An ellipse has two foci, plural of the word "focus. you can draw ellipses by putting pins or nails through a piece of paper tying strings to the pins and nails, then tie the other ends of the strings to one pencil or marker, Draw an ellipse around the nails, pins. Change the distances between the pins/nails and draw anew ellipse. Changing the distance between the foci changes the eccentricity of the ellipse and the orbit. For planetary and comets, the sun is ALWAYS one of the foci of the orbit even when the orbit of the comet is a parabola or hyperbola. Comets with parabolic or hyperbolic orbits never return. The get close to the sun one time and eventually leave the solar system entirely. Returning comets, like Halley, have closed elliptical orbits with high eccentricities, but the sun is at one of the focuses of the ellipse.
All you need to do the hands on demonstration is a piece of cardboard, a sheet of paper, thumb tacks, push pins, or nails, thread or string, and a pencil, pen, or marker, You'll understand the geometry a lot better if you actually do the hands on demonstration. You don't have draw perfect ellipses. Drawing perfect ellipses isn't the point of the demonstration, but it will be a lot more "real to you if you actually do the experiment rather than just thinking about it in your head.
