var viewer = new Cesium.Viewer('cesiumContainer');
viewer.canvas.addEventListener('click', function(e){
    var mousePosition = new Cesium.Cartesian2(e.clientX, e.clientY);

    var ellipsoid = viewer.scene.globe.ellipsoid;
    var cartesian = viewer.camera.pickEllipsoid(mousePosition, ellipsoid);
    if (cartesian) {
        var cartographic = ellipsoid.cartesianToCartographic(cartesian);
        var longitudeString = Cesium.Math.toDegrees(cartographic.longitude).toFixed(2);
        var latitudeString = Cesium.Math.toDegrees(cartographic.latitude).toFixed(2);

        alert(longitudeString + ', ' + latitudeString);
    } else {
        alert('Globe was not picked');
    }
	
}, false);