As the Earth Wobbles

By Dennis Mammana

June 28, 2018 4 min read

Week of July 1-7, 2018

Of all the stars in the heavens, one appears to stand firmly in place. It's not the brightest of all stars, as many people sometimes believe; in fact, you may even have trouble finding it under the light pollution of a large city. But it's always there, in the same position no matter what time of night or which season of the year you happen to glance skyward. It marks the north celestial pole, the spot toward which our planet's rotational axis points.

Its name is Polaris, more commonly known as the North Star.

Throughout centuries, writers and philosophers have recognized Polaris as a sign of constancy and faithfulness. To Northern Hemisphere 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 literally "fixed star."

Even the 16th-century English poet and playwright William Shakespeare weighed in on the steadfastness of this celestial beacon in his Sonnet 116: "Love ... is the star to every wandering bark, / Whose worth's unknown, although his height be taken."

Astronomers, of course, 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, causes stars to appear to wheel endlessly around it.

But this is purely a cosmic coincidence; in fact, as permanent a feature as Polaris might appear, it 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. How much time? It takes about 25,800 years to complete one cycle.

For much of modern history, the northern pole of our Earth's axis has pointed roughly toward Polaris. But this hasn't always been the case. For example, 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'll see the pole star gradually change again. Over the next few millennia Polaris will drift farther from its central position as other fainter 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.

Of course, Polaris will return to its starring role as our North Star, but for that you'll have to wait until the 277th century!

 The Earth wobbles and the north celestial pole changes.
The Earth wobbles and the north celestial pole changes.

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