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Friend of the Traveler
The Galaxy >
One of the most easily found and well known stars in the night sky is also a luminary that has saved many, many lives for the last centuries. Can you guess which one? It has also been christened Stella Maris (the Star of the Sea), and Angel Stern (the Pivot Star). It is sometimes known by another Latin name, Navigatoria. The English have known it by several names --- The Steering Star and the Leading Star, or Lodestar.
Of course you have figured it out by now: we know it as the North Star, or Polaris, the Pole Star.
Polaris is easy to find --- if you can find the Big Dipper first, just after twilight. The two end stars on the "cup" part of the Dipper, also called Pointer Stars as every scout knows, point up to Polaris, the lone star just a hand span or two away.
The star itself is not so spectacular in the sky. Really it is very bright, but being located over 400 light years away dims that brightness down to just an average star in the sky.
It is its position on the celestial sphere around us that makes it such a lifesaver. Here's how:
Our planet rotates or spins on an axis, an imaginary line from North Pole to South. In your mind's eye extend that line out both ends of the earth and shoot it off into space. Above the North Pole that line rises into the sky and practically pierces Polaris. It is because Polaris is there in the sky, directly above the North Pole, that it appears not to move all day long, all year long, all century long, as we spin and spin day after day underneath it.
The laws of geometry tell us that the "pivot point" at Polaris will be due north for all people on the planet who can see it. And it will always be north, at least during our lifetime and the lives of our children and their children. Eventually the wobbling of the Earth, called precession, will move the pivot point away from Polaris, but we'll be OK for centuries.
It is due to the fact that Polaris is always due north that so many lives have been saved over the last millennium. Sailors tossed and turned and disoriented in the waters after a storm can quickly spot Stella Maris and begin to recalculate their bearings. Hikers lost in the forests can spot the Pole Star and find their way towards a river they recall is due east.
But it is more than just a handy dandy compass. It can also tell one his or her latitude. For example, at the North Pole Polaris is directly above one's head. That is 90 degrees from the horizon, thus one's position there is 90 degrees north latitude. Travel all the way down to the equator, and one can only see the North Star on the horizon. It is zero degrees above horizon, thus the latitude at the equator is zero degrees.
In San Diego, one can spot Polaris about 32 degrees up in the sky. Therefore, San Diego is at about 32 degrees north latitude. You can bet seafarers and land-lubbing wayfinders have known this trick forever and utilized it to their advantage.
Your curious self can go out tonight and estimate your own latitude this way. And while you are in an observing mode, try going out every hour or so and see for yourself how all the stars have moved in the sky with one famous exception, the friend of travelers --- Polaris.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2003.09.13 01:12 PM | Comments (0)
