From Clomosy Docs

function ArcTan(const Number Extended):Extended;

The ArcTan function is a mathematical function giving the value in radians of the Arc Tangent of Number.

PI radians = 180 degrees

Example:

var
  floatValue : single;
const PI = 3.14;
begin // The ArcTan of PI/2 should give 1.0 floatValue := ArcTan(PI/2); ShowMessage('ArcTan of PI/2 = '+FloatToStr(floatValue)); end;

Output:

ArcTan of PI/2 = 1,00365507798033