What is the shape of our orbit?
Aim: What are the Laws of
Planetary Motion?
Topic: Unit 8 Astronomy
Main Idea: Johannes Kepler & Sir Isaac Newton
What is Kepler’s First Law of
Planetary Motion (Law of Orbits)?
• Planets travel around the sun in slightly
flattened circles called ellipses.
• Eccentricity measures how flattened the
circle is
• e=d/L
• 0=perfect circle; 1=straight line
• The sun is always at one focal point
Parts of an ellipse
Kepler’s Second Law (Law
of Equal Areas)
• A planet
will sweep
equal
amounts
of area in
equal
amounts
of time.
What date are we traveling
the fastest in our orbit?
Slowest?
Kepler’s Third Law (Law of Periods)
• Planets
closer to the
sun go
around it
faster
because
there is more
gravitational
pull.
Newton’s Universal Law of
Gravitation
• Closer objects =
bigger
attraction
• Bigger objects =
bigger
attraction
Is there a relationship between distance to the sun
and eccentricity of orbit?