Mapping the Paths of the Planets

Johannes Kepler's observations of the planets were so detailed and precise that he was able to determine that planets travel around the sun in ellipses, not circles, according to "laws of planetary motion."
This elliptical orbit can be demonstrated by using string, tape, a pencil and paper. The boundaries caused by the string are the same as the gravitational boundaries on the planet's orbit.

To do this demonstration, a 5-inch string is taped at both ends so that the taped ends are 4 inches apart. Keeping the string tight at all times. the pencil moves from one piece of tape to the other, drawing a curve. The process is repeated, this time starting above the two pieces of tape. The result will be an oval which mathematicians call an ellipse. Do this a second time, except use an 8-inch piece of string.

"An ellipse is defined by two points called foci. The foci were the two pieces of tape. The sum of the distances from any point on the ellipse to each of the foci is always the same. If you were to take any point on the first ellipse and measure the distance from that point to each piece of tape, the sum of those two distances would be 5 inches. That's the property of an ellipse." -Exploring Creation with General Science by Dr. Jay Wile.

The second ellipse is much more circular than the first one and the ellipses in which the planets travel around the sun are much more like the second ellipse. The sun is at one of the two foci.

source: Exploring Creation with General Science, Jay Wile