North Star, or Pole Star, a bright star near the north celestial pole (the point in the sky toward which the northern end of the earth's axis points). The North Star is the only star in the sky that does not appear to change position as the hours and days pass. It can therefore be used as a guide to direction on any clear night. For centuries sailors have used the North Star to help them navigate.

At the North Pole, the North Star appears directly overhead. At the Equator, the North Star is seen on the horizon; south of the Equator it cannot be seen at all. At any point in the Northern Hemisphere, the height of the North Star above the horizon, measured in degrees, is very nearly equal to the latitude of the observer. The North Star, therefore, serves as a method of determining position as well as direction.

No one star remains the North Star. The reason is that although the north celestial pole does not appear to move, it actually does, because the earth's axis wobbles in somewhat the same manner as the axis of a spinning top. The wobble of the earth's axis (called precession), caused by the gravitational pull of the sun and the moon, is very slow. It takes about 25,800 years to complete one cycle. Any bright star near the path of the pole becomes in turn the North Star. When no bright star is near the pole, there is no North Star, just as there is no South Star.

At present Polaris, the last star in the handle of the Little Dipper, is the North Star. illustration.) Polaris is not exactly at the north celestial pole; it is about one degree away. (Its very slight movement around the pole cannot be detected visually but it does show up in time-exposure photographs.) In about 150 years Polaris will be only half as far from the pole as it is now. After that the distance between the star and the pole will increase. About 5,000 years ago Alpha Draconis was the North Star; 12,000 years from now the North Star will be Vega.