Constant as the northern star

(NASA/HST)

The sky’s most famous star? No contest: It’s Polaris, the North Star. And like most celebrities, it’s enveloped in misconception.

People often assume, for example, that it’s brilliant. But Polaris is just medium-bright – about 45th on the sky’s list of luminaries – capable of appearing over light-polluted cities but never brilliant enough to catch one’s eye. Polaris may be amazing, but its brightness is not its calling card. Its uniqueness becomes clear only after it has been ignored for a few hours.

By then, Earth’s rotation will have whirled the sky around, arcing the stars in their grand and endless ballet. But not Polaris. Our axis of spin points in its direction, causing Polaris to appear glued in place. “Constant as the northern star,” said Shakespeare’s Julius Caesar, longing perhaps to link to it his own dreams of immortality.

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It’s an unlikely reality, this conspicuous distant sun sitting within a single degree of the celestial pole, the precise spot around which everything pivots. Given the 41,253 square degrees of sky, the odds against such a noticeable star occupying the right spot is nearly a thousand-to-one. It’s then 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 in spring, because the Big Dipper hovers at its highest of the year. Follow its two leftmost “pointer” stars downward to the only star of the same brightness as they; that’s it.

Over the centuries, the slow 25,780-year wobble of Earth’s axis allows a procession of stars to take turns being closest to the site around which the sky performs its counterclockwise spin. But never in those 26 millennia is there a star as bright and as close to true north as Polaris. We live in the unlikely era of the best possible North Star. This implausible situation continues to improve as Polaris slowly creeps toward its half-degree flyby of the celestial pole 87 years from now.

In one’s imagination, it’s fun to drop a stone straight down from Polaris, because it then touches true north to an accuracy of better than a degree. A compass, by contrast, follows magnetic lines of force and is badly in error from most locations. From the mid-Hudson Valley, compasses point a whopping 14 degrees to the left of north.

Turns out, 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 1,000 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 Magnitude Two. But you can’t have an unreliable standard star, and the system moved on.

As if petitioning for another chance, its fluctuations have been strangely diminishing. While Polaris varied its light by .1 magnitude when the 20th century began, in our new century the flickerings are less than .01 magnitude. One Canadian researcher has predicted that the North Star’s variations will soon come to a complete and permanent standstill, granting it yet another curious distinction: as the first Cepheid variable to give up its unsteady habits.

Check it out during 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.

Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.

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