As you know an ellipse equation is:
$(x/a)^2 + (y/b)^2 = 1$ in Cartesian system. (center of the system is at the center of the ellipse)
but also it is:
$r=a(1-e^2)/(1+e \cos (\theta))$ in polar system. ($r$ is distance to one of the focal points and $\theta$ is the angle between the arbitrary point, focal point and periapsis, i.e. if we name the point $Q$, the focal point $F$ and the periapsis $P$, $\angle QFP$)
If we have $x = x_0 , y= y_0$ (we know the numerical values) and also the $e$, How can we find the $r_0$ and the $\theta_0$ for this specific point.
