1. Seasons

2. Reasons for the Seasons:
Tilt of Earth’s axis
Axial parallelism
Sphericity
Axial parallelism: axis maintains alignment during orbit around sun (FIXED); North Pole points to Polaris (North Star)

3. Landmark dates ... December Solstice March Equinox June Solstice
September Equinox,
September 22/23

4. Landmark latitudes ... Equator = 0o Tropic of Cancer = 23.5oN
Arctic Circle = 66.5oN
Tropic of Capricorn = 23.5oS
Antarctic Circle = 66.5oS

5. On each day,….. 1. What is subsolar point (SSP)?
SSP is latitude where sun is directly
overhead; where sun’s rays are vertical.
For each landmark date: answer these questions…

8. 2. At what latitude will sun’s rays be oblique/tangent?

10. 3. Where is Circle of Illumination?
separates light and dark halves

12. December Solstice 1.SSP? 23.5oS 2. tangent rays ? 66.5oN, 66.5oS
3. C.I. is tangent?
at 66.5oN, 66.5oS

13. March Equinox 1.SSP? 0° 2. tangent rays? 90oN, 90oS
3. C.I. is tangent?
at 90°N, 90°S

14. June Solstice SSP? 23.5oN 2. tangent rays? 66.5oN, 66.5oS
3. C.I. is tangent?

15. September Equinox 1.SSP? 0° 2. tangent rays ? 90oN, 90oS
3. C.I. is tangent?
at 90°N, 90°S

16. Animated tour of the seasons

17. Annual March of the Seasons
What is range of SSP?

19. Angle of noon sun in a location=
90 - difference in degrees between latitude of location and SSP

20. Latitude?

21. Solution:
Angle of noon sun (March 22) = 50
SSP (March 22) = 0
50 = 90 – (? – 0)
? = 40
for June 21:
73 = 90 – (?-23.5)
?-23.5 = 27
? = 40.5

22. What is the angle of the noon sun in Duluth on September Equinox?
= 43 degrees

23. What is the angle of the noon sun in Duluth on December Solstice?
= 90 – 70.5
= 19.5 degrees

24. What is the angle of the noon sun in Duluth on June Solstice?
= 90 – (47N – 23.5 N)
= 90 – 23.5
= 66.5 degrees

25. What is the angle of the noon sun in Duluth today (January 27)?
SSP today is approx. 20˚S.
= 90 – (47N – 20 S)
= 90 – 67
= 23 degrees