Kepler's Laws
of
Planetary Motion
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.
In this orbit you can see that the sun is at one end of the
long, narrow ellipse. There is nothing at the other focus of
the ellipse.
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.
© 1999. 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 eviau@earthlink.net.
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