A simple Cesium example of drawing a label at the horizon point.

DrawHorizonPoint.js

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

var scene = viewer.scene;

var ellipsoid = scene.globe.ellipsoid;
var entity = viewer.entities.add({
    label : {
        show : true,
        outlineColor: Cesium.Color.BLACK,
        verticalOrigin : Cesium.VerticalOrigin.BOTTOM,
        style : Cesium.LabelStyle.FILL_AND_OUTLINE
    }
});

function updateHorizonPoint(){
    var canvas = viewer.canvas;
    var height = canvas.height;
    var width = Math.floor(canvas.width/2);
    for(var i = 0; i < height; i++) {
        var cartesian = viewer.camera.pickEllipsoid(new Cesium.Cartesian2(width, i), ellipsoid);
        if (cartesian) {
            if(i === 0){
                //Sky not visible
                entity.label.show = false;
                return;
            }
            var cartographic = ellipsoid.cartesianToCartographic(cartesian);
            var longitudeString = Cesium.Math.toDegrees(cartographic.longitude).toFixed(2);
            var latitudeString = Cesium.Math.toDegrees(cartographic.latitude).toFixed(2);

            entity.position = cartesian;
            entity.label.show = true;
            entity.label.text = '(' + longitudeString + ', ' + latitudeString + ')';
            return;
        }
    }
    entity.label.show = false;
}


scene.preRender.addEventListener(updateHorizonPoint);