What Is The North Star?

Learn about why the North Star got its name, why the North Star changes over time, and the current North Star Polaris.

The North Star, also called the Pole Star or Polaris, is the star that the earth's axis points toward in the Northern sky. For many years, people have been fascinated with this star and the fact that it doesn't seem to move in the sky. Some have created legends explaining why the star stands still. As more detailed scientific instrumentation has become available, scientists have begun to study more about Polaris. Surprisingly, it is a rich subject consisting of a binary star system.

Why Do We Care about the North Star?

For many years, the North Star has been used as a navigation aid and to chart navigational maps. It has also been used to measure astronomical latitude since we map latitudes to the equivalent sky positions: the North Pole equates to +90 degrees latitude on Earth as does its projection into the sky. In addition to these functional uses, over time many cultures have built folklore around the North Star. Even people with little interest in astronomy or mapmaking know about the North Star, and some have created stories explaining why it seemingly never moves.

The most famous story about the North Star is the Native American myth explaining why the North Star stands still. In this story, a brave son Na-Gah tried to impress his father by climbing the tallest cliff he could find. Through difficult conditions he persisted until he found himself at the top of a very high mountain. The mountain was so tall that Na-Gah looked down on all the other mountains. Unfortunately, there was no way down. When his father came looking for him, he found Na-Gah stuck high above. Not wanting his son to suffer for his bravery, he turned Na-Gah into a star that can be seen and honored by all living things.

Polaris: The Current North Star

Today the Earth's axis points within one degree of Polaris, the brightest star in the constellation Ursa Minor (also called the Little Bear or the Little Dipper). Polaris appears to be in a fixed position in the sky throughout the year. All other stars and constellations seem to revolve around the North Star.

To find Polaris in the sky, locate the Big Dipper and follow the two stars at the end of the basin upward. This should lead you directly to Polaris. It is the last star in the tail of the Little Dipper.

Why isn't the North Star Fixed?

Over the course of time, the North Star changes. Right now Polaris is within one degree of true north, but at other times the North Star has been and will again be Thuban (the brightest star in the constellation Draco), Vega (the brightest star in the constellation Lyra), and Alpha Cephei (the brightest star in the constellation Cepheus).

The North Star changes over time because the direction of the earth's axis changes slowly over time. Since by definition the North Star is the star most closely aligned with the earth's axis, as the axis moves the nearest star changes too.

This type of axis movement is similar to that of a spinning top. As the top slows, the axis of rotation changes as the top draws out each rotation; that is to say that the stem of the top itself traces out a circular pattern rather than pointing at a single spot or staying mostly still. If you draw an imaginary line of the earth's axis and continue it up to the sky, it will make a similar path. This type of axis rotation is called precession.

In the case of the earth, precession is caused by the gravitational pull of the sun and the moon. The earth's axis makes one complete rotation over the course of approximately 26,000 years. If you trace the path of the axis in the sky, you will find that Polaris, Vega, Thuban, and Alpha Cephei all fall on or very close to it. So when the earth's axis is at a point on the path near Vega, Vega becomes the North Star while Thuban is the North Star when the axis is near it on the path.

Five thousand years ago, Thuban was the North Star. Five thousand years from now, the North Star will be Alpha Cephei. Seven thousand years after that, it will be Vega. Nine thousand years after that, Thuban will be the North Star again. At these dates, the various stars will be at the closest to absolute north. For some time before, the relevant star will be approaching due north and it will be receding for some time after the time listed. In these interim times, the North Star is whichever star is closest to north.

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