April 29, 2003

Comfort Stars
Photo credit: Don Pettit, ISS Expedition 6 Science Officer, NASA

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April 29, 2003: It's not the brightest bunch of stars in the heavens, but it's surely the most familiar: The Big Dipper. People who have never seen a dark night sky can trace its outline, and it's often the first star pattern kids learn. Draw a line through the first two stars in the bowl. It leads to Polaris, the north star. You're never truly lost if you can see the Big Dipper.

It must be comforting to look out the window from the International Space Station (ISS) and see that the Big Dipper looks just the same as it does from Earth. ISS science officer Don Pettit, who hasn't been home since Nov. 2002, captured this picture of the Dipper last month.

Actually, he notes, there is one difference: "The stars don't twinkle." Twinkling is caused by irregularities in Earth's atmosphere that refract starlight to and fro. But in orbit there is no atmosphere. Stars are remarkably sharp and steady. Mizar and Alcor close together in the crook of the Dipper's handle are good examples: On Earth it's a good test of eyesight to "split the pair." On orbit they're easy to distinguish.

The Big Dipper is most familiar to northerners. Southern sky watchers have their own famous constellation: the Southern Cross. The crew of the ISS can see both. Says Pettit: "The Southern Cross looks the same from orbit, too." Which means there's a little bit of comfort up there for everyone.

Editor's note: The Big Dipper is not a constellation, it's an asterism. Asterisms are easily-recognizable patterns of stars that don't necessarily respect constellation boundaries. The Big Dipper is a subset of stars in Ursa Major. Another well-known asterism, the Summer Triangle, includes stars from three constellations: Cygnus, Aquila, and Lyra.