A spinning top “precesses” http://en.wikipedia.org/wiki/Precession

4. Precession The Earth’s axis of rotation precesses, so makes a big circle, with a period of 26,000 years (so it goes once around every 26,000 years) The Earth’s pole moves about one degree in 72 years. The precession is a result of the fact that the Earth’s axis of rotation is tilted with respect to the ecliptic, and with respect to the orbit of the Moon. The Moon’s and Sun’s gravity tug on the Earth and cause the pole to precess.

5. Currently the North Celestial Pole is very near the North Star, Polaris. In 3000 BC, the pole star was Thuban, in the constellation Draco. But it is moving away from Polaris: in 14000 AD, it will be pointed towards Vega, as it was in 12000 BC Polaris will be the North Star again in 27800 AD. Direction of the North Celestial Pole as a function of year, AD

6. Direction of the South Celestial Pole as a function of year, AD

7. Hipparchus of Rhodes (190 BC-120BC) Precession was first observed by Hipparchus Also was “father” of trigonometry Made one of the first star catalogs

8. Phases and Eclipses

9. We always see the SAME SIDE of the Moon because the moon orbits the Earth and rotates on its axis with EXACTLY the same period (29 ½ days). The “DARK SIDE OF THE MOON” was not seen by any human until spacecraft were sent to the moon and photographed it, in the 1950s.

10. Rotation of the Moon Applet Animation

11. 24 hour cycle of the Earth24 hour cycle of the Earth

12. PHASES

13. Phases of the Moon youtube.com rap. The Moon appears to go through a series of PHASES during each month: new moon, crescent, gibbous, full, gibbous, crescent, new.

14. Lunar Eclipse = Earth's shadow falls on the Moon

16. Solar Eclipse = Moon's shadow falls on the Earth

17. Cause of Eclipse Animation

18. Path of some Solar Eclipses on the Earth

19. Solar Eclipses: Total Partial Annular

20. Examples of Beautiful Eclipses!

23. Why don’t we have a lunar and solar eclipse every month? (1) The Moon’s orbit is tilted with respect to the ecliptic (orbit of the Earth around the Sun) by about 5 degrees (2)Also, the Moon’s orbit PRECESSES: the period of precession is about 18.6 years There is a long cycle for the eclipses, called the Saros Cycle.

24. Tilt of the Moon’s Orbit

25. PRECESSION of the Moon’s Orbit

26. Summary: Key Concepts 1.We always see the SAME SIDE of the Moon because the moon orbits the Earth and rotates on its axis with EXACTLY the same period 2.Phases of the Moon: New, Waxing Crescent, Quarter, Waxing Gibbous, Full, Waning Gibbous, Quarter, Waning Crescent, New, 3.Eclipses: Penumbra and Umbra 4.Solar Eclipses: Total, Partial, and Annular 5.Saros Cycle

27. Why Are There Seasons?? http://livewithpurposecoaching.com/wp-content/uploads/2010/04/seasons1.jpg

28. NOT NOT NOT because the Earth is closer to the Sun in summer and farther in winter. OK???? Why are there Seasons?

29. The Reasons for the Seasons 1. The Earth's axis is tilted, and during the year the axis points basically in the same direction (except for tiny motion or precession). 2. During summer in the northern hemisphere, the northern part of the Earth points TOWARDS the Sun. sunlight is direct, more intense, heats the earth days are longer sunlight has less atmosphere to go through so is more effective in warming the ground and vise versa for winter.

31. 7/14/2015The Seasons31

32. 7/14/2015The Seasons32 Earth on June 22

33. 7/14/2015The Seasons33 Earth on December 22

34. 7/14/2015The Seasons34 Equinoxes and Solstices The Vernal (Spring) Equinox (about March 21): The location where the Sun crosses the equator when going from south of the equator to north of the equator. – Position of the Sun: R.A. 0 h, Dec 0 o The Summer Solstice (about June 21): The location where the Sun is at its furthest north. – Position of the Sun: R.A. 6 h, Dec +23.5 o

35. 7/14/2015The Seasons35 Equinoxes and Solstices The Autumnal (Fall) Equinox (about September 21): Where the Sun crosses the equator when going from north to south. – Position of the Sun: R.A. 12 h, Dec 0 o The Winter Solstice (about December 21): The location where the Sun is at its furthest south. – Position of the Sun: R.A. 18 h, Dec –23.5 o

36. 7/14/2015The Seasons36

37. 7/14/2015The Seasons37 Tropics

38. Note: When it's summer in the northern hemisphere, it's winter in the southern hemisphere, and vise versa.

39. 7/14/2015The Seasons39 The Annual Path of the Sun On the summer solstice: – the Sun will appear directly overhead to someone at 23.5 degrees north latitude. This latitude is called the Tropic of Cancer. – The Sun does not set for people within 23.5 degrees of the North pole (above the Artic circle) – The Sun does not rise for people within 23.5 degrees of the South pole (below the Antarctic Circle).

40. 7/14/2015The Seasons40 Earth on June 21

41. 7/14/2015The Seasons41 Standing on the North Pole

42. 7/14/2015The Seasons42 Midnight Sun

43. 7/14/2015The Seasons43 Standing on the Tropic of Cancer

44. 7/14/2015The Seasons44 The Annual Path of the Sun On the autumnal equinox: – The Sun will appear directly overhead to someone on the equator at 0 degress latitude. – The Sun sets at the North Pole, ending 6 straight months of day and beginning 6 straight months of night. – The Sun rises at the South Pole ending 6 straight months of night and beginning 6 straight months of day.

45. 7/14/2015The Seasons45 Standing on the Equator

46. 7/14/2015The Seasons46 The Annual Path of the Sun On the winter solstice: – The Sun will appear directly overhead to someone at 23.5 degress south latitude, the Tropic of Capricorn. – The Sun does not set for people within 23.5 degrees of the South Pole (below the Antarctic Circle) – The Sun does not rise for people within 23.5 degrees of the North Pole (above the Arctic Circle)

47. 7/14/2015The Seasons47 Earth on December 21

48. 7/14/2015The Seasons48 The Annual Path of the Sun On the vernal equinox: – The Sun will appear directly overhead to someone on the equator at 0 degress latitude. – The Sun rises at the North Pole, ending 6 straight months of night and beginning 6 straight months of day. – The Sun sets at the South Pole ending 6 straight months of day and beginning 6 straight months of night.