1. Announcements Don’t forget to take the test! (today, tomorrow, Friday before class) Don’t discuss the test with classmates until class on Friday. Pick up graded homework. First project due in 9 days!

2. Earth as a Planet 4 October 2006

3. Course Outline Naked-eye astronomy Crash course in physics Our solar system The stars Structure and history of the universe

4. Course Outline Naked-eye astronomy Crash course in physics Our solar system The stars Structure and history of the universe You are here

5. Today: Some important characteristics of Earth to keep in mind as we compare it to other planets

6. The planets, to scale

7. Earth My favorite planet Largest known rock Very geologically active Atmosphere of nitrogen and oxygen 3/4 covered with water Nice, comfortable surface temperature Only known home of life

8. Earth: Geology

11. Earth Made of rock (why?) Hot inside (why?) Atmosphere of nitrogen and oxygen (why?) 3/4 covered with water (why?) Nice, comfortable surface temperature (why?)

12. Some partial answers Made of rock because heavy atoms (Si, O, C, Ca, Na, K, Fe, Mg) tend to form rocks, and light atoms (esp. H, He) are uncommon on earth (why?) Hot inside because of radioactive decay of rare atoms (U, Th, K, Rb), plus some residual heat from earth’s formation; heat builds up and escapes slowly due to earth’s size Atmosphere of nitrogen (N 2 ) and oxygen (O 2 ) because plant life has removed most carbon dioxide (CO 2 ) and produced oxygen 3/4 covered with water because of size and surface temperature Nice, comfortable surface temperature because of our distance from the sun

13. Earth: Life

14. Earth’s Moon 1/4 earth’s diameter 1/80 earth’s mass Covered with craters, ancient lava flows Geologically inactive No atmosphere Essentially no water Hot days, cold nights Only other world humans have visited