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

For Jupiter, the clouds near the equator rotate almost 1% faster than clouds nearer the poles. Radio observations allow the rotation period of Jupiter’s magnetic field to be determined and this is only 0.03% faster than the polar rotation. It’s the radio period that’s given as the standard measurement of Jupiter’s day.