Back to the
Ask An Astronomer page.
On Mon, 16 Oct 2000, Jim wrote:
> I can understand why a planet turns around one focus of an ellipse but
> why does it trun around the other focus and come back whenthere is
> nothing there?
The focus of an ellipse is a mathematical definition, and there needn't be
anything there to make the ellipse an ellipse. The sun is at one focus of
the ellipse traced out by a planet, and it is the sun's gravity that pulls
a planet back. The other focus has nothing to do with the turning around.
Let's try a thought experiment to understand what is happening. If we had
a hole through the center of the Earth, and if you were to drop a rock
down the hole, the rock would move faster and faster (due to Earth's
gravity) until it reached the center of the Earth. Then, as it began to
travel up the other side, Earth's gravity would pull it back down again,
and the stone would begin to slow down until it finally stopped and went
back toward the center of the Earth again.
A very similar thing happens when a planet orbits the sun in an
ellipse. As the planet moves closer to the sun, the sun's gravity makes
it move faster. After the planet makes its closest approach to the sun,
it moves away from the sun, and so slows down until its motion away from
the sun stops, and it falls back inward again. (Note that this doesn't
mean the planet stops moving through space; rather, only the component of
its motion toward or away from the sun changes.)
The elliptical path of objects orbiting the sun comes about because of
gravity. If you take the time to solve the gravitational calculus
equations Isaac Newton discovered, you would find that ellipses are a
naturally occuring solution.
I hope this clears things up a little!
Sincerely,
Kurtis Williams
Back to the
Ask An Astronomer page.