GeoOverlay.js

var viewer = new Cesium.Viewer('cesiumContainer');

//Normally you would define the default styles
//in CSS, but for the example it's easier to 
//just do it here.
var geoOverlay = document.createElement('div');
viewer.container.appendChild(geoOverlay);
geoOverlay.style.display = 'none';
geoOverlay.style['background-color'] = 'white';
geoOverlay.style.position = 'absolute';
geoOverlay.style.top = '0';
geoOverlay.style.left = '0';
geoOverlay.style.width = '20px';
geoOverlay.style.height = '20px';
geoOverlay.style['pointer-events'] = 'none'; //Disable mouse inter

//The geolocation that we want to the element t olive.
var anchor = Cesium.Cartesian3.fromDegrees(0, 0, 0);

//Every frame, figure out if the geolocation is on the screen
//and move the element accordingly.
var tmp = new Cesium.Cartesian2();
viewer.scene.preRender.addEventListener(function(){
    var result = Cesium.SceneTransforms.wgs84ToWindowCoordinates(viewer.scene, anchor, tmp);
    if(Cesium.defined(result)){
        geoOverlay.style.display = 'block';
        geoOverlay.style.top = tmp.y + 'px';
        geoOverlay.style.left = tmp.x + 'px';
    } else {
        geoOverlay.style.display = 'none';
    }
});