Jimmy Westlake: The pivot of the sky
April 25, 2011
Steamboat Springs — If you were to take a camera, mount it on a stable tripod, aim it at the northern sky after dark, and open the shutter for an hour or so, your photo would come out looking something like an archery target, full of concentric circular arcs. The slow rotation of the Earth on its axis causes the stars to trace out circles throughout our sky, completing one trip around in 23 hours and 56 minutes.
On close inspection of your photo, you'd notice that there is a single bright star right at the center of the bull's-eye. That star is Polaris, our North Star. Just by coincidence, the Earth's axis points almost directly at Polaris such that as Earth rotates, Polaris remains nearly motionless throughout the night. It's the pivot point for the whole sky. While it isn't the brightest star in the sky, it is the only star that doesn't move, so it can be used to pinpoint the direction north better than any compass.
Polaris is the alpha star in our constellation of Ursa Minor, the Little Bear, but most people in the U.S. see it as the star marking the end of the handle of the unofficial star pattern called the Little Dipper. To locate it, just follow a line through the two stars at the end of the nearby Big Dipper's bowl, the so-called "pointer stars," Dubhe and Merak. You can always count on them to point the way to Polaris.
Here are a few things about our pole star that maybe you didn't know:
■ Polaris is the closest pulsating Cepheid variable star to Earth. Stars like Polaris are well advanced in age and are going through a stage of instability, causing them to beat like a heart. Its brightness fluctuates by about 15 percent during a period of four days, but this slight variability is not easily perceptible to the naked eye.
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■ Polaris also is the brightest member of a trinary star system located 431 light years from Earth. In other words, what looks like one star to the unaided eye is really three stars in an intricate dance with one another. One star orbits Polaris with a period of about 30 years and the other takes thousands of years to complete its orbit.
■ Polaris has been known by many names through the centuries. Its current popular title comes from the Latin name "Stella Polaris," meaning "Pole Star." Other names include Cynosura (the Dog's Tail), Lodestar, Navigatoria, and Angel Stern (Angel Star).
■ Polaris has not always been our pole star. Thanks to the slow, 26,000-year wobble of the Earth on its axis, Polaris exists as our pole star only for a few brief centuries before the axis wanders on to another pole star. Polaris will be closest to the true pole of the sky around the year 2100. Then, it will start moving away. Better enjoy it while it lasts.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Visit Westlake's website at http://www.jwestlake.com.