Planetary Motion

By Carol Greco

Why do planets move the around the sun the way they do?

First you need to understand that scientists have discovered an invisible force that exists between two objects that pulls these two objects towards each

other. They call this force gravity. Click to see attraction.

Centripetal Motion Next, the gravitational force the Sun pulls on a planet is perpendicular to the

direction of the planet's movement. This gravitational force constantly changes the direction of the planet's forward motion so that it revolves in a

nearly circular orbit around the Sun. This force is described ascentripetal motion. Click to see this motion.

Kepler’s First LawIn the early 1600’s a scientist named Johannes

Kepler observed that planets moved in ellipses, or ovals, with the Sun at one focus.

Click to see this law.

When the planet gets closer to the sun the gravitational pull between them increases.

Kepler’s Second LawHe also observed that the radius vector describes equal areas in equal times.

Or you could say it this way, that if you drew an imaginary line connecting the planet to the Sun it would sweep out equal areas in equal times.

Click to see an example of this law.

NASA’s current missions exploring our neighboring planets

MESSENGER

Venus Express

LRO Lunar Reconnaissance

Orbiter Missions to Mars

Dawn

Cassini

New Horizons

Click on each picture to learn about that mission!

Resources

• The Solar System (slide 1)http://www.powerhouseanimation.com/PRG/EART037.swf

• NASA Kepler Mission (slide 4 picture and information)http://kepler.nasa.gov/Mission/JohannesKepler/

• Keplar’s Laws (slides 4 and 5 animations)http://www.drennon.org/science/kepler.htm

• Solar System Explorations: Missions (slide 6-13)http://solarsystem.nasa.gov/missions/index.cfm