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.

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?

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

Go on to Kepler's Second Law

 Astronomy Information Menu Kepler's Second Law Top of Page