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Sky Watch: 'Fixed' North Star only appears so

Of all the stars in the heavens, one stands firmly in place. It's not the brightest, as many people sometimes believe; in fact, you may have trouble finding it under lights of a large city. But it's always there, in the same position, no matter what time or which season. It marks the "north celestial pole" -- the point about which all other stars appear to turn.

Its name is Polaris, but most people know it as the North Star.

Throughout the centuries, writers and philosophers have recognized Polaris as a sign of constancy and faithfulness. To navigators, it was a steady light by which they could safely guide their ships. Cultures throughout Asia long recognized its prominent position as the pinnacle of the cosmic "Mountain of the World" or "Axis of the Universe." In traditional Indian astronomy, its Sanskrit name is "dhruva tara," which means "fixed star."

Even the 16thstyle/yr century English poet and playwright William Shakespeare weighed in on the steadfastness of this celestial beacon in his Sonnet 116: " is the star to every wandering bark / Whose worth's unknown, although his height be taken."Astronomers know Polaris as the star that shines nearly directly over Earth's North Pole and appears to stand firmly above our northern horizon.

But this is a cosmic coincidence; as permanent a feature as Polaris might appear, it has not always occupied the role of North Star. This is because Earth doesn't spin perfectly on its axis, but wobbles as it travels its cosmic journey.

We can easily see this "precession" effect in a spinning top. As the top rotates about its axis, it eventually slows down and wobbles. The axis about which the top spins no longer points in one direction but traces a larger circle.

Earth behaves similarly, and over time, its rotational axis traces a giant circle among the stars. Most people don't know about precession because a full cycle takes 25,800 years to complete.

and you'll see the pole star change again. Over the next few millennia, Polaris will drift from its central position as other stars take over its role. In another 55 centuries, the star Alderamin in Cepheus, the king, will occupy that prominent position. And by 14,000 AD, the bright star Vega will lie closest to the north celestial pole.

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