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HOW COME? Sailors use Polaris to navigate

How come sailors use the star Polaris to navigate? asks Adina Davis, of Great Neck

Imagine setting up a camera near the North Pole in winter, pointing it at the sky and then taking pictures over one (long, dark) day. Here's what you'd see in the time-lapse video: Polaris, almost directly overhead, like a beacon. And, over 24 hours, the rest of the stars appearing to slowly circle it.

The stars' circling is really an illusion, of course. It's the Earth that's spinning under the stars, taking 24 hours to turn once around. While the Earth turns, Polaris appears to stand still only because of its position in the sky: lined up almost perfectly with our planet's axis.

How does it work? Earth's North (celestial) pole traces a small circle over 24 hours as the planet turns. Since the pole happens to be pointed at Polaris, the medium-bright star is always directly overhead there. That makes Polaris the Earth's North Star. (There is no corresponding South Star, simply because the South celestial pole isn't pointing at any easily visible star.)

Since Polaris stands above the North Pole like a glowing directional beacon, it's the star to steer by in the Northern Hemisphere. Sailors, hikers, and even birds have used it to find their way in the dark for many centuries.

While Polaris is directly overhead at the top of our planet, it sits on the horizon at the Earth's equator. Just south of the equator, Polaris disappears from view.

Between the North Pole and the equator, Polaris is at an in-between position in the sky, corresponding to your latitude north of the equator. Which can help you figure out where you are, even in the middle of the sea.

The New York City area is located almost 41 degrees north of the equator. And Polaris can be seen about 41 degrees above the northern horizon on a clear, dark night. To find it, simply make a fist at the end of your outstretched arm. Four fists above the horizon is about 40 degrees. Polaris can be found in the Northern sky near that height each night, at the tip of the handle of the Little Dipper.

But the role of the Pole Star is played by different celestial actors over time. As the Earth rotates, it wobbles on its axis, rather like a spinning toy top. It takes nearly 26,000 years for the Earth to execute a full wobble. By the year 13,727, the fifth-brightest star in the night sky, Vega, will reign as our new Pole Star -- until the Earth tilts away again.

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