太空中拍摄的极光和星迹

Space Station Star Trails
Photo credit: ISS Expedition 6 science officer Don Pettit, NASA
with assistance from astronomers Robert Reeves and Rob Matson


May 13, 2003: Mount your camera on a tripod. Point it toward the North Star (Polaris) at night. Open the shutter for a while--a few tens of seconds or even a few hours--then develop the film. The picture you get will look like a vortex with Polaris in the middle. Astronomers call the concentric arcs "star trails."

Star trails are caused by the rotation of our planet. The sky turns circles above our heads, streaking the images of stars across film. Only Polaris at the North Celestial Pole seems stationary. (There's a South Celestial Pole, too, but no bright star lies at the stationary point.)

On April 30, 2003, International Space Station (ISS) science officer Don Pettit took this picture of star trails from the ISS. The exposure was brief, only about 30 seconds, so the star trails are stubby and subtle, but they are there. The telltale vortex hangs in the black sky above the limb of the earth, which is lit by red and green auroras.

The vortex itself looks like thousands of others photographed by amateur astronomers on Earth. But there's a difference: it's not centered on either of Earth's celestial poles.

Rob Suggs of the NASA/Marshall Space Flight Center explains: "The ISS usually orbits our planet in a level attitude--belly-down like an airplane. The station therefore flips relative to the stars once per trip around Earth." This flipping motion is what gives the space station its star-trailing spin. The spin of the ISS is not aligned with the spin of Earth; the station's celestial poles are therefore different.

The space station was above the south Pacific Ocean with its spin axis tilted toward the constellation Grus when Don took the picture. The stars are swirling around ... nothing particular.

"The station's celestial poles wobble around the sky once every 70 days," says Suggs. They move because the gravity of Earth's equatorial bulge tugs on the space station and causes its orbital plane to rotate. "Earth's polar axis wobbles, too," he notes "but one wobble of Earth takes 26,000 years rather than 70 days."

So while Earth's celestial poles seem constant, the space station's are in constant motion. Every day there's a new batch of star trails and maybe a new North Star. What more could an astrophotographer want?

不是很清楚

好象还有颗流星。。。