Chapter 20: Formation of the Solar System
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.
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.
• 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.
• 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
• Kepler’s Third Law of Motion Kepler observed
that planets more distant from the sun, such
as Saturn, take longer to orbit the sun.
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.
• 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.
• 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.