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Polaris, the North star, is a very bright star.

Bad Astronomy: The North Star, Polaris, is a very bright star.

Good Astronomy: Polaris is a middling-bright star, easily missed. And it won't even be the North Star forever!


The other day a friend was asking me about an object he saw in the sky, and as he described it I got confused. He said it was close to Polaris, the North Star, but he also said it was in the East. "It can't be in the East if it was near Polaris," I told him. "But it was near this really bright star!" he replied. It was then I knew I was onto more Bad Astronomy.

A lot of people think that Polaris is a very bright, if not the brightest star in the sky. It isn't. As a matter of fact, it's only about average. From my own back yard I have a hard time seeing it! (I live a few miles north of Washington, DC, and the sky is bright from city lights).

It's natural, I suppose, to think that such an important star should be bright, but really, that's a conceit, like thinking that the Earth is at the center of the Universe. If more people knew just why Polaris is the North Star, this Bad Idea would stop.

The Earth is spinning on its axis like a top, once a day. We define the points where this axis intersects the Earth's surface as the North and South Poles. If you stood at the North Pole (brrrr!) over the course of one day you would spin around once. Now, since we are stuck on the surface of the Earth, we don't really perceive this spin. We do see the sky, however, and our spin makes it look like the sky is revolving around us once a day.

Imagine someone standing on the equator. If he looks straight up, he will see stars fly past him all night long as the Earth spins, sweeping him around the circumference of the Earth. The stars would appear to rise in the East and set in the West. However, if he were at the North Pole, it would look like the stars are spinning around a point straight up in the sky. This point is called the North Celestial Pole (NCP), and is basically the same as the North Pole on the Earth projected up into the sky. All the stars seem to spin around this point, just as the Earth spins around its own North Pole.

So we can see that this point on the sky is only special to us on the surface of the Earth. A different planet, with an axis pointing in a different direction, would see a different spot on the sky as its NCP. It just so happens we see a star near this spot. This star is part of the Little Dipper (as it's called in the U.S.), and is about 2nd magnitude, which means it ain't all that bright. It is the brightest star in the Little Dipper, but that isn't saying much since the Little Dipper is a much fainter counterpart of its bigger brother, the Big Dipper.

The reason Polaris is important is because it is so close to the NCP. As the night progresses, Polaris does not rise or set, but seems to be glued to the sky! So at any time in the night you can find Polaris, and it is always in the North. If you get lost, you can always figure out your direction by finding Polaris (provided that you are in the Earth's northern hemisphere!).

Now remember, there is a South Celestial Pole as well. It lies in the constellation called Octans, and the nearest star to it is a a very faint 5th magnitude star, which is near the limit of the human eye to see unaided. More proof of our Polaris conceit!

Note that stars near the two celestial poles will make little circles around the pole as the Earth spins. The farther you get from the poles, the bigger the circle. At some point, the circle will be big enough so that it just touches your horizon. Stars inside that circle are said to be circumpolar, and they never set! They just seem to circle the pole endlessly. Note that the stars you see as circumpolar depend on your latitude. If you were at the North Pole, all the stars you can see in the sky are circumpolar, but at the equator, no stars are! At the equator, both celestial poles lie on the horizon, and all the stars in the sky rise and set.

spinning top animation One more thing: have you ever watched a top spin, and seen it wobble? The wobble is due to a force called torque, which is like a twisting pull. When torque is applied to a spinning object, the spin axis will precess, or make a little circle as the top spins. Well, the same thing is happening to the Earth! The gravity of the Moon and Sun provides a torque on the Earth, causing the axis to wobble. The Earth's axis takes 26,000 years to make a complete circle, and as it moves it points to different parts of the sky. It just so happens that right now it is pointing near Polaris (actually, Polaris is about a degree away from the actual North Celestial Pole). In a few thousand years, the Earth's pole will be pointed at the bright star Vega, which is one of the ten brightest stars in the sky. Imagine how hard it will be to convince people that's just a coincidence when that happens!

Bad Addendum
Bad Reader Andrew Sincinito brought to my attention the lyrics of a song by Gerry Rafferty, called "Right Down the Line":

  I know how much I lean on you
  Only you can see
  The changes that I've been through
  Have left a mark on me
  You've been as constant as a northern star
  The brightest light that shines
  It's been you
  Woman
  Right down the line
I hate to give him more credit than he might deserve, but those lines could be interpreted as "she is as constant as the north star, and she is the brightest light that shines;" he might not have been talking about Polaris. However, he might have meant that Polaris is the brightest star in the sky. If so, we have another victim of Bad Astronomy. If not, well, the lyrics are still kind of Bad. ;-)

Incidentally, Mark Bellis of Canada tells me that Joni Mitchell had an even earlier song with similar lyrics in it. I guess this idea goes back pretty far. Matter of fact, Shakespeare made the same error in "Julius Caesar"! So I guess Joni and Gerry are in good company.



©2008 Phil Plait. All Rights Reserved.

This page last modified Saturday, 05-Mar-2011 18:03:18 UTC


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