Find the radius in an ellipse from its angle Basically, I want to know the length of the radius of an ellipse, based on the angle this radius makes with either of the 2 main radius.

Is that possible to do?
The equation of an ellipse is $(\frac xa)^2+(\frac yb)^2=1$.  If you know the angle $\theta$ from the $x$ axis, you have $y= x \tan (\theta)$.  Now substitute in to get $x^2(\frac 1{a^2}+\frac {\tan^2 \theta}{b^2})=1$  This gives you $x$, then you can find $y$, then $r=\sqrt {x^2+y^2}$
$$\left(\frac{x}{a}\right)^2+\left(\frac{y}{b}\right)^2=r^2$$
$$x=acos(t),y=bsin(t)$$
$$r=\sqrt{x^2+y^2}=\sqrt{a^2cos^2(t)+b^2sin^2(t)}\text{, where "t" is the angle from }0\text{ to }2\pi.$$
If the angle from the minor axis is $\phi$ and the angle from the major axis is $\psi$, then $\phi=\frac{\pi}{2}-t$ and $\psi=\pi-t$.
$$r=\sqrt{a^2cos^2(\pi-\psi)+b^2sin^2(\frac{\pi}{2}-\phi)}$$