How to get coordinates of a point on a circle and its angle to the center I have a circle. I know the radius (800) and I know the point coordinates (0, -800) under the circle. I double the point and move this one to the right. And a second point now has coordinates (500, -800). I have to define y (z coordinates according to my screenshot) value like a point is located on the circle and define an angle to the center (0, 0, 0).
I have tried:

*

*If the bottom point has an angle of 0°, then a point in the middle of the right has a 90° angle. If movement of X axis is located within |0, R| then I can say the angle equals $$(90/R)*500$$. R is the radius. But then I got 56.25°. It is incorrect because the direction vector doesn't point to the center.

*I tried to define an angle using the equation: $$cos(x)*R=500$$ and then I got 0.624999996583 rad = 35.8098619999856°. It is also incorrect for the same reason.

How can I define y value and the angle?

 A: Thank you all for your comments. It was my bad. I thought in equation $$800\cos(x) = 500,$$$x = \frac58$, but $$x = \arccos\left(\frac58\right).$$Therefore, I can define the angle this way. So then the y value equals $$R\sin(\text{angle}).$$
A: The following is very important for everyone working with 2D polar and Cartesian coordinates, especially programmers:
$$\left\lbrace ~ \begin{aligned}
x &= r \cos \theta \\
y &= r \sin \theta \\
\end{aligned} \right. \quad \iff \quad \left\lbrace ~ \begin{aligned}
r &= \sqrt{x^2 + y^2} \\
\theta &= \operatorname{atan2}(y, x) \\
\end{aligned} \right.$$
where $\operatorname{atan2}()$ denotes the two-argument arctangent:
$$\operatorname{atan2}(y, x) = \begin{cases}
\arctan\left(\frac{y}{x}\right), & x \gt 0 \\
\arctan\left(\frac{y}{x}\right) + 180°, & x \lt 0, ~ y \ge 0 \\
\arctan\left(\frac{y}{x}\right) - 180°, & x \lt 0, ~ y \lt 0 \\
+180°, & x = 0, ~ y \gt 0 \\
-180°, & x = 0, ~ y \lt 0 \\
\text{undefined}, & x = 0, ~ y = 0 \\
\end{cases}$$
although most programming languages implement $\operatorname{atan2}(0, 0) = 0$.
