In the construction, the green dot is the primary focus of the ellipse about which the planet orbits; the blue dot is the secondary focus. The black dot is on a circle at a distance in radius equal in length to the major axis of an ellipse. A line is drawn from the blue dot to the black dot and its perpendicular bisector is constructed. The point where this perpendicular bisector intersects the line from the green dot (primary focus) to the black dot (circle) is a point on the ellipse. The perpendicular bisector is tangent to the ellipse at this point. A further interesting point of the construction is that the length of the line from the blue dot (secondary focus) to the black dot (circle) is proportional to the velocity of the orbiting planet at this point. In the Demonstration, half this length is represented by the black vector traveling in the direction of the planet.

All of the possible ellipses with the given major axis are contained in the circle. You can adjust the eccentricity and rotation of the ellipse.