Laws of Planetary Motion

First Law

• The planets revolve about the sun along an elliptical path. This idea went against the predominant belief of the time that planets followed a simple circle. In addition, the sun is located at one focus of the ellipse, giving the planets a perihelion and aphelion.

Second Law

• Gravity from the sun causes the planets to move more quickly when they approach the sun and slower when they're farther away. The result is that the planets "sweep out" the same area of the ellipse during the same amount of time.

Third Law

• Kepler's Third Law of Planetary Motion was formulated years after the publication of the first two. It simply provides mathematical proof that planets further from the sun travel more slowly than those that are closer. It states that the square of the orbital period of any planet is proportional to the cube of the planet's average distance from the sun.

Significance

• The laws of planetary motion proved very significant for space travel, providing necessary answers to a number of questions. It told astronomers how long it would take to enter into orbit around another planet and how to put a satellite into synchronous orbit around the Earth.

Delayed Success

• It wasn't until 50 years after Kepler's laws were hypothesized that Newton finally proved them correct with his Law of Universal Gravitation.

Incomplete Data

• Kepler's Laws of Planetary Motion were formulated before the world knew of the three outer bodies of the solar system: Uranus, Neptune and Pluto. But outer planets follow the rules just as did the six planets known at the time, further validating Kepler's ideas.