The Ever-changing North Pole Star

This shows Polaris at an angle 40 degrees up in the sky; hence it is being observed from an Earth latitude of 40 degree. Carolyn Collins Petersen

Stargazers are familiar with the concept of the "pole star". In particular, they know about the north star, with its formal name of Polaris. For observers in the northern hemisphere and parts of the southern hemisphere, Polaris (formally known as α Ursae Minoris because it's the brightest star in the constellation), is an important navigational aid. Once they locate Polaris, they know they're looking north. That's because the north pole of our planet appears to "point" at Polaris. There is no such pole star for the southern celestial pole, however. 

What's the Next North Pole Star?

An artist's concept of how the Polaris system looks. Based on HST observations. NASA/ESA/HST, G. Bacon (STScI)

Polaris is one of the most searched-out stars in the northern hemisphere sky. It turns out that there's more than one star at Polaris.  It's really a triple star system that lies around 440 light-years away from Earth. The brightest is what we call Polaris. Sailors and travelers have used it for navigational purposes for centuries because of its constant-seeming position in the sky.

Because Polaris is located very close to the point where our north polar axis points, it appears motionless in the sky. All the other stars appear to circle around it. This is an illusion caused by Earth's spinning motion, but if you've ever seen a time-lapse image of the sky with an unmoving Polaris at the center, it's easy to understand why early navigators gave this star so much attention. It has often been referred to as a "star to steer by", particularly by early sailors who traveled the uncharted oceans and needed celestial objects to help them find their way. 

Why We Have a Changing Pole Star

Precessional movement of Earth's pole. Earth turns on its axis once a day (shown by the white arrows). The axis is indicated by the red lines coming out the top and bottom poles. The white line is the imaginary line the pole traces out as Earth wobbles on its axis. NASA Earth Observatory adaptation

Polaris hasn't always been our north pole star. Thousands of years ago, the bright star Thuban (in the constellation Draco), was the "north star". It would have been shining over the Egyptians as they began building their early pyramids. Over the centuries the sky slowly appeared to shift and so did the pole star. That continues today and will do so into the future.

Around the year 3000 AD, the star Gamma Cephei (fourth-brightest star in Cepheus) will be closest to the north celestial pole. It will be our North Star until about the year 5200 ​AD, when Iota Cephei steps into the limelight. In 10000 AD, the familiar star Deneb (the tail of Cygnus the Swan) will be the North Pole star, and then in 27,800 AD, Polaris will take up the mantle again. 

Why do our pole stars change? It happens because our planet is wibbly-wobbly. It spins like a gyroscope or a top that wobbles as it goes. That causes each pole to point at different parts of the sky during the 26,000 years it takes to make one complete wobble. The actual name for this phenomenon is "procession of Earth's rotational axis".

How to Find Polaris

How to find Polaris using the Big Dipper's stars as a guide. Carolyn Collins Petersen

To locate Polaris, find the Big Dipper (in the constellation Ursa Major). The two end stars in its cup are called the Pointer Stars. Draw a line between the two and then extend it out about three fist-widths to get to a not-too-bright star in the middle of a relatively dark area of sky. This is Polaris. It's at the end of the handle of the Little Dipper, a star pattern also known as Ursa Minor.

An interesting note about the name of this star. It's actually a shortened version of the words "stella polaris," which is a Latin term for "polar star." The names of stars are often about the myths associated with them, or, as with Polaris, are given to illustrate their practicality. 

Changes in Latitude...Polaris Helps Us Figure Them Out

This illustrates Polaris at an angle 40 degrees up from the horizon of the observer, who is looking from an observing site located at 40 degrees latitude on Earth. Carolyn Collins Petersen

There's an interesting thing about Polaris — it helps people determine their latitude (unless they are too far south to see it) without needing to consult fancy equipment. This is why it has been so useful to travelers, particularly in the days before GPS units and other modern navigational aids. Amateur astronomers can use Polaris to "polar align" their telescopes (if needed).

After Polaris is found, it's easy to do a quick measurement to see how far above the horizon it is. Most people use their hands to do it. Hold a fist out at arm's length and align the bottom of the fist (where the little finger is curled up) with the horizon. One fist-width equals 10 degrees. Then, measure how many fist-widths it takes to get to the North Star. Four fist-widths means 40 degrees north latitude. Five indicates fifth degrees north latitude, and so on. And, an added bonus: when people find the north star, they know they're looking north. 

What about the south pole? Don't the southern hemisphere folks get a "south star"? It turns out that it does. Right now there is NO bright star at the south celestial pole, but over the next few thousand years, the pole will point at the stars Gamma Chamaeleontis (the third-brightest star in Chamaeleon, and several stars in the constellation Carina (the Ship's Keel), before moving on to Vela (the Ship's Sail). More than 12,000 years from now, the south pole will point toward Canopus (the brightest star in the constellation Carina) and the North Pole will point very close to Vega (the brightest star in the constellation Lyra the Harp).