Planetary Motion

Johannes Kepler's Three Laws of Orbits

  1. Each planet orbits the Sun in an ellipse
  2. Straight line joining a planet and the Sun sweeps out equal areas in space and equal areas in time
    • Eg. in 60 days a planet will move the same amount of area, no matter how close it is to the body it orbits
    • The Law of Equal Areas
    • The Law of Equal Areas
    • Discovered by observing orbits speeding up as a planet gets closer to the Sun
    • Consequence of the conservation of angular momentum
    • Figure skater law
  3. A planet's orbital period (PP) squared is proportional to the semi-major axis (aa) of its orbit cubed
    • P2āˆa3P^2 \propto a^3
    • When PP is measured in years and aa is measured in astronomical units, the two sides are equal
    • Applies to all objects orbiting the Sun
    • Provides means for calculating objects relative distances from the Sun
    • Law of harmonies

Newton's Addition to the Third Law

Space Astronomy Physics