 <p>You do not use arctan or <code>atan</code> to find an angle (except where explicitly demanded), but the argument function <code>arg(x+i*y)</code> usually found under <code>atan2(y,x)</code>.</p> <p>The angle around the origin between two points <code>a</code> and <code>b</code> can be found as </p> <pre><code>arg( (b.x+i*b.y)/(a.x+i*a.y) ) </code></pre> <p>or in real components</p> <pre><code>atan2(a.x*b.y-a.y*b.x, a.x*b.x + a.y*b.y) </code></pre>