There is also the extremely ingenious invention of the hodograph.
So that we have in the circling electro-magnetic Aether currents a physical explanation for the hodograph of any planet.
In the motion of a projectile under gravity the hodograph is a vertical line described with constant velocity.
The annexed fig. 70 exhibits the various cases, with the hodograph in its proper orientation.
The pole O of the hodograph is inside on or outside the circle, according as the orbit is an ellipse, parabola or hyperbola.
Thus for a circular orbit with the centre of force at an excentric point, the hodograph is a conic with the pole as focus.
For applications of the hodograph to the solution of kinematical problems see Mechanics.