A Day's A Day The World Around — But Shorter On Saturn : The Two-Way Researchers have finally determined the length of a day on the ringed planet (gas shrouds any landmarks, so it was tough). Precision matters: A faster spin influences the speed of surface winds.
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A Day's A Day The World Around — But Shorter On Saturn

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A Day's A Day The World Around — But Shorter On Saturn

A Day's A Day The World Around — But Shorter On Saturn

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SCOTT SIMON, HOST:

I think we've heard enough Earth-based stories for today. Let's take Saturn for a spin. This week, researchers announced they had precisely determined the length of a day on Saturn. NPR's Geoff Brumfiel reports they hope this'll teach them more about the past of the ringed planet.

GEOFF BRUMFIEL, BYLINE: Planets spin, and a day is simply how long it takes to spin all the way around. Ravit Helled is a planetary scientist at Tel Aviv University in Israel. She says it's easy to measure a day from space.

RAVIT HELLED: You can, you know, identify a mountain or a stone or whatever it is and just check how long it takes it to come back and that will be, you know, one day.

BRUMFIEL: On Earth, one rotation takes 24 hours - actually, 23 hours, 56 minutes and four seconds if you want to be precise. And Helled does want to be precise because that's her job, which brings us to Saturn. It has those pretty rings, but the planet itself is a beige ball of gas. It's surface is covered in clouds. You can't see any landmarks. So when the Voyager spacecraft first flew by in the 1980s, it estimated the day using the planet's magnetic field.

HELLED: The number was 10 hours, 39 minutes.

BRUMFIEL: Then in 2004, another mission arrived on Saturn and measured a different value.

HELLED: They found a rotation period of 10 hours and 45 minutes.

BRUMFIEL: Six minutes makes a big difference. The speed at which Saturn's rocky core rotates determines how fast the winds blow above. It also changes estimates of the giant planet's internal structure.

HELLED: If you want to understand, you know, giant planet formation and the origin of the solar system, these 10 minutes or six minutes are actually quite crucial.

BRUMFIEL: So Helled and her colleagues set out to make a better estimate of Saturn's day. They measured the planet's gravitational pull, combined that with estimates of the core's density and did some very tricky statistical analysis.

HELLED: It's tough, you know. (Laughter) The concept is not - yeah, it's a bit complicated. I'm sorry.

BRUMFIEL: But the bottom line published in the journal Nature - a day on Saturn is 10 hours, 32 minutes and 45 seconds. Helled eventually wants to use the technique to learn the length of days on planets outside our solar system, though, she adds, she'll need quite a few Earth days to figure out exactly how to make it work. Geoff Brumfiel, NPR News.

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