cesium地图自传，自动旋转

     //旋转
      viewer.clock.multiplier = 55;//速度
      viewer.clock.shouldAnimate = true;
      let previousTime = viewer.clock.currentTime.secondsOfDay;
      function onTickCallback() {
        let spinRate = 1;
        let currentTime = viewer.clock.currentTime.secondsOfDay;
        let delta = (currentTime - previousTime) / 1000;
        previousTime = currentTime;
        viewer.scene.camera.rotate(Cesium.Cartesian3.UNIT_Z, -spinRate * delta);
      }
      viewer.clock.onTick.addEventListener(onTickCallback);