'Math - How to calculate arctangents in all four quadrants 
'Create a new project with a form and place a label (label1) on that.
'
'And place the following code:
 
Option Explicit
 
Private Sub Form_Load()
    AutoRedraw = True
    ForeColor = vbRed
    Line (0, ScaleHeight / 2)-Step(ScaleWidth, 0)
    Line (ScaleWidth / 2, 0)-Step(0, ScaleHeight)
    ForeColor = vbBlack
    Picture = Image
End Sub
 
'Atan2(): Four-Quadrant Arc-Tangent                       *
'
'Given the coördinates, {x, y}, of a point, this function returns the angle,
'in radians, between the positive x-axis and a line from the origin to the
'point. The point can be anywhere in the plane. Atan2(0,0) is undefined, but
'returns 0. The arguments are called by value so there is no chance of the
'function corrupting them.
'
'Invocation and Domain of the result:
'
'                           -pi < Atan2(y,x) <= +pi
'
'The function is invoked exactly as is the atan2() function supplied with the
'C and C++ runtime libraries.
'
Public Function Atan2(ByVal Y As Double, ByVal X As Double) As Double
    Dim theta As Double
 
    If (Abs(X) < 0.0000001) Then
        If (Abs(Y) < 0.0000001) Then
            theta = 0#
        ElseIf (Y > 0#) Then
            theta = 1.5707963267949
        Else
            theta = -1.5707963267949
        End If
    Else
        theta = Atn(Y / X)
 
        If (X < 0) Then
            If (Y >= 0#) Then
                theta = 3.14159265358979 + theta
            Else
                theta = theta - 3.14159265358979
            End If
        End If
    End If
    Atan2 = theta
End Function
 
Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
    Const PI = 3.14159265
    Dim radians As Single
    Dim degrees As Single
    Cls
    Line (ScaleWidth / 2, ScaleHeight / 2)-(X, Y)
    ' Reverse the Y distance's sign to get a clockwise measurement.
    radians = Atan2(-(Y - ScaleHeight / 2), X - ScaleWidth / 2)
    degrees = radians / PI * 180
    Label1.Caption = Format$(radians, "0.00") & ", " & Format$(degrees, "0.00")
End Sub