How can astronomers find a planet's rotational period when they cannot see its surface?

The rotation periods of Jupiter, Saturn, Uranus, and Neptune range from roughly 10 to 17 hours. Estimating giant planets’ rotation rates, however, is not easy. Because they don’t have solid surfaces, we can’t infer their periods from following the reoccurring surface features. We must use alternative methods. One way is to observe the clouds and see how long it takes them to reappear (cloud tracking). But it’s unclear whether clouds rotate at the same rate as the planet.

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