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

A day is one rotation of a planet around it’s own axis, how would the existence or lack of surface factor into this?

Answer 2

Ngl I didn’t read all the comments but I did the first few and it literally says ELI5 and some yall just wanna sound smart on reddit lol the answers arnt wrong but damm u didn’t make it simpler to understand aha

Answer 3

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.