Thread by @BadAstronomer

We *just* figured out how long a day is on Saturn. Like, a couple of weeks ago.

Gas giant planets are so WEIRD.

syfy.com/syfywire/how-l…

Something I thought a lot about while reading the journal paper about this, and writing my own article: If a planet isn’t solid, does it even *have* a definable day?

(any planetary scientists wanna chip in here, I’m listening…)

Imagine a planet made of two layers: An inner one (call it the core) and an outer one. Say they spin at different rates. What’s the planet’s day? The time it takes the core to rotate, or the outer layer?

Now take a gas giant with a weird hot dense metal hydrogen core, a fuzzy ocean mantle that maybe has ices in it, and an atmosphere that’s thousands of km deep. Without a solid surface, what do you use as a landmark? What’s a day?

You can measure how long it takes the magnetic field to rotate once to great accuracy, and that’s tied to stuff way below the surface. We do this for Jupiter, for example. It’s harder for Saturn (I explain why in the article: syfy.com/syfywire/how-l…)

The atmosphere is layered by latitude, too. Stuff at the equator may go around the planet faster or slower than stuff at higher latitudes. It’s a mess.

My point is you have to have different ideas for what a “day” is, and maybe be flexible and not be too pedantic about exact definitions.

That’s good advice for anything, really.