The Sciences

Constant as the Northern Star

The pole star won't always point north.

By Bob BermanMay 1, 1995 5:00 AM


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The sky’s most famous star? No contest: It’s Polaris, the North Star. And as with most celebrities, misconceptions surround it. People often assume, for example, that it’s a brilliant star. But Polaris is only about fiftieth on the sky’s list of luminaries--capable of appearing over light-polluted cities but never brilliant enough to really stand out. Polaris may be amazing, but brightness is not its métier. Its uniqueness becomes clear only after Earth’s rotation has whirled the sky around, with most of the stars arcing in their grand and endless ballet. But not Polaris. Our axis of spin points, by chance, in its direction, causing Polaris to appear glued in place. I am constant as the northern star, says Shakespeare’s Julius Caesar, longing perhaps to link to it his own dreams of immortality.

It’s an improbable coincidence, this conspicuous distant sun sitting within a single degree of the celestial pole, the precise spot around which everything pivots. Given 41,253 square degrees of sky, the odds that such a noticeable star would occupy the right spot are nearly a thousand to one. It’s not surprising that those living south of the equator, where Polaris is invisible, do not have a South Star to mark the sky’s other pole. Polaris can be identified most easily right now at nightfall, because the Big Dipper hovers at its highest point of the year. Follow its two leftmost stars downward to the only star of similar brightness, the one on the end of the Little Dipper’s handle. That’s Polaris.

Polaris won’t always occupy its unique position. The 25,800-year wobble of Earth’s axis allows a procession of stars to take turns being polestar. But never in those millennia is there a star as bright and close to true north as Polaris. We live in the unlikely era of the best possible North Star. This implausible situation gets better as Polaris slowly creeps toward its one-half-degree flyby of the pole a century from now.

Already Polaris is a much better guide to true north than any compass. Polaris points to true north with an accuracy of better than a degree. A compass, by contrast, follows magnetic lines of force and often errs badly. In Maine, compasses point a whopping 20 degrees to the left of north. In Washington State they point as erroneously in the other direction.

But that distant celebrity to which Earth’s axis happens to point is remarkable in its own right. For starters, it’s not an ordinary sun but a Cepheid variable--a giant, pulsating star shining with the light of at least a thousand suns. This brightness varies over a four-day period by an amount too small for the eye to notice. The tiny flickering was unfortunate at first, since the entire system of measuring the brightness of stars originally used Polaris as its standard. Polaris defined second magnitude. But you can’t have an unreliable standard star, and the system moved on, demoting Polaris to a slightly lower magnitude in the process.

As if the star were petitioning for another chance, its fluctuations have been strangely diminishing. A year ago Canadian researchers predicted that the North Star’s variations would come to a permanent standstill by the year 2000, which would have given the star yet another curious distinction as the first Cepheid to give up its unsteady habits. New measurements, though, suggest it will continue to vary, albeit minutely.

Check it out this month, when it’s simplest to find. Or put it off until whenever--Polaris is going nowhere. For, in every sense, constant as the northern star is becoming truer than ever.

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