How to calculate an angle from points?

Question

I want to get a simple solution to calculate the angle of a line (like a pointer of a clock).

I have 2 points:

cX, cY - the center of the line.
eX, e         


        

Answer 1

You find here two formulas ,one from positive axis x and anticlockwise

and one from the north and clockwise.

There is x=x2-x1 and y=y2=y1 .There is E=E2-E1 and N=N2-N1.

The formulas are working for any value of x,y, E and N.

For x=y=0 or E=N=0 the result is undefined.

f(x,y)=pi()-pi()/2*(1+sign(x))*(1-sign(y^2))

     -pi()/4*(2+sign(x))*sign(y)

     -sign(x*y)*atan((abs(x)-abs(y))/(abs(x)+abs(y)))

f(E,N)=pi()-pi()/2*(1+sign(N))*(1-sign(E^2))

     -pi()/4*(2+sign(N))*sign(E)

     -sign(E*N)*atan((abs(N)-abs(E))/(abs(N)+abs(E)))