Among all the stars in the heavens, one stands firmly in place. Its name is Polaris, and it marks the north celestial pole – the point about which all other stars turn during the night.
Throughout the centuries, writers and philosophers have seen Polaris as a sign of constancy and faithfulness. To navigators, it was a steady light by which they could safely guide their ships. And cultures throughout Asia have long recognized its prominent position as the pinnacle of the cosmic “Mountain of the World” or “Axis of the Universe.”
Astronomers know Polaris as the star that shines nearly directly over the Earth's North Pole and, as the world whirls on its daily and annular paths, Polaris appears to stand firmly above our northern horizon.
But Polaris has not always occupied the role of North Star. This is because our Earth doesn't spin perfectly on its axis, but rather wobbles a bit as it travels on 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 begins to wobble. The axis about which the top spins no longer points in a single direction, but instead traces a much larger circle.
Our 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.
For much of modern history, the northern pole of Earth's axis has pointed roughly toward Polaris. But when the ancient Egyptians were building pyramids 50 centuries ago, they saw a different North Star. At that time, the star Thuban in the constellation of Draco, the Dragon, appeared quite close to the north celestial pole and served this important function.
Wait around for a while and you will see the pole star gradually change again. Over the next few millenniums, Polaris will drift farther from its central position as other fainter stars take over its role. In 55 centuries the star Alderamin in Cepheus, the King, will occupy that prominent position. And by the year 14,000, the bright star Vega will lie closest to the north celestial pole. Polaris eventually will return to its starring role, but that won't happen for another 25,800 years.
© Copley News Service
