Neptunians presumably already knew this.
A day on the gaseous giant apparently doesn't last 16 hours and six minutes, as we Earthlings have long thought.
Instead, according to University of Arizona astronomer Erich Karkoschka, the planet completes a rotation every "15 hours, 57 minutes and 59 seconds."
As NPR's Richard Harris tells us, "it's not easy figuring out the length of a day on Neptune, since the entire planet is swathed in clouds. And different clouds rotate at different rates, so they don't necessarily tell you what's happening with the planet hidden down below."
Karkoschka, though, "took advantage of what one might call the scraps of space science: publicly available images of Neptune from the Hubble Space Telescope archive," his university writes. "Other scientists before him had observed Neptune and analyzed images, but nobody had sleuthed through 500 of them" that had been taken over 20 years.
He noticed that "two features in Neptune's atmosphere" — the "South Polar Feature and the South Polar Wave" — "appear exactly every 15.9663 hours, with less than a few seconds of variation."
Digging further, Karkoschka used images taken by Voyager 2 in 1989 to identify six other features on the planet that rotate with the same regularity.
So why is this important?
The university writes that "in addition to getting a better grip on Neptune's rotational period, the study could lead to a better understanding of the giant gas planets in general."
"We know Neptune's total mass but we don't know how it is distributed," Karkoschka says in the university's report. "If the planet rotates faster than we thought, it means the mass has to be closer to the center than we thought. These results might change the models of the planets' interior and could have many other implications."
For an animated look at Neptune's rotation, check out this video that Karkoschka produced.
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NPR's Richard Harris explains.
RICHARD HARRIS: Figuring out the length of a day on Neptune has been a giant headache. The problem is, as is true of the other gas giants, you can't see its surface. When you look at the greenish face of Neptune all you see is clouds, clouds, clouds. And Eric Karkoschka, at the University of Arizona, says that's a problem for astronomers like him who have been trying for many years to figure out how fast the planet is rotating.
ERIC KARKOSCHKA: Just like when Earth clouds sometimes move to the east, sometimes to the west, and sometimes they accelerate, and so you cannot measure accurate rotation.
HARRIS: Karkoschka figured maybe there was another way, so he poured over images of Neptune taken over more than 20 years, to look at odds spots that are perpetually embedded in the clouds - and Eureka.
KARKOSCHKA: I could find two feature on Neptune in images taken, starting from large images in 1989 until 2010, in Hubble's Space Telescope images. And they were so regular that I think that must indicate the interior rotation.
HARRIS: And if you time one full rotation, you have the length of a day on Neptune.
KARKOSCHKA: Its 15.9663 hours, and that should be more accurate than half a second.
HARRIS: Richard Harris, NPR News. Transcript provided by NPR, Copyright NPR.