The gas planets (Jupiter, Saturn, Uranus, and Neptune) have no surface to speak of, so how was it determined how long a day is?

Question

The gas planets (Jupiter, Saturn, Uranus, and Neptune) have no surface to speak of, so how was it determined how long a day is?

In: 549

Answer 1

You can measure the average rotation rate of all the particles. It’s like looking at how long it takes the great red spot to go around Jupiter. Each individual but of it moves at a different rate, but overall it balances out.