The North Star and magnetic north

Our two most recent columns discussed aurora and alpenglow so we felt it appropriate to look toward the Montana sky and horizon again to see what we missed. Polaris, commonly called the North Star is a perfect point of night light for our Northern Rockies and prairie. Once you learn to find it, your eyes will easily pick it out, especially during our clean sharp winter nights.

Through the ages, looking northwards, Polaris has been used as a tool of navigation for it represents true north and remains in place directly over the Earth’s north pole while other heavenly bodies travel in a circular arc. Explorers could determine their latitude from the North Star by using a sextant, a device that incorporates a telescope and an angular scale. The number of degrees above the horizon the star appeared was the degrees of north latitude the wanderer was located on.

Looking skyward from what would become Helena, the Four Georgians (the men who discovered gold in Last Chance Gulch) would have seen the North Star 47 degrees above the north horizon and Meriwether Lewis from his lower portage camp just beyond Great Falls would have noted Polaris twinkling at about 48 degrees north latitude.

In ancient times, navigators measured the angle of the North Star at their home port. And according to one historian, “To return after a long voyage, these seamen only needed to sail north or south to bring the star back to the angle of their home — then turn left or right and sail down the latitude, keeping the angle the same all the way.”

Polaris is the end star in the handle of Ursa Minor — the Little Dipper, but this grouping tends to be faint and usually only Polaris shows well. The easiest way to locate the North Star is to look at Ursa Major — the Big Dipper — which clearly stands out under our darkened big sky. Follow a line from the two outside stars of the cup going from the bottom to the top edge (where the water would pour out) and that line will lead you to the navigation star — Polaris — you can’t miss it.

The earth wobbles a bit so the Polaris hasn’t always been the star of the north and in the very distant future it will retire and give its job to another gathering of plasma called a star.

Wanderers trying to determine their direction south of the equator have a much tougher time. A south star doesn’t exist but the constellation Crux, referred to as the Southern Cross does point to the South Pole.

The North Star may reside over the North Pole but if you open your compass at night and aim the point of north at Polaris, the compass needle will not point to the star. If you are doing this in Great Falls you will be off by close to 15 degrees because a magnetic north as well as true north coexist. If you were in the vicinity of the Mississippi River, true north and magnetic north are the same and your compass would be accurate.

The further west you go, the more the needle is pulled to the east. Missoula, further west, for instance, is 15.5 degrees off to the east and Glendive, much further east needs only a 10.7 degrees change.

Magnetic north (same thing happens on the other end of the planet) is the point where lines of the magnetic field (remember our discussion on the Northern Lights and the role the magnetic field plays) line up vertically and dive straight into the earth acting much like a super sized magnet. And the place doesn’t stay still but rather is inching ever so slowly across the Canadian Arctic and is now somewhere west of Greenland and in the northern reaches of Canada’s newest territory, Nunavut. Estimates are that it is currently moving in a northwesterly direction at about 20 to 25 miles a year.

Back to Great Falls — your compass should have an adjustment on it that allows you to compensate for the magnetic declination — move the travel arrow 15 degrees east and your compass will be properly aligned. Test it again with the North Star. USGS and other maps usually have the true north-magnetic north scale on them. However as we said, the magnet is moving and so a check on the internet can give you the exact reading. Your compass may not allow for tenths of a degree, but your setting will be close enough to plot a course across the landscape without becoming hopelessly off course.

Rick & Susie Graetz are Montana publishers and photojournalists. Rick teaches three course during the autumn semester at The University of Montana. Their latest book is Montana High, Wide and Handsome. Their e-mail address is thisismontana@aol.com.