of Planetary Motion

                                    First Law

        This is an ellipse.

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

                                     focus

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?

Go on to Kepler's Second Law

---