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

Gaseous planets do not experience uniform rotation rates at their cloud tops across all latitudes. You can take an average for the rotation of a section of gas as it rotates about the center of the planet (For example, identifying a storm system and watching it rotate around the planet), but usually the rotation rate of the planet’s internal magnetic field is used to define a sidereal day for them. If you look at the wikipedia page for planetary rotations, you can see that several different figures can be used for different latitudes.

https://en.m.wikipedia.org/wiki/Rotation_period