Thuban: Ye Olde Northe Starre
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There is a special star in the sky which, in days of old, had a place of great prominence. Great civilizations revered it. Monuments were built by its position in the heavens. Alas! Nowadays it is nothing but an average member of the congregation of starry hosts, hardly discernible from the thousands of other stars in the night sky.
This star is Thuban. It can be found, but not easily, in the northern skies surrounded by its more renowned neighbors. Go outside tonight and do this: Find the North Star, Polaris. It is --- obviously --- due north and about two hand spans (held at arm's length) above the horizon. It is essentially by itself so it isn't too difficult to pick out.
Now find the Dipper. Using both Polaris and the stars at the end of the handle of the Dipper as your "end stars," can you find the two stars right between them? They are relatively easy to pick out and, for the sake of trivia, are part of the bowl of the Little Dipper.
Now you need to set your night vision to "sensitive." Right between these two stars and the stars at the handle of the Big Dipper is a dim star. That is Thuban. (A helpful star map can be found here.)
For the science-minded reader, Thuban is actually a tremendously big, very bright star. It is an A-class star (translation: hot and bleeding energy profusely) but it is a good distance away, over 300 light years. That explains its relative dimness.
So how is it that a dim, insignificant A-class star was once an A-list star to the ancients?
In days of old, nearly 4600 years ago, Thuban --- not Polaris --- was the North Star! All the constellations of the northern skies traversed around it.
About that time, the eminent empire of the Egyptians was erecting the great pyramids. All indications are that a pyramid built at that time was architecturally lined up with Thuban, the then-North Star.
Going down Pyramidal passageways, many of which were lined up in a north-south direction, one could look down the hall and see Thuban sparkling outside all night.
So how is it that Thuban was pushed from this limelight? Precession!
The earth spins like a top, with the top of our top always pointing in the same direction. Well ... not always. It just appears that way. During your lifetime our planet will spin around thousands of times, all that time with its axis seeming to point at that same star, our North Star, Polaris.
But like a top that is slowing, our planet wobbles about while spinning. This wobble is called precession. And although we can see the wobble in a spinning top after just after a few tens of spins, our planet takes 26000 years worth of spinning to wobble around just once!
Now imagine a spinning top with a laser light beaming out the top of it --- along its "axis of rotation."
Precession would cause the lightbeam to slowly draw out a circle on your ceiling. Earth does something similar, only without the laser light, of course. In the four-and-a-half millennia since Thuban reigned as North Star, the "point" directly above our spinning planet has circled over and around to Polaris, our current North Star.
Where is this point above our axis headed for now? See if you can find that bright star almost directly above our heads tonight. That is Vega, a bright blue hot star just 25 light years away. Stick around until about A.D. 14000 and you'll be able to use Vega as your wayfinding North Star.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2003.07. 5 01:23 PM | Comments (0)
