Measuring True North
The direction of geographic north can also be determined by
measuring the shadow of a gnomon at local noon. This can be used to
calculate the position of the Earth's magnetic north pole. See the measuring the circumference of the earth page for
information about measuring sun angles and gnomons.
When you are outside measuring the length of the shadow cast by a gnomon at noon, notice the direction the shadow lies when it is the shortest. It points due north. Why? Consider the position of the sun in the sky at noon. On any given day, the latitude of the sun is anywhere between -23.4 and +23.4 degrees. The longitude of the sun changes as the sun "moves" across the sky (the earth is rotating beneath the sun). Local noon occurs when the sun's longitude equals your longitude. This is as close as the sun gets to being directly overhead. At this moment, the sun is due south of anyone north of the Tropic of Cancer. All shadows are cast due north.
To visualize this, tape a crayon or a matchstick onto a large globe so that it is perpendicular to the surface. The crayon is the gnomon. Darken the room and shine a flashlight on the globe. Make sure the flashlight is oriented so that the bulb is even with a point on the globe that is between the tropics (why?). Slowly rotate the globe. When is the shadow the shortest? In what direction does the shadow point?
Collaborative ProjectThis leads into an excellent collaborative project. From Earth Science, you should remember that at any spot on the earth, there is a difference between the direction a magnetic compass will point and the direction towards the north pole. Stated simply, if you follow a compass, you will not end up at the North Pole. This is because the geographic north pole and the magnetic north pole are not located at the same place! The geographic north pole is located at the top of the globe. This is the place where the longitude lines converge and its direction is called true north. The magnetic north pole is located somewhere in Canada.
Not only are magnetic north and true north different, the amount of variation between magnetic north and true north changes as you travel from place to place, especially as you move north towards the pole. You can easily measure this difference. True north is the direction the gnomon's shadow points at local noon. Magnetic north is measured using a magnetic compass.
Using collaborative methods, we can compile a small database of how magnetic north and geographic north differ across the country (or hemisphere) and use this information to locate the earth's magnetic north pole. More advanced classes can take these angle differences and trace two geodesics (great circles) across the surface of the globe. Geodesics corresponding to the compass directions will intersect at the magnetic north pole. This is an interesting exercise becasue the analysis must be done on a globe. Drawing lines on a flat map will not work.
Checking Your Results: Noon, True North, Magnetic NorthNoon
Before submitting data to a collaborative group project, you should check to see that your measurements are accurate. For example, notice how the shadow of the gnoman changes around local noon. What changes more, its direction or its size? How often and how carefully do you need to measure the shadow's position?
You can verify your measurements of due north in an old fashioned but accurate way: sight Polaris. Pick any clear night and look for the north star.
Magnetic north is the easiest thing to measure, but there may be some hidden problems. First, make sure your compass works. Many of the compasses at our school have been zapped by the large horseshoe magnets they are stored next to. Also, beware of localized magnetic disturbances. Often, concrete is reinforced with metal that is magnetic (or there are small bits of metal ground up in the concrete). Placing your compass on the sidewalk or blacktop could result in poor readings. This was our experience. Try doing your measurements on a large wooden table instead.
You can always check your results on a nautical navigation chart.