The North Star Club

As Ken Croswell notes in “All about the North Star,” Polaris occupies a privileged place in our sky. Calculations show the North Star lay 2° from true north in 1757, and was 1° away in 1944. Even now, as wayfarers rely on Global Positioning Satellites, Polaris continues to improve as an indicator of true north. The star comes closest in 2102, when it will lie 27.6′ — less than half a degree — from the North Celestial Pole.

Precession on the web

J. Giesen’s Precession page contains Java applets that allow you to precess star positions and animate the North Celestial Pole’s motion for the next 14,000 years.

But the North Star isn’t really moving: Earth’s rotational axis is. Like a spinning top, Earth has a slight wobble that slowly makes its spin axis sweep out an arc in the direction opposite the planet’s rotation. This motion, called precession, is a rotating object’s response to a twisting force. In the case of a top, this force is friction with the floor. For Earth, the force is the gravitational pull of the Sun and Moon on our planet’s bulging equator. Over the course of nearly 25,800 years, Earth’s spin axis — and, thus, the North Celestial Pole — traces a circle through the constellations. This circle is centered on the North Ecliptic Pole, which is the point perpendicular to Earth’s orbit around the Sun.

Precession also means Polaris hasn’t always been in its current privileged position as the North Star. Instead, many stars along the track of Earth’s axis have taken turns playing this role. Some 5,000 years ago, when the ancient Egyptians were constructing their pyramids, the closest bright star to true north was Thuban in Draco. Ursa Minor’s second brightest star, Kochab, took its turn the following millennium, but its distance from the sweep of Earth’s axis kept it from being as helpful as Thuban or Polaris.

Two thousand years hence, the “north star” will be Alrai at the tip of Cepheus. After another 3,500 years, the North Celestial Pole will lie near Cepheus’ brightest star, Alderamin. From then on, membership in the North Star Club seems to dwindle. No really bright stars take the title until the pole moves near bright Deneb 8,000 years from now. A few thousand years later, the star Vega in Lyra will point spectacularly, if inaccurately, to true north.