1. Warm-up Explain the difference between precession and nutation.
(check your notes from last week)
TEST FRIDAY

2. The Universe

3. Organization (from large to small)
Universe - made of galaxies
Galaxies - made of many stars
Some stars have planet systems
Earth orbits one particular star (our Sun)

4. Galaxies
Galaxy- a collection of stars, dust, and gas bound together by gravity
Barred
Elliptical
Spiral
Irregular

5. Big Bang theory
The theory that the universe began as a point and has been expanding ever since.
Not an explosion into space but an expansion of space
Matter going along for the ride

6. Big Bang theory
Competition between the outward expansion of the universe and inward force of gravity as the matter in the universe slows the expansion

8. Nebula- A large cloud of gas and dust in interstellar space; a region in space where stars are born or where stars explode at the end of their lives

9. Solar System Formation
Collapse of the solar nebula caused heating at the center
Planetesimals (small planets) formed in the surrounding space
The central mass of the nebula became the Sun
Planets formed from the surrounding materials

10. Solar System Formation
There are an estimated 100 billion stars in the Milky Way alone!

11. Terrestrial Planet Formation
Coalescence of small particles
High temperatures developed, melting interiors

12. Rotation- the spin of a body on its axis
Revolution- the motion of a body that travels around another body in space; one complete cycle of the orbit
Orbit- the path that a body follows as it travels around another body in space

13. Kepler’s First Law
Johannes Kepler, using Brahe’s accurate observations of planetary positions, demonstrated that planets orbit the Sun in an ellipse

14. Kepler’s Second Law
A line between the Sun and a planet sweeps equal areas in equal times

15. Kepler’s Third Law
The square of the orbital period (P) of a planet equals the cube of the semi-major axis of the orbital ellipse (a)
P2 = a3

16. Kepler’s Third Law
As planets get farther from the sun, the length of their orbit increases
The amount of time to complete one orbit around the sun (period) also increases

17. Astronomical Unit AU Earth’s average distance from the Sun
1.5 x 108 km
Used as base for measuring the distance between all planets
Used in the next diagram – third law

18. Why Do Planets Orbit?
In 1684, Isaac Newton published his mathematical and physical explanation of the motions of celestial bodies.
Law of Universal Gravitation – any two bodies attract each other with a force that depends on their masses and the distance between the two bodies.