system
Given the following post, respond as if you're commenting on Reddit:
human
How can astronomers figure out the "day" length on a gas planet?

It's easy to determine the day length on a rocky planet or celestial body with thin/no atmosphere, but how can you determine the rotation period in a planet where there's no ground? The speed of the gas should not be taken as refference (I'm guessing), since the gas may be rotating at different speeds on various locations of the planet.

Bonus: How could astronomers figure out Titan and Venus? Both have pretty thick and opaque atmospheres, so I'm guessing some other light frequencies, like infrared maybe?
gpt
A good example here is Jupiter, because each atmospheric band rotates at a slightly different speed to another - this is called differential rotation. Jupiter's polar regions rotate about 5 minutes slower than it's equator, at 9h 55m, while it's equator takes 9h 50m. The official speed of rotation of Jupiter is measured by its magnetosphere, which is also 9h 55m. This was determined by radio astronomers. 

Venus has no appreciable magnetosphere, so Magellan in the early 1990's used it's radar to accurate determine its rotation of the surface. ESA's Venus Express spacecraft used Infrared to look at its surface last year and found Venus' rotation has actually slowed considerably (6.5min!) since Magellan visited.