Edit in JSFiddle

var paper;

window.onload = function(){
    paper = Raphael(0,0,"100%","100%");
    paper.path(
        getArc($(window).width()/2,$(window).height()/2,70,120)
    ).attr({"stroke-width":10});        
};


function getArc(cx,cy,r,value) {
    var a =(90-value)*Math.PI/180,
    x = cx+r*Math.cos(a),
    y = cy-r*Math.sin(a),
    path;
    
    if(value==360){
        path=[["M",cx,cy-r],["A",r,r,0,1,1,cx-0.001,cy-r]];
    }else{
        path=[["M",cx,cy-r], ["A",r,r,0,+(value>180),1,x,y]];
    }
    return path;
}

<script type="text/javascript" src="http://dl.dropbox.com/u/63305355/raphael-min.js"></script>
<div id="canvas">
    <div class="text">
        120°
        <div id="result"></div>
    </div>
</div>

body,html
{
    height:100%;
    margin:0;
}

#canvas 
{
    height:99%;
    background:#fff;
    text-align:center;
}

.text{
    letter-spacing: 0px;
    color:#fff;
    font-size:150%;
    position:absolute;
    text-align:center;
    top:40%;
    left:0%;
    width:100%;
    z-index:1;
    background:#000;
    opacity:0.5;
}
#result{
    font-size:60%;
    letter-spacing: 5px;
}
input{
    padding:3px;
}