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

They really don’t have surfaces? I thought that gas planets just meant they had a ton of gas on a planetary scale. I didn’t know that meant they LITERALLY have no surface.