海王星昼夜“变短”

ROBERT SIEGEL, host: And from the inner circles of Washington to the outer reaches of the solar system. This just in from Neptune(海王星): The Neptunian day has gotten shorter, at least as we understand it.

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.

Mr. ERIC KARKOSCHKA (Planetary Scientist, University of Arizona): 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: We do have some idea about the length of a Neptunian day. Back in 1989, the Voyager 2 spacecraft flew past Neptune. Astronomers measuring the planet's magnetic field from that probe figured that Neptune's core was rotating once every 16 hours, six and a half minutes. That seemed like a reasonable measure of a day length, until astronomers studying the other gas giants discovered that the magnetic fields aren't nearly as reliable as clockwork（钟表的机械; 发条装置）.

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 the clouds - and Eureka.

Mr. 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.

Mr. KARKOSCHKA: It’s 15.9663 hours, and that should be more accurate than half a second.

HARRIS: So 15 hours 57 minutes and 59 Earth seconds make a day on Neptune. Put it another way, the day on Neptune just got eight and a half minutes shorter than we'd thought. Fortunately, as far as we know, there are no clocks on Neptune that need to be adjusted.

Richard Harris, NPR News.