Friday January 15th 1999
Announcements:

Lecture notes:
Four Basic Important Questions

• Why day and night?
• What causes the seasons?
• Why different stars in summer and winter?
• What makes phases of moon?

Four Basic Important Facts

• Earth orbits sun in 1 yr
• Earth rotates in 1 day
• Rotation axis tilted from plane of orbit by 23.5 degrees
• Moon orbits Earth in about 27.3 days

Title of today's lecture is Darkness and Light Winter and Summer

discuss and answer 5 basic but important questions

To answer them we will frequently use 4 important facts

Question 1 - Why day and night?

Circumpolar rotation = the stars appear to move around a particular star called the North Star (Polaris) during course of night

Point out Big Dipper and pointer stars
    Little Dipper and Polaris
    Arcturus - students name it

Takes 24 hours - 24 hour rotation of Earth
What would sky at North Pole look like - pointer stars still point to Polaris

Question 3 - What causes seasons?
What makes days longer in summer than in winter?

Use fact #3 - rotation axis is tilted from orbital plane by 23.5 degrees
Earth's rotation vector points towards Polaris and is fixed there
Changes angle at which sun's rays strikej different parts of Earth's surface

Extreme of North Pole and South Pole
In far North during summer doesn't get dark at all.

Seasons produced by combination of tilt of Earth's axis and the revolution about the sun.

No tilt = no seasons

Summer solstice is June 21 = longest daylight of year in far Northern hemisphere

Winter solstice = December 21 = shortest daylight of year in far Southern hemisphere

Fall and spring equinoxes
Spring equinox = March 21
Autumn equinox = September 23

Why different stars in summer and winter?
Can't see stars located behind sun.

Phases of Moon
not Earth blocking sunlight from Moon - this only happens during eclipses