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
to draw a circle.
The push pin marks the focus of the
Notice that every part of the outside of
the circle is the same distance from the focus at the
An ellipse looks a bit like a
An ellipse has two foci instead of one
The green dots inside the
mark the foci in this small
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
Our sun is very close to being at the
center of the ellipse.
Some comets, however, have long
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
on to Kepler's Second Law
© 1999, 2003. Elizabeth
Anne Viau. All rights reserved. This material may be used by
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