Rho Aquilae

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Aquila, the eagle, soars across the south tonight. Its brightest star is Altair, the southern point of the Summer Triangle. But one of the constellation's other stars is missing. No, the star didn't explode. Instead, it up and moved to another constellation.

Every star in the Milky Way orbits the galaxy's center. But every star follows its own orbit through space, and none exactly matches the path of the Sun. As a result, the positions of the stars gradually change through the years -- an effect known as proper motion.

The northeastern quadrant of Aquila bears a star named Rho Aquilae -- or, rather, it used to. In 1992, the star's northeasterly proper motion carried it across the border into neighboring Delphinus, the dolphin. It's as if Chicago suddenly moved over and became part of Wisconsin.

Unlike Aquila, Delphinus is a faint constellation, with no prominent stars. So Aquila's loss has been Delphinus's gain.

Still, Rho Aquilae is hardly a spectacle. It's fifth magnitude, which means that on a dark, moonless night away from city lights, you can barely see it with the unaided eye. The star is white, like Altair, but it's nearly 10 times farther from Earth, which explains its faintness.

Even so, the star has made astronomical history. It's the first star with a Greek-letter name to move from one constellation to another. It won't happen again for another four centuries.

Script by Ken Croswell, Copyright 2008

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