World Builders™                                                                        Session One  --  Astronomy              
        Kepler's Laws       

                                     of Planetary Motion

                                    First Law

This law tells us about the shape of the paths of the planets as they travel around their suns.

Kepler's First Law

The orbit of a planet around the Sun is an ellipse with the Sun at one focus.

What does this mean?

The orbit is the path that the planet follows as it journeys around the sun.

        This is an ellipse.

You can see that an ellipse looks rather like a stretched out circle.


In a circle, the focus is at the very center, where the green dot is in this picture.


You can use a push pin and some string 

                                                             to draw a circle.

The push pin marks the focus of the circle.


Notice that every part of the outside of the circle is the same distance from the focus at the center.

An ellipse looks a bit like a stretched-out circle.

 An ellipse has two foci instead of one focus. 

The green dots inside the ellipse 

                mark the foci 
in this small picture.



You can draw an ellipse like this one, using two push pins and a piece of string.

In our own solar system the planetary orbits are close to being circular. 

Our sun is very close to being at the center of the ellipse.


Some comets, however, have long elliptical orbits.


It is possible that some planetary systems have planets with orbits that are long and narrow. How would that affect life on those planets?

Let's look at Kepler's law again. Does it make sense now?

The orbit of a planet around the Sun is an ellipse with the Sun at one focus.

Go on to Kepler's Second Law

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© 1999, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at