Why This code not rotate the image?

Code:
<!DOCTYPE html>
<html>
<body>

<p>Image to use:</p>
<img id="scream" src="http://www.webdeveloper.com/forum/images/webdev-logo2.gif" alt="The Scream" width="220" height="277">

<p>Canvas:</p>
<canvas id="myCanvas" width="250" height="300" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.</canvas>

<script>

var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
var img=document.getElementById("scream");
ctx.drawImage(img,10,10,150,180);
ctx.rotate(30*Math.PI/180);
</script>

</body>
</html>
base on the code on:
http://www.w3schools.com/tags/tryit....vas_drawimage2

Thanks
Nadav