How do we measure the rotation speed of a planet? Exactly as you would expect.  Watch the surface, look for markable features, and time how long it takes until those features pass the same point again in the future.  But how can we possibly nail down this information when the planet has little to no visible surface features.  Gas giant planets are great examples of this.  Jupiter is a bit easier since it has plenty of storms and separated cloud layers along the planet’s rotation axis, but the other three are much tougher.  Aside from hard-to-spot features, gas giants also...