If the Earth is moving around the sun, and the Earth's magnetic field reverses polarity every 15,000 years or so, then how come the North Star has not appeared to move at all since people first started keeping astronomical records and calendars?

kevinatilusa

Actually, the North Star (or Polaris) does appear to move, just VERY slowly. There's three reasons (at least) why objects in the sky seem to move around. First off, that the earth is rotating around itself every 24 hours. However, a star which lay directly on the axis on which the earth rotated (a true "north star") would not be affected by this. Polaris is close enough to true North that we don't see much of a change. Secondly, the Earth is moving around the sun. This DOES affect Polaris, but only by a small amount. To see why this is the case, imagine two people, one of them standing a foot in front of you and the other one standing 100 feet away from you. If you take a large step to the left, the person standing a foot away from you is no longer anywhere near straight ahead, while the person 100 feet away doesn't seem to move nearly as much. Over the course of 6 months, the earth takes a gigantic step from one side of the sun to the other, but Polaris is so far away that a person not keeping very careful track won't notice that it's shifted ever-so-slightly. Astronomers do notice such things, however. In fact, they have a formula relating how much a star appears to move between spring and autumn and how far the star is away from the Earth. This is one way to measure the distance to fairly close astronomical objects. Sep 05 04, 6:47 AM

kevinatilusa

(continued) The third way that Polaris is moving has to deal with a very slow, wobble in the Earth's orbit known as precession (similar to the slow wobble in tops even when they're spinning quickly). It takes nearly 26,000 years for the Earth to finish each of these wobbles, but the wobble is large enough that 13,000 years from now Vega will be closer to Celestial North than Polaris. (See http://starchild.gsfc.nasa.gov/docs/StarChild/shadow/questions/question64.html ) for more on Precession. The Earth's magnetic field doesn't really come into things here. The North Star is defined in relation to the north around which the earth rotates (the geographic north pole) and the magnetic north pole doesn't have to be located in the same place. Sep 05 04, 6:47 AM

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