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

I have a related question of how the “surface” of a gas planet is determined for purpose of measuring things like the radius, circumference, surface area, etc. If you look up any of these planets on wikipedia you’ll see numbers for all these things which means there must be some “official” way of determining it.