The Many North Stars of Earth

One of the first skills learned by a young scout (or amateur astronomer) is how to find north using the night sky.  From the Big Dipper (more properly, the constellation Ursa Major), drawing a line between the two stars at the end of the dipper will lead to a moderately bright star, which marks the location of Earth's north pole, projected into the sky.  The accompanying diagram will help in understanding these directions.

From our location in Connecticut, and most of the Northern hemisphere, Ursa Major is a "circumpolar" constellation, meaning it is always above the ideal horizon.  In reality, trees and hills can block Ursa Major from late summer through early spring.  To find the North Star without Ursa Major, if you already know which way is north (or you have a compass handy), face north on a clear night, and look a little less than half way up the sky for a somewhat bright star.  Polaris is not dazzlingly bright, but is usually visible from even urban locations on any clear night.

The North Star (Polaris) lies at the end of the handle of the Little Dipper (Ursa Minor).  This time of year, in the evening hours, Ursa Minor will be to the left of Polaris.  The stars of Ursa Minor are mostly quite dim, so a clear evening without a bright moon, and a location away from bright city lights will be needed to see Ursa Minor easily.

The Earth rotates about its axis once a day, traveling from west to east.  The stars, Moon and Sun appear to rotate about this axis once each day, from east to west.  Polaris, being near the axis, does not appear to move (much) throughout the day or year.  Once you have found the North Star from a given location, you can be sure it is always in that spot in the sky — whether it is 10 p.m. next spring, or 10 a.m. tomorrow morning.  

But what about over thousands of years?  Well, over that span of time, other factors come into play.  Time for a wee bit of geometry, and a dash of physics.

Earth orbits the sun in a nearly circular path, which lies in a flat sheet of space — the ecliptic plane.  Earth's axis is tilted with respect to this plane — which, if you recall from elementary school, causes the seasons — by an angle of about 23 degrees.  

This arrangement of a tilted axis pointing at a fixed location in space would be stable over millions of years, with only slight changes caused by the motion of our sun orbiting the center of the galaxy, if the Earth were perfectly round. However, because Earth is a spinning object, it is not a perfect sphere but is slightly fatter around the equator (7,926 miles diameter) than through the north and south poles (7,900 miles).  

This "equatorial bulge," though seemingly very slight, complicates the motion of the spinning Earth.  The bulge acts as a sort-of handle, which is pulled upon by both the Moon and the Sun's gravity, pulling the bulge toward the ecliptic plane. The combined effect of a spinning Earth and this gravitational pull on the bulge is analogous to the behavior of a toy top, or gyroscope, whose axis begins to slowly wobble when it is tilted with respect to the floor.  And so, the Earth's axis wobbles — very slowly.

The Earth's "wobble", properly called precession, completes one cycle every 26,000 years.  The effect of the precession is to change the direction of the North Pole with respect to the stars along a circle whose diameter is about 1/4 of the visible sky (46 degrees), once every 26,000 years.  This means that our "north star" Polaris will keep this distinction only for a few hundred years, and has not been the North Star for earlier civilizations.

In particular, the ancient Egyptians, at the time of the construction of the Great Pyramid at Giza used a very different star to align the walls and ceremonial interior passages of the structure to the Earth's axis.  In the 1960s it was noticed that an interior passageway from the "King's Chamber", deep within the Great Pyramid, was aligned to where the star Thuban, in the constellation Draco would have appeared in 2500 BC when the pyramid was built.  Thuban was indeed a better North Star than Polaris will ever be, marking the northern axis almost exactly.  The Egyptians fully understood this, and attached religious significance to this star that never changed its position in the sky.

The circle described by the Earth's North Pole in the sky as it precesses passes a small handful of other noticeable stars.  The axis will be closest to Polaris in March of 2100, when it will be about one diameter of the full moon away.  By the year 4000, the much dimmer star Errai in the constellation Cepheus will become a less accurate pole star, followed in 7000 by the slightly brighter Alderamin, also in Cepheus.  About 13000 AD, the upper wing star of will be a rather good pole star, followed by Vega, the bright star seen overhead in August and September evenings, by the year 16000.  For the next 9,000 years, no bright star will be near the pole, until around 25,000 AD, when Thuban will once again become the North Star.

September is a great time of the year to map out the circle traced by the Earth's precessing axis in the evening sky.  Starting at Polaris, follow the arc of the Little Dipper to the end of its bowl.  Halfway from the end of the bowl to the second star in the handle of the Big Dipper we can find Thuban.  From there, looking overhead we find Vega, and lastly, high in the east we find Cygnus.  Using the star map attached to this post, on a clear night, you can trace the path the Earth's pole will follow through the sky over the next 26,000 years.