1. SEASONS
Chapter 3

2. WHAT IS THE REASON THAT WE HAVE SEASONS ON EARTH?
(A) The Earth’s orbit is elliptical with a 3%
variation in Earth-Sun distance - summer
when we are closest to the Sun.
The Earth’s axis is tilted - sometimes it points
towards the Sun (summer) and sometimes
away (winter).
There is a 1 year variation in the energy
output of the Sun - when output highest -
summer.
There is a 1 year cycle in weather patterns
due to coriolis force - cold air forced from Arctic in winter, up from tropics in summer.

3. SEASONS on EARTH
Earth’s orbit is elliptical with a 3% variation in Earth-Sun distance
Is this the cause of the seasons?

4. EARTH’S ELLIPTICAL ORBIT
Sun
Aphelion
Perihelion A B C D
Elliptical orbit cannot be reason for seasons as if it were both
hemispheres would have summer when Earth closest to Sun.
Earth at perihelion in January - cold in North

5. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)

6. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)
Seasons DO NOT arise because of changes in Earth-Sun distances. (Earth is closest to Sun early in January)

7. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)

8. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)
Seasons DO NOT arise because of changes in Earth-Sun distances. (Earth is closest to Sun early in January)

9. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)

10. SEASONS on EARTH
Seasons arise on Earth because the spin axis of Earth is TILTED with respect to the ECLIPTIC (the apparent path of the Sun on the Celestial Sphere, or the plane of Earth’s orbit around the Sun)
Seasons DO NOT arise because of changes in Earth-Sun distances. (Earth is closest to Sun early in January)

11. VARIATION in the SUN’S RAYS
If Sun high in sky (left) energy not spread out significantly so
warming effect of Sun is strongest - summer.
If Sun low in the sky (right) energy spread out over larger area
so that warming effect smaller.

12. FLASHCARD
WHAT WOULD THE SEASONS BE LIKE ON EARTH IF THE EARTH’S AXIS WAS NOT TILTED WITH RESPECT TO THE ECLIPTIC?
A) The seasons would be the same as they are now.
B) The seasons would be switched. Fall would be spring and spring would be fall.
C) There would be no seasons
D) Earth’s axis HAS to be tilted so the question is silly.

13. SEASONS on EARTH

14. Sun's motion defines the seasons

15. SEASONAL PATHS of the SUN
In summer Sun is far to North, so it rises North of East and sets North
of West. For northern observer, it is above horizon for > 12 hours.
In winter the Sun is far to the South so it rises South of East and sets
South of West. For northern observer, it is above horizon < 12 hours.
At the equinoxes the Sun rises directly East and sets directly West.

16. SUMMER SOLSTICE
At summer solstice Sun overhead for those at +23 degrees latitude.
All points North 67 degrees see Sun for 24 hours.
All points South 67 degrees never see Sun.

17. WINTER SOLSTICE
At winter solstice Sun overhead for those at -23 degrees.
All points North 67 degrees do not see Sun at all.
All points South 67 degrees see Sun for 24 hours.

18. FLASHCARD
WHERE DO YOU HAVE TO BE ON EARTH TO SEE THE SUN DIRECTLY OVERHEAD AT SOME TIME DURING THE YEAR?
A) On the equator.
B) At one of the poles.
C) At one of the tropics.
D) At any point between the tropics.

19. FLASHCARD
IF YOU ARE AT SOME POINT BETWEEN THE TROPICS, HOW MANY TIMES DURING THE YEAR WILL THE SUN BE OVERHEAD?
A) Once.
B) Twice.
C) Three times.
D) Four times.

20. FLASHCARD
FROM VANCOUVER (LATITUDE 49 DEGREES NORTH), AT NOON HOW MANY DEGREES ABOVE THE SOUTHERN HORIZON IS THE SUN ON THE WINTER SOLSTICE?
A) 18 degrees.
B) 23.5 degrees.
C) 41 degrees.
D) 49 degrees.

21. WINTER SOLSTICE

22. FLASHCARD
FROM VANCOUVER (49 DEGREES NORTH), HOW HIGH ABOVE THE HORIZON DOES THE SUN GET AT THE SUMMER SOLSTICE?
A) 64 degrees.
B) 49 degrees.
C) 41 degrees.
D) 23.5 degrees.

23. SUMMER SOLSTICE

24. PLANETARY INCLINATIONS
Inclination Equator to Orbit
Seasons?
Mercury
0 degrees No
Venus
177.4 degrees
Yes retrograde
Earth
23.5 degrees
Yes
Mars
25.2 degrees
Yes (like Earth)
Jupiter
3 degrees
Saturn
27 degrees
Uranus
97.9 degrees
Yes (strange)
Neptune
29.6 degrees
Pluto
122 degrees
Most planets
will experience
seasons as
their axes are
inclined to
the ecliptic

25. URANUS’ ORBIT
Seasons on Uranus are strange - its axis lies almost in its orbital plane.
North - 21 years of summer (right) with the pole always facing Sun.
North - 21 years winter (left) with pole always facing away from Sun.