For decades, it was a nagging mystery — how long does a day last on Saturn?

Earth pirouettes around its axis once every 24 hours or so, while Jupiter spins comparatively briskly, once in roughly 9.8 Earth-hours. And then there is Venus, a perplexingly sluggish spinner that takes 243 Earth-days to complete a full rotation.

With Saturn, it turns out the answer rippled in plain view, in the planet’s lustrous rings.

After reading small, spiraling waves in those bands, sculpted by oscillations from Saturn’s gravity, scientists reported this month in the Astrophysical Journal that one Saturnian day is a mere 10 hours, 33 minutes and 38 seconds long, measured in Earth time.

“The rings are not only beautiful, they’re very diagnostic of what’s going on inside the planet,” said Linda Spilker, project scientist for NASA’s Cassini mission, which studied Saturn for more than a decade.

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Saturn has been stubbornly secretive about its days. Its buttery c louds don’t bear helpful markings that scientists might use to track the planet’s rotation, and they can't easily use its nearly vertical magnetic axis — as they have for Jupiter's more off-kilter alignment — to gather clues about the planet's interior.