The Reason for the Seasons

1. The Reason for the Seasons

2. The Sun in the Sky Ecliptic is the Sun’s apparent path through the celestial sphere.

3. We can recognize solstices and equinoxes by Sun’s path across sky: Summer (June) solstice: highest path; rise and set at most extreme north of due east Winter (December) solstice: lowest path; rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west.

4. But…CLOSER means MORE right?

5. CLOSER means MORE right? • Heat • The closer you are the hotter it is

6. CLOSER means MORE right? • Heat • The closer you are the hotter it is • Sound • The closer you get, the louder it is

7. CLOSER means MORE right? • Heat • The closer you are the hotter it is • Sound • The closer you get, the louder it is • Light • The closer you get, the brighter it is

8. Complete this statement! When the Sun is ______ it is summer, and when the Sun is _______ it is winter.

9. Complete this statement! When the Sun is CLOSER it is summer, and when the Sun is FARTHER it is winter.

10. When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

11. When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER

12. When the Sun is low in the sky, the amount of direct sunlight received is less. This results in WINTER When the Sun is high in the sky, the amount of direct sunlight received is greater. This results in SUMMER

13. What causes the seasons?

14. Summary: The Real Reason for Seasons • Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sunchanges as Earth orbits the Sun. • Summer occurs in your hemisphere when sunlight hits it more directly and is in the sky longer; winter occurs when the sunlight is less direct and is not in the sky as long. • AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.

15. Why doesn’t distance matter? • Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt.

16. How do we mark the progression of the seasons? • We define four special points: • summer solstice • winter solstice • spring (vernal) equinox • fall (autumnal) equinox

17. We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west.

18. Phases of the Moon • Why do we see phases of the Moon? • What causes eclipses? Our goals for learning:

19. The changing phases of the Moon inspired the concept of the month

20. Phases of the Moon: 29.5-day cycle • Waxing • Moon visible in afternoon/evening • Gets “fuller” and rises later each day • Waning • Moon visible in late night/morning • Gets “less full” and sets later each day

21. Although the Moon is always ½ lit by the Sun, we see different amounts of the lit portion from Earth depending on where the Moon is located in its orbit.

22. Moon is illuminated (always ½) by Sun • We see a changing combination of the bright and dark faces as Moon orbits the Earth Animations at links from our website

23. The full moon rises at approximately: • Midnight • Sunset • Sunrise • 9 or 10 p.m. • 3 or 4 a.m.

24. If you were on the Moon, would the Earth, • Show no phases • Show phases the same as the moon (when it is full Moon it is full Earth, etc.) • Show phases opposite to the Moon (when it is full Moon it is new Earth, etc.) Make a sketch to decide!

25. We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth