1. Chapter 20
Section 4 Planetary Motion
Bellringer
A mnemonic device is a phrase, rhyme, or anything that helps you remember a fact. Create a mnemonic device that will help you differentiate between planetary rotation and revolution.
Record your mnemonic device in your science journal.

2. Chapter 20
Section 4 Planetary Motion
Objectives
Explain the difference between rotation and revolution.
Describe three laws of planetary motion.
Describe how distance and mass affect gravitational attraction.

3. A Revolution in Astronomy
Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy
Each planet spins on its axis. The spinning of a body, such a planet, on its axis is called rotation.
The orbit is the path that a body follows as it travels around another body in space.
A revolution is one complete trip along an orbit.

4. Chapter 20
Section 4 Planetary Motion
Earth’s Rotation and Revolution

5. A Revolution in Astronomy, continued
Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
Johannes Kepler made careful observations of the planets that led to important discoveries about planetary motion.
Kepler’s First Law of Motion Kepler discovered that the planets move around the sun in elliptical orbits.

6. Chapter 20
Section 4 Planetary Motion
Ellipse

7. A Revolution in Astronomy, continued
Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
Kepler’s Second Law of Motion Kepler noted that the planets seemed to move faster when they are close to the sun and slower when they are farther away.

8. A Revolution in Astronomy, continued
Chapter 20
Section 4 Planetary Motion
A Revolution in Astronomy, continued
Kepler’s Third Law of Motion Kepler observed that planets more distant from the sun, such as Saturn, take longer to orbit the sun.

9. Chapter 20 Newton to the Rescue!
Section 4 Planetary Motion
Newton to the Rescue!
Kepler did not understand what causes the plans farther from the sun to move slower than the closer planets.
Sir Isaac Newton’s description of gravity provides an answer.

10. Newton to the Rescue! continued
Chapter 20
Section 4 Planetary Motion
Newton to the Rescue! continued
The Law of Universal Gravitation Newton’s law of universal gravitation states that the force of gravity depends on the product of the masses of the objects divided by the square of the distance between the objects.
According to this law, if two objects are moved farther apart, there will be less gravitational attraction between them.

11. Newton to the Rescue! continued
Chapter 20
Section 4 Planetary Motion
Newton to the Rescue! continued
Orbits Falling Down and Around Inertia is an object’s resistance to change in speed or direction until an outside force acts on the object.
Gravitational attraction keeps the planets in their orbits. Inertia keeps the planets moving along their orbits.

12. Chapter 20 Section 4 Planetary Motion
Gravity and the Motion of the Moon