Rich Schuler, an adjunct instructor and outreach coordinator in the physics and astronomy department at the University of Missouri-St. Louis, explains.
Image:NASA A STAR MAP shows the relative position of Polaris in Ursa Minor. |
The North Star, or Polaris, is the brightest star in the constellation Ursa Minor, the little bear (also known as the Little Dipper). As viewed by observers in the Northern Hemisphere, Polaris occupies a special place. The point in the night sky where the projection of the earth's axis lies is known as the North Celestial Pole (NCP). As the earth rotates on its axis (once every 24 hours), the stars in the northern sky appear to revolve around the NCP. Polaris lies roughly one half degree from the NCP, so this particular star appears to remain stationary hour after hour and night after night.
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Because the earth is spherical, the position of Polaris relative to the horizon depends on the location of the observer. Consequently, the angle between the northern horizon and Polaris is equal to the observer's latitude. For example, when viewed from the equator (0 degrees latitude), Polaris lies on the northern horizon. As an observer moves northward--say, to Houston, Tex. (30 degrees latitude)-- Polaris is located 30 degrees above the northern horizon. This trend continues until the traveler reaches the geographic (not magnetic) North Pole. At this point (90 degrees latitude), Polaris is 90 degrees above the northern horizon and appears directly overhead.
Elmore further asked, "Did travelers in the past actually depend on the North Star to guide them?"
A traveler on land or sea need only measure the angle between the northern horizon and Polaris to determine his or her latitude. Thus, Polaris is a handy tool for finding the northern extent of one's position, or latitude, and was therefore heavily utilized by travelers in the past¿especially sailors.
Unfortunately, latitude alone is insufficient to pinpoint a location on the surface of the earth. Lines of constant latitude circle the earth parallel to the equator. With only latitude in hand, an individual knows just that he or she is on a particular "latitude circle." Without an accompanying longitude coordinate, an observer could be located at any position on the circumference of the planet consistent with the measured latitude. Although many cultures succeeded in making long ocean voyages using only the stars, weather and currents, the "longitude problem" plagued sailors for millennia and remained unsolved until the invention of a clock that could keep accurate time while a ship rolled, pitched and yawed on the sea (circa 1740).
There is currently no star in the Southern Hemisphere that coincides with the South Celestial Pole. Furthermore, Polaris is not an absolute guide to measuring latitude on the earth for Northern Hemisphere observers. In addition to the daily 24-hour rotation cycle, the axis of the earth precesses in a conical motion. Thus, the projection of the earth's axis traces a circle in the northern (and southern) sky with a period of 26,000 years. The location of the North (and South) Celestial Pole is defined by projecting the axis of the earth onto the celestial sphere; consequently, as the axis changes position, so, too, does the "North Star." As a result, 5,000 years ago the earth's axis pointed toward Draco, and the North Star was Thuban. Similarly, in 12,000 years the star Vega (in the constellation Lyra) will be the North Star.
Answer originally posted February 18, 2002.