What causes the seasons?

What are the differences between days in summer and winter?

Longer days

stronger Sun

in summer than in winter

How long to go all the way around?

d. 1 year

c. 1 month

b. 1 week

1 day

There are two ways to think about the motion of the skey: - the C.S. rotates - or the Earthrotates while the sky sits still.

Sometimes one works better thanthe other …

Questions

• Where on Earth would stars circle straight overhead around NCP?

• Where on Earth can you see both poles?

Latitude and longitude make aconvenient coordinatesystem for locatingobjects on the Earth.

Altitude and Azimuthgive us the directionto look on the celestialsphere.

Are there stars in the sky during the daytime?

The altitude of Polaris above the horizon is approximately the

same as the observer's latitude in the

Northern Hemisphere.

Are the seasons due to the changing distance from the sun?

Distance doesn't matter here!(Not to scale)

Seasons are due to the tilt of the earth.

The sun rises in the northeast and sets in the northwest!

When is the sunrise due east?

The sun rises in the southeast and sets in the southwest!

The sun is above our horizon longer in summer than in winter.

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

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. 30o north

d. 60o south

c. 60o north

b. 30o south

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

Assigned reading

• Chapter 2