How to make earth spin

Hi,
I’m trying to have my globe gently rotate in space.
I tried with that:
// Globe rotation
viewer.clock.shouldAnimate = true;
viewer.clock.multiplier = 100
viewer.camera.setView({
destination: Cesium.Cartesian3.fromDegrees(c.data.longitude, c.data.latitude, 20000000)
});
but instead of having my globe rotate, the sky is spinning instead. it seems like the camera is rotating together with the globe.
What would be the way to achieve that effect?
Thanks,
Nicola