1. What causes the seasons?
What are the differences between days in summer and winter?
Longer days
stronger Sun
in summer than in winter

2. How long to go all the way around?
1 day
b. 1 week
c. 1 month
d. 1 year

3. There are two ways
to think about the
motion of the skey:
- the C.S. rotates
- or the Earth rotates while the sky sits still.
Sometimes one works better than the other …

4. Questions
Where on Earth would stars circle straight overhead around NCP?
Where on Earth can you see both poles?

5. Latitude and longitude make a convenient coordinate system for locating objects on the Earth.
Altitude and Azimuth give us the direction to look on the celestial
sphere.

6. Are there stars in the sky during the daytime?

7. The altitude of Polaris above the horizon is approximately the same as the observer's latitude in the Northern Hemisphere.

8. Are the seasons due to the changing distance from the sun?
(Not to scale)
Distance doesn't matter here!

9. Seasons are due to the tilt of the earth.

15. The sun rises in the northeast and sets in the northwest!
When is the sunrise due east?

16. The sun rises in the southeast and sets in the southwest!
The sun is above our horizon longer in summer than in winter.

17. Suppose you are in Australia in August. Which of the following is true:
It’s winter and the Sun rises in the Northeast
It’s summer and the Sun rises in the Northeast
It’s winter and the Sun rises in the Southeast
It’s summer and the Sun rises in the South east

18. You are dropped onto an unknown location on Earth (with no GPS)
You are dropped onto an unknown location on Earth (with no GPS). Cleverly, you note that Polaris is about 30o above the horizon. What's your latitude?
a o north
b. 30o south
c. 60o north
d. 60o south

19. Discussion Questions/Issues
What would happen to the seasons if the earth were tipped at 35 degrees, instead of 23.5 degrees?
How about if it were not tipped?
Tilt does change: from 22 to 24 deg (currently 23.5) every 41,000 yr
Ellipticity (distance from Sun) also changes by 1.7% every 100,000 yr
Think what happens when effects combine: more tilt and extreme distances from Sun

20. Assigned reading
Chapter 2