XVIII  

  XIX  

This means to calculate exactly on the circumference of the ellipse, which is certainly wrong. Nevertheless, let's take the risk:

  XX  

We simplify the equation because on the ellipse is difficult to specify (Fig. 16).

  XXI  

This equation is valid because and are the result of tilting a circle that is travelled at constant speed.
Hence:

  XXII  

So we have reached the point at which the calculation cannot be continued because equals