For centuries, mariners and intrepid explorers have navigated using Polaris because it is a celestial constant -- it never rises or sets. If we could turn off the Sun for a moment and speed up the motion of the sky so that a day was 10 seconds long, wed see all the stars wheeling about this one star in dizzying concentric circles. This motion, of course, is simply the result of Earths rotation around its pole. Because Earths pole happens to point toward Polaris (it has to point somewhere in space), the whole sky appears to turn around that star.

You can reveal this motion using a typical 35mm camera and 400-speed film. Simply place the camera on a stable surface or mount it on a tripod, focus on Polaris in the viewfinder and open the shutter for a half-hour or so. When you develop your film, youll see colorful concentric star trails centered on Polaris.

Ah! But if you carefully examine the image of Polaris in your star-trail photos, youll notice that it, too, displays a slight arc. Even from the North Star, its still possible to go farther north in the sky.

True celestial north is offset three-quarters of a degree (48 minutes of arc, to be more precise) from Polaris in the direction of Alkaid (Eta Ursa Majoris) -- the star marking the end of the handle of the Big Dipper. Although 48 arcminutes may seem like a trivial amount, it looms large in the life of astrophotographers who align their telescope mounts precisely on this point in the sky in order to take sharply focused, long-exposure photographs of deep-sky objects. If the polar axis of the telescope mount is off by even half that amount, the stars in the photographs will appear not as distinct points but as commas of trailed light.

The north celestial point will remain in this vicinity of the sky for another thousand years or so, but it is not destined to remain there forever. Earths rotational axis wobbles slightly owing to the combined gravitational pull of the Sun and Moon on our planets equatorial bulge.

This, "precession," in turn, causes the true north point to transcribe a great circle in the sky over a period of 25,800 years. Currently, it is drawing nearer to Polaris and will come closest to that star around the year 2100. By the year 13500 true north will be inclined toward Vega in Lyra the Lyre, though it will be offset by about 5 degrees.