Sun’s annual motion and the Seasons

1. Sun’s annual motion and the Seasons Living on Earth may be expensive, but it includes a free yearly trip around the Sun.

2. Review Question From a latitude of 40 degrees north, describe where in the sky you would find the zenith, the meridian and the north celestial pole.

3. Review Question From 40 degrees north, describe where in the sky you would find the south celestial pole?

4. Review Question From 40 degrees north, describe where in the sky you would find the celestial equator?

5. Review Question From a latitude of 40 degrees south, describe where in the sky you would find the zenith, the meridian and the south celestial pole and the celestial equator.

6. Review Question Describe the diurnal motion of the Sun.

7. Discussion About what time of day is depicting in this drawing? How can you tell?

8. Discussion About where in the sky will the Sun be at 6 pm?

9. Review Question Describe how the stars move through the sky if you were at the earth’s south pole.

10. Star trails…

11. Review Question Describe how the stars move through the sky if you were at the earth’s equator.

12. Star trails…

14. Review Question Which star (A, B, C, or D) will spend the least amount of time above the horizon during the course of a day?

15. Review Question As viewed from the northern hemisphere, which stars spend a greater amount of time above the horizon, those north of the celestial equator or those south of the celestial equator?

16. Is the camera facing north or south? Star trails over the Gemini Northern Telescope dome. The glow from the eruption of the Kilauea volcano is seen on the left.

17. Review Question What is the definition of a solar day? What is the definition of a sidereal day?

19. Discussion If I told you the camera was pointed east, which way are the stars moving?

20. Discussion If I told you the camera was pointed east, are you north or south of the equator?

21. Discussion How can you use the picture to determine your latitude?

22. Discussion In what constellation will the Sun be in at sunset?

23. Review Question What time of year is the solar day the longest? What time of year is the sidereal day the longest?

24. Review Question If the Earth moves faster in its orbit what happens to the length of the sidereal day?

25. Review Question If the Earth moves faster in its orbit what happens to the length of the solar day?

26. Review Question In what constellation will the Sun be in one month from the time depicted in this drawing?

27. Review Question What is the ecliptic?

29. The Seasons and tilt The seasons are caused by the 23.5 degree tilt of the Earth’s rotation axis relative to the normal (perpendicular) of Earth’s orbital plane.

30. Conservation of angular momentum Anything that spins on an axis or revolves around another object has angular momentum. Conservation of angular momentum requires that the rate of spinning remains constant with time. Also, the axis of rotation of any spinning object remains in a fixed direction in space.

31. Discussion How does conservation of angular momentum help you when you are riding a bicycle?

32. Near edge-on view

34. Discussion Where is the Sun today on the previous picture?

35. Discussion What is the evidence that the seasons are not caused by the Earth’s distance from the Sun?

36. The seasons and ellipticity The Earth’s orbit is nearly circular – distance from the Sun varies by only 3% Earth is closest to the Sun in January and furthest from the Sun in July Summer in northern hemisphere is winter in southern hemisphere

37. Discussion What are the two reasons it is warmer during the summer than during the winter?

38. Why is summer warmer? • The Sun, being above the celestial equator, remains in the sky longer during the summer, the longest daylight time occurring on the summer solstice. • The Sun rays hit the earth more directly during the summer months, i.e. the summer hemisphere receives more energy per square meter.

43. Discussion If I make the Earth’s orbit a perfect circle, do the seasons in Cleveland stay the same, become more extreme or become less extreme?

44. Discussion From Cleveland, latitude 42, on what day does the Sun pass through the zenith?

45. Discussion From Cleveland on September 22 the Sun sets directly in the west. Two weeks later, about where would we see the set?

46. The arctic circle The arctic circle is the northern latitude at which on the summer solstice the Sun never sets and on the winter solstice the Sun never rises above the horizon.

48. Discussion What is the latitude of the arctic circle?

49. Discussion The tropics mark the farthest points north and south where the Sun can appear at the zenith. What is the latitude of the tropics?

50. Discussion What is the highest the Sun can get above the southern horizon at a latitude of 42 degrees?