cesium去掉黑色背景，只保留地球

 var viewer = new Cesium.Viewer('cesiumContainer',{
      allowDataSourcesToSuspendAnimation: false,
      timeline: false,//是否显示时间轴
      sceneModePicker: false,//是否显示3D/2D选择器
      baseLayerPicker: false,//是否显示图层选择器
      geocoder: false,// 搜索
      scene3DOnly: true,//如果设置为true，则所有几何图形以3D模式绘制以节约GPU资源
      animation: false,//左下角仪表盘
      navigationHelpButton: false,//右上角的帮助按钮
      homeButton: false,//是否显示Home按钮
      infoBox: false,//?
      fullscreenButton: false,//是否显示全屏按钮
      showRenderLoopErrors: true,//是否显示渲染错误
      //imageryProvider:new Cesium.SingleTileImageryProvider({
        //url:'../../../Specs/Data/Cesium3DTiles/1.jpg'//透明图片
      //}),
      orderIndependentTranslucency:true,
      contextOptions: {
        webgl: {
            alpha: true,
        }
      },
      skyBox:false,
      //关闭月亮、星星等
      skyBox: new Cesium.SkyBox({  
        show: false
      }),
      //关闭地球光环
      skyAtmosphere: false
    });