Webgl-Earth: rotate earth only horizontally in both directions

Question

I am developing an app, where I am using webglearth for drawing earth. now I am able to rotate it only horizontally through javascript while mouse is dragged. JSFiddle

earth.on('mousedown', function(e) {
    console.log(e.latitude + ', ' + e.longitude);
    if (e.longitude && e.latitude) {
      earth.on('mousemove', function(e) {
        console.log(e.latitude + ', ' + e.longitude);
        if (e.longitude) {
          //console.log('in',earth.getCenter(0),e.longitude);
          //earth.setView([earth.getCenter()[0],e.longitude]);
          earth.setCenter([earth.getCenter()[0], e.longitude]);
          //earth.setCenter([e.latitude,e.longitude]);
        }
      });
    }
  })
  earth.on('mouseup', function() {
   earth.offAll('mousemove');
  })

The problem is that the earth rotates to any two points clockwise or anticlockwise, whichever is closest. I don't find any method in the documentation where I can rotate it clockwise or anticlockwise. Has anyone does that before. Any help is appreciated.